Antithrombin Deficiency
Antithrombin deficiency is an inherited or acquired reduction in antithrombin activity that impairs the inhibition of thrombin and factor Xa, leading to a hypercoagulable state with increased risk of venous thromboembolism.
Antithrombin Deficiency
Antithrombin deficiency is a hereditary or acquired disorder characterised by a quantitative reduction or qualitative dysfunction of antithrombin (AT, historically called antithrombin III / AT-III), a serine protease inhibitor (serpin) that is the principal physiological inhibitor of thrombin (factor IIa) and factor Xa, as well as factors IXa, XIa, and XIIa [1][2]. The resulting loss of this natural anticoagulant shifts the haemostatic balance toward a prothrombotic state, predisposing to venous thromboembolism (VTE) — deep vein thrombosis (DVT) and pulmonary embolism (PE) — and, less commonly, arterial thrombosis.
The name itself tells you the pathology: "anti" = against, "thrombin" = the central enzyme of coagulation. Antithrombin is literally the body's built-in brake on clot formation. When that brake fails, clots form unchecked.
2. Epidemiology
- Prevalence in the general population is approximately 0.02–0.2% (1 in 500 to 1 in 5,000) [1][2].
- Among patients presenting with a first unprovoked VTE, inherited AT deficiency accounts for roughly 1–2% of cases.
- AT deficiency is the rarest but most thrombogenic of the three classic inherited thrombophilias (AT deficiency, protein C deficiency, protein S deficiency) [1][2].
- In the Chinese population, inherited thrombophilias (AT, protein C, protein S deficiency) are very rare causes of VTE [1][3].
- Factor V Leiden is NOT found in Chinese — this is a Caucasian-predominant mutation [1][2]. This is critical for HKUMed exams: do not include Factor V Leiden in the differential for thrombophilia in a Chinese patient.
- Prothrombin G20210A mutation is also virtually absent in East Asians.
- Despite rarity, AT deficiency should still be considered in young Chinese patients with unprovoked or recurrent VTE, VTE at unusual sites, or family history of thrombosis.
| Feature | Value |
|---|---|
| General population prevalence | 0.02–0.2% |
| Relative risk of VTE | ~16.3× compared to normal (highest among inherited thrombophilias) |
| VTE by age 60 | ~70% of affected individuals will have had at least one VTE event |
| Mode of inheritance | Autosomal dominant with variable penetrance |
3. Anatomy and Function of Antithrombin
Antithrombin (AT) is a single-chain glycoprotein of ~58 kDa, belonging to the serpin superfamily (serine protease inhibitor). It circulates in plasma at a concentration of approximately 150 µg/mL (functional level ~80–120%).
- Antithrombin is synthesised by the liver [1]. This is why severe liver disease also leads to acquired AT deficiency — the failing liver simply cannot make enough.
- SERPINC1 gene, located on chromosome 1q25.1 [2].
- "SERPIN" = serine protease inhibitor; "C1" = complement/coagulation inhibitor subfamily, member 1.
AT acts as a suicide substrate inhibitor (a "molecular trap"): it binds covalently to the active site of target serine proteases and forms an irreversible 1:1 complex, permanently inactivating them.
Antithrombin is a protease that neutralises (inactivates) the following clotting factors: [1]
- Thrombin (factor IIa) — the most important target
- Factor Xa — the second most important target
- Factor IXa
- Factor XIa
- Factor XIIa
"Basically some common pathway, then most of the intrinsic pathway" [1]
Think of it this way: AT polices the middle and end of the coagulation cascade. It does NOT significantly inhibit factor VIIa (the initiator of the extrinsic pathway — that's the job of tissue factor pathway inhibitor / TFPI).
Under physiological conditions, AT's inhibitory activity is slow. The endogenous heparan sulphate proteoglycans on the endothelial cell surface (and therapeutically administered heparin) accelerate AT's activity by ~1,000–4,000 fold.
Heparin relies on antithrombin to exert its activities — hence, without antithrombin, heparin has nothing to enhance, and thus cannot inhibit anything. [1]
"If your patient has a homozygous deficiency of the [antithrombin] gene, then they will have primary resistance towards heparin." [1]
The lecture analogy is apt: "Antithrombin is the army, heparin is the king — without an army, no matter how smart the king is, cannot defeat the enemy." [1]
How heparin works mechanistically:
- Heparin (a negatively charged glycosaminoglycan) binds to a specific pentasaccharide sequence on AT, inducing a conformational change in AT.
- This conformational change exposes the reactive centre loop of AT, drastically accelerating its binding to thrombin and factor Xa.
- For thrombin inhibition specifically, heparin must be long enough (≥18 saccharide units) to simultaneously bind both AT and thrombin (a "ternary bridging complex"). This is why unfractionated heparin (UFH) inhibits both thrombin and Xa, while low-molecular-weight heparin (LMWH) primarily inhibits factor Xa (shorter chains cannot bridge).
- Fondaparinux (a synthetic pentasaccharide = the minimal heparin sequence needed to bind AT) activates AT selectively against factor Xa only.
Why Heparin Resistance Matters Clinically
AT deficiency sits within the broader framework of the body's three major natural anticoagulant pathways:
| System | Key Components | Targets Inhibited | Where Deficiency Causes Problems |
|---|---|---|---|
| Antithrombin pathway | Antithrombin + heparan sulphate | Thrombin (IIa), Xa, IXa, XIa, XIIa | Venous > arterial thrombosis |
| Protein C pathway | Protein C + Protein S + Thrombomodulin | Factor Va, Factor VIIIa | Venous thrombosis; warfarin-induced skin necrosis |
| Tissue Factor Pathway Inhibitor (TFPI) | TFPI | Factor VIIa–TF complex, Xa | Rarely clinically significant deficiency |
4. Aetiology
This is the form most commonly discussed in the thrombophilia context.
Inheritance: Autosomal dominant with variable penetrance [1][2]
- Heterozygous deficiency: the clinically relevant form. AT levels typically 40–60% of normal. Symptomatic with variable expressivity.
- Homozygous Type I deficiency: essentially incompatible with life (lethal in utero or neonatal period) — very rare, almost never seen clinically.
- Homozygous Type II (heparin-binding site) deficiency: rare but can be viable, though severely prothrombotic.
Classification of Hereditary AT Deficiency
The risk of thrombosis depends on the type of antithrombin deficiency: [1]
| Type | Subtype | Mechanism | AT Antigen Level | AT Activity Level | Thrombotic Risk |
|---|---|---|---|---|---|
| Type I (Quantitative) | — | Reduced synthesis or increased clearance of a normal AT molecule | ↓ (~50% in heterozygotes) | ↓ (proportionally reduced) | High (OR ~5–16.3×) |
| Type II (Qualitative) | IIa — Reactive site (RS) | Mutation in the reactive centre loop (the "business end" that binds thrombin/Xa) | Normal | ↓ | High |
| IIb — Heparin-binding site (HBS) | Mutation in the heparin-binding domain → AT cannot be activated by heparin | Normal | ↓ (especially heparin-cofactor activity) | Lower than IIa; homozygous state can occur | |
| IIc — Pleiotropic | Mutations affecting multiple functions (both reactive site and heparin binding, or conformational) | Normal or slightly ↓ | ↓ | Variable |
Why does this classification matter?
- Type I and Type II-RS have the highest thrombotic risk because the AT molecule is either absent or functionally dead at its critical business end.
- Type II-HBS has a lower thrombotic risk in heterozygotes because the AT molecule can still inhibit thrombin/Xa (just more slowly without heparin enhancement). However, homozygous Type II-HBS individuals lose the heparin-augmentation entirely and are at significant risk.
Acquired causes are far more common than hereditary causes and must be excluded before labelling a patient as having inherited AT deficiency.
| Mechanism | Causes |
|---|---|
| Decreased synthesis (liver origin) | Liver cirrhosis, acute liver failure, hepatitis, neonatal immaturity |
| Increased consumption | DIC, major surgery, extensive thrombosis, sepsis, burns, pre-eclampsia/eclampsia/HELLP |
| Increased loss | Nephrotic syndrome (AT is lost in urine — AT molecular weight ~58 kDa is similar to albumin ~67 kDa, so it leaks through the damaged glomerular basement membrane) [2][4] |
| Drug-induced | Heparin therapy (paradoxically, heparin accelerates AT consumption), L-asparaginase (used in ALL treatment — inhibits hepatic protein synthesis), oestrogen-containing OCP |
| Dilutional | Massive transfusion, large-volume fluid resuscitation |
Nephrotic Syndrome and Thrombosis
In nephrotic syndrome, loss of natural anticoagulants (especially antithrombin) in the urine contributes to the hypercoagulable state. [2][4] This is why nephrotic syndrome is associated with renal vein thrombosis, DVT, and PE. The urinary loss of AT parallels albumin loss — when you see heavy proteinuria, think "losing clot brakes too."
5. Pathophysiology
The pathophysiology is straightforward once you understand AT's role:
-
Normal state: AT continuously patrols the blood, neutralising any thrombin and factor Xa that are generated. This prevents inappropriate clot propagation. Endothelial heparan sulphate potentiates this on vessel walls, creating an anticoagulant surface.
-
AT deficiency: With reduced AT quantity or function, the "braking system" on coagulation is impaired:
- Thrombin accumulates → converts more fibrinogen to fibrin → clot grows
- Factor Xa accumulates → generates more thrombin via prothrombinase complex → positive feedback amplification
- Factor IXa, XIa, XIIa accumulate → further amplification of intrinsic pathway
- Net result: unopposed thrombin generation → hypercoagulable state → VTE
-
Why venous thrombosis predominantly?
- Venous flow is slow (stasis), allowing coagulation factors to accumulate locally
- The coagulation cascade (thrombin/fibrin-driven) is the dominant mechanism in venous thrombosis (as opposed to arterial thrombosis, which is more platelet-driven)
- AT deficiency specifically impairs the coagulation cascade inhibition → preferentially predisposes to venous clots
-
Heparin resistance:
- Since heparin's mechanism of action is to potentiate AT, patients with AT deficiency may show VTE often resistant to normal doses of heparin [2]
- This manifests as failure to achieve therapeutic APTT prolongation despite adequate/escalating UFH doses
- Or subtherapeutic anti-Xa levels despite adequate LMWH dosing
AT deficiency alone (in heterozygotes) is often not sufficient to cause thrombosis — most affected individuals need an additional provocation (Virchow's triad):
- Stasis: immobilisation, long-haul flights, post-operative period
- Endothelial injury: surgery, trauma, central venous catheters
- Additional hypercoagulability: pregnancy (physiological increase in procoagulant factors II, VII, VIII, X, fibrinogen + decreased protein S), oral contraceptive pills, concurrent malignancy
This explains why ~70% have VTE before age 60 [2] — over a lifetime, the cumulative exposure to "second hits" eventually triggers an event.
6. Classification
| Category | Subtypes |
|---|---|
| Hereditary | Type I (quantitative), Type II (qualitative: IIa reactive-site, IIb heparin-binding site, IIc pleiotropic) |
| Acquired | Decreased synthesis, increased consumption, increased urinary/GI loss, drug-induced, dilutional |
| Zygosity | AT Level (approx.) | Clinical Significance |
|---|---|---|
| Heterozygous Type I | ~40–60% | Clinically significant; most patients encountered |
| Homozygous Type I | Near 0% | Usually lethal in utero |
| Heterozygous Type II-RS | Normal antigen, ↓ activity | Clinically significant |
| Heterozygous Type II-HBS | Normal antigen, ↓ heparin-cofactor activity | Lower risk in heterozygotes |
| Homozygous Type II-HBS | Normal antigen, absent heparin-cofactor activity | Viable but significant risk |
Common inherited thrombophilias (all autosomal dominant) [1][2]:
| Condition | Gene / Locus | Physiological Role | Prevalence | VTE Risk | Key Clinical Pearls |
|---|---|---|---|---|---|
| Antithrombin deficiency | SERPINC1, 1q25.1 | AT inactivates thrombin and Xa (↑ by heparin) | 0.02–0.2% | 16.3× | Heparin resistance; need higher LMWH dose ≥100 U/kg/d; AT concentrate for refractory cases |
| Protein C deficiency | PROC, 2q13-14 | Activated protein C inactivates Va, VIIIa | 0.2–0.5% | 7× | A/w warfarin-induced skin necrosis in first few days |
| Protein S deficiency | PROS, 3q11.1 | Acts as co-factor of protein C | Low (unclear) | Similar to protein C | Individualise decision for indefinite anticoagulation |
Warfarin-Induced Skin Necrosis
Initial phase of warfarin use is associated with protein C and S depletion, leading to cutaneous vascular thrombosis in those with protein C deficiency. [2] This occurs because warfarin inhibits vitamin K-dependent factors (II, VII, IX, X) AND natural anticoagulants (protein C, protein S). Since protein C has the shortest half-life (~6–8 hours) among these, it drops fastest when warfarin is started → transient prothrombotic state → skin necrosis. This is why we always bridge with heparin when initiating warfarin, especially in suspected protein C/S deficiency.
This is NOT a feature of antithrombin deficiency (AT is not vitamin K-dependent).
7. Clinical Features
| Symptom | Pathophysiological Basis |
|---|---|
| Unilateral leg swelling, pain, warmth | DVT — thrombus in deep veins of the lower extremity (ilio-femoral > calf) causes venous outflow obstruction → increased hydrostatic pressure → oedema, and inflammatory response in vessel wall → pain and warmth |
| Sudden-onset dyspnoea, pleuritic chest pain, haemoptysis | PE — embolised thrombus from DVT lodges in pulmonary vasculature → V/Q mismatch → hypoxaemia → dyspnoea; pulmonary infarction → pleuritic pain and haemoptysis |
| Recurrent VTE | Persistent hypercoagulable state due to chronic AT deficiency; each episode of VTE damages venous valves, further predisposing to stasis and recurrence |
| VTE at young age (< 40–50 years) | Inherited deficiency present from birth; cumulative lifetime exposure to provocative triggers eventually results in thrombosis at an earlier age than sporadic VTE |
| VTE at unusual sites (mesenteric vein, portal vein, cerebral venous sinus, renal vein) | The hypercoagulable state is systemic, not limited to lower extremity veins; unusual-site VTE should always prompt thrombophilia screening |
| VTE during pregnancy/puerperium | Pregnancy is itself a hypercoagulable state (increased factors II, VII, VIII, X, fibrinogen; decreased protein S); AT deficiency compounds this → significantly elevated VTE risk during pregnancy and post-partum |
| VTE precipitated by minor provocations (short flights, minor illness) | Low AT levels mean the threshold for clot formation is reduced; trivial provocations that would not cause VTE in healthy individuals can trigger events |
| VTE resistant to standard heparin dosing | Heparin requires AT to function; with AT deficiency, heparin's anticoagulant effect is blunted → "heparin resistance" [1][2] — clinically, this manifests as failure to achieve target APTT or anti-Xa levels despite adequate dosing |
| Sign | Pathophysiological Basis |
|---|---|
| Leg swelling with pitting oedema, calf tenderness (DVT) | Venous outflow obstruction → elevated venous hydrostatic pressure → transudation of fluid into interstitial space |
| Positive Homan's sign (pain on dorsiflexion of foot) | Stretching of the gastrocnemius compresses the thrombosed deep veins → pain. Note: low sensitivity and specificity; not reliable |
| Dilated superficial veins | Collateral venous drainage develops to bypass obstructed deep veins |
| Tachycardia, tachypnoea, hypoxia (PE) | Pulmonary vascular obstruction → dead-space ventilation, V/Q mismatch → hypoxaemia → compensatory tachycardia and tachypnoea |
| Raised JVP, right parasternal heave (massive PE) | Acute right ventricular strain → RV dilatation → elevated right-sided pressures |
| Hypotension, cardiovascular collapse (massive PE) | Massive clot burden obstructs > 50% of pulmonary vasculature → acute RV failure → reduced LV preload → cardiogenic shock |
| Post-thrombotic syndrome (chronic) | Venous valve damage from prior DVT → chronic venous hypertension → leg oedema, skin changes (haemosiderin staining, lipodermatosclerosis), venous ulceration |
| No specific physical signs of AT deficiency itself | AT deficiency is a laboratory diagnosis; the clinical manifestations are entirely those of its thrombotic complications |
Indications for thrombophilia screening (which would detect AT deficiency) [3]:
- Young patients with idiopathic venous thrombosis
- Suspected antiphospholipid syndrome, e.g. recurrent miscarriage
- Unusual sites of thrombosis (mesenteric, renal, portal vein, cerebral venous sinus)
- Warfarin-induced skin necrosis (protein C/S deficiency, but screen full panel)
- First-degree relative with documented inherited thrombophilia
- Recurrent VTE despite adequate anticoagulation
- Heparin resistance
| Feature | AT Deficiency | Protein C Deficiency | Protein S Deficiency |
|---|---|---|---|
| Thrombotic risk | Highest (~16.3×) | Moderate (~7×) | Moderate (similar to protein C) |
| VTE by age 60 | ~70% | ~50% | ~50% |
| Heparin resistance | Yes (characteristic) | No | No |
| Warfarin-induced skin necrosis | No | Yes (classic association) | Yes (can occur) |
| Neonatal purpura fulminans | No (homozygous lethal) | Yes (homozygous protein C deficiency) | Rare |
| Vitamin K-dependent? | No (AT is NOT vitamin K-dependent) | Yes | Yes |
Why AT Deficiency Does NOT Cause Warfarin-Induced Skin Necrosis
Warfarin inhibits vitamin K-dependent factors (II, VII, IX, X) and vitamin K-dependent anticoagulants (protein C, protein S). Since AT is NOT vitamin K-dependent, warfarin does not affect AT levels. Therefore, the transient prothrombotic state from early protein C depletion on warfarin initiation is irrelevant to AT deficiency.
On standard coagulation testing (PT and APTT), antithrombin deficiency does NOT cause any abnormality: [5]
- PT: Normal (extrinsic pathway intact)
- APTT: Normal (all intrinsic pathway factors are present; AT is not a clotting factor but an inhibitor)
- Fibrinogen: Normal
- Platelet count: Normal
This is a critical teaching point: a normal PT and APTT do NOT exclude a hypercoagulable state. Standard clotting tests only detect factor deficiencies (bleeding tendency). They do not detect deficiencies of natural anticoagulants. Specific assays (AT activity, protein C, protein S) are needed.
The only scenario where AT deficiency might indirectly affect the APTT is when heparin is administered — you would expect UFH to prolong the APTT, but in AT deficiency, the APTT fails to prolong adequately (heparin resistance).
"Heparin can inhibit the intrinsic pathway (prolonging APTT), as well as inhibits thrombin (factor IIa, in common pathway) — antithrombin activated, inactivates thrombin-induced activation of factors V, VIII, and XI" [5]
8. Important Conceptual Links
In DIC, there is widespread activation of the coagulation cascade → massive consumption of clotting factors AND natural anticoagulants including AT [3][6]. This creates a vicious cycle:
- Reduced AT → less inhibition of thrombin → more thrombin generation → more fibrin deposition → more consumption of AT
- This is why AT levels are measured in DIC and why AT concentrate has been studied (though not standard therapy) as adjunctive treatment
Pregnancy is a physiological prothrombotic state. In women with AT deficiency:
- Baseline thrombotic risk is already elevated
- Pregnancy further increases procoagulant factors and reduces protein S
- Risk of VTE during pregnancy is estimated at 30–50% per pregnancy in untreated AT-deficient women
- Prophylaxis is required in pregnancy, surgery, or post-VTE [2]
Both cause hypercoagulability but through different mechanisms:
- AT deficiency: loss of a natural anticoagulant → reduced inhibition of coagulation cascade
- APS: autoantibodies against phospholipids/β2-glycoprotein I → complex mechanism involving endothelial activation, complement activation, and platelet activation → both venous AND arterial thrombosis
- APS paradoxically prolongs APTT in vitro (lupus anticoagulant interferes with phospholipid-dependent clotting tests) but causes thrombosis in vivo
- AT deficiency does NOT affect APTT
High Yield Summary
Antithrombin Deficiency — Key Points for Exams:
- Autosomal dominant inheritance, SERPINC1 gene on chromosome 1q25.1 [1][2]
- AT is synthesised in the liver; it is a serine protease inhibitor (serpin) that neutralises thrombin (IIa), Xa, IXa, XIa, XIIa [1]
- Heparin works BY potentiating AT → without AT, heparin cannot work → heparin resistance is the hallmark clinical clue [1]
- Prevalence: 0.02–0.2%; confers the highest VTE risk (~16.3×) among inherited thrombophilias [2]
- ~70% of affected individuals develop VTE before age 60 [2]
- Type I (quantitative: ↓antigen and ↓activity) vs Type II (qualitative: normal antigen, ↓activity) [1]
- NOT vitamin K-dependent → NOT associated with warfarin-induced skin necrosis (that's protein C/S)
- Normal PT and APTT — standard clotting tests do NOT detect natural anticoagulant deficiencies
- Acquired causes: liver disease, DIC, nephrotic syndrome (urinary loss), heparin therapy, L-asparaginase
- Treatment: higher-dose LMWH (≥100 U/kg/day), AT concentrate for refractory VTE, long-term anticoagulation, prophylaxis in high-risk situations [2]
- Factor V Leiden is NOT found in Chinese — do not include in differential for Chinese patients [1][2]
- Thrombophilia screening should NOT be done during acute VTE or while on anticoagulants [3]
Active Recall - Antithrombin Deficiency
[1] Lecture slides: Block A - Leg swelling and chest pain: deep vein thrombosis; pulmonary embolism; Thrombophilia.pdf (Inherited thrombophilia section) [2] Senior notes: Ryan Ho Haemtology.pdf (Section 4.5 - Other Thrombotic Disorders); Adrian Lui Pediatrics Notes.pdf (Section 10.2.4 - Other Thrombotic Disorders) [3] Senior notes: Maksim Medicine Notes.pdf (Haematology - Thrombophilia screening) [4] Senior notes: Block A - Hematology Data Interpretation.pdf (Nephrotic syndrome complications - loss of antithrombin) [5] Senior notes: Block A - Abnormal bleeding after tooth extraction_ bleeding tendency; thrombocytopenia.pdf; Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (Coagulation cascade and interpretation) [6] Senior notes: Maksim Medicine Notes.pdf (DIC section)
Differential Diagnosis of Antithrombin Deficiency
When we talk about the "differential diagnosis of antithrombin deficiency," we are really asking two linked clinical questions:
- A patient presents with VTE (or recurrent VTE) — what are the possible underlying prothrombotic causes? (i.e., DDx of the hypercoagulable state)
- A patient has a low antithrombin level on laboratory testing — is this truly hereditary AT deficiency, or is the low AT level caused by something else? (i.e., DDx of a low AT level)
Both angles are examinable. Let's work through each systematically.
A. Differential Diagnosis of the Hypercoagulable State (Thrombophilia)
This is the broader clinical question: a young patient presents with unprovoked VTE, VTE at an unusual site, recurrent VTE, or VTE with a strong family history. What are the possible aetiologies?
| Category | Condition | Mechanism / Why It Causes Thrombosis |
|---|---|---|
| Stasis | Prolonged immobilisation (bed rest, long-haul flights, plaster casts) | Virchow's triad — slow flow allows coagulation factor accumulation and prevents dilution/clearance by hepatic blood flow |
| Hormonal | High-dose oestrogen (OCP, HRT, ovulation induction) | Oestrogen ↑ hepatic synthesis of procoagulant factors (fibrinogen, VII, VIII, X, vWF) and ↓ natural anticoagulants (protein S, AT) |
| Pregnancy and puerperium | Same mechanism as oestrogen + mechanical compression of IVC/iliac veins by gravid uterus + endothelial injury at delivery [2][7] | |
| Malignancy | Visceral malignancy (especially adenocarcinoma — pancreas, stomach, ovary, lung) | Most important cause of unprovoked VTE! [2] Tumour cells secrete tissue factor and mucin (activate extrinsic pathway), cancer-associated inflammation, immobility, and direct venous compression/invasion |
| Myeloproliferative neoplasms (PV, ET, PMF) | Hyperviscosity, abnormal platelet function, JAK2-driven endothelial activation | |
| Autoimmune | Antiphospholipid syndrome (APS) (primary or secondary to SLE) | Autoantibodies (lupus anticoagulant, anti-cardiolipin, anti-β2-GPI) → endothelial activation, complement activation, platelet activation → both venous AND arterial thrombosis |
| Inherited thrombophilia | Antithrombin deficiency | Loss of the principal thrombin/Xa inhibitor → unopposed thrombin generation |
| Protein C deficiency | Loss of activated protein C → cannot inactivate factors Va and VIIIa → excessive thrombin generation | |
| Protein S deficiency | Loss of protein C's cofactor → impaired protein C function |
GC High Yield – Malignancy as Cause of VTE
Malignancy, especially visceral malignancy, is the most important cause of unprovoked VTE. [2] In exams, if a middle-aged or elderly patient presents with unprovoked VTE, always consider occult malignancy as the top differential — even before inherited thrombophilia. Occult cancer screening (CT TAP, age-appropriate cancer screening) is warranted in genuinely unprovoked VTE in patients > 40 years old.
These causes are extremely uncommon in Chinese. Do not include these as differential diagnosis in Chinese patients. [2]
| Condition | Why Rare in HK / Chinese? | Mechanism |
|---|---|---|
| Factor V Leiden | NOT found in Chinese — prevalence 3–8% in Caucasians, essentially 0% in East Asians [2][7] | Point mutation (Arg506Gln) renders factor Va resistant to inactivation by activated protein C ("activated protein C resistance" / APCR) |
| Prothrombin G20210A mutation | Caucasian-predominant (~2% prevalence), near-absent in Chinese | Gain-of-function mutation → ↑ prothrombin levels → more thrombin available |
| Paroxysmal nocturnal haemoglobinuria (PNH) | Very rare in the Chinese [2] | PIGA gene mutation → loss of GPI-anchored complement regulatory proteins (CD55/CD59) → complement-mediated haemolysis + thrombosis (mechanism of thrombosis is multifactorial: free Hb scavenging NO, platelet activation, complement on platelet surface) |
| Homocystinaemia | No data in the Chinese [2] | Homocysteine damages endothelium, activates tissue factor, impairs thrombomodulin and protein C activation |
| Nephrotic syndrome | Not ethnicity-specific but an acquired cause | Loss of natural anticoagulants (especially antithrombin) in urine [2][4] — AT MW ~58 kDa leaks through damaged GBM alongside albumin (~67 kDa) |
Exam Pitfall — Factor V Leiden in Chinese Patients
Factor V Leiden is NOT found in Chinese. [2][7] If a question asks about thrombophilia DDx in a Chinese/HK patient, do NOT include Factor V Leiden or prothrombin G20210A mutation. This is a common trap in MCQs. The relevant inherited thrombophilias in Chinese are AT deficiency, protein C deficiency, and protein S deficiency.
Common inherited thrombophilias [1][2][7]:
| Feature | Antithrombin Deficiency | Protein C Deficiency | Protein S Deficiency |
|---|---|---|---|
| Inheritance | AD, variable penetrance | AD | AD |
| Gene / Locus | SERPINC1, 1q25.1 | PROC, 2q13-14 | PROS, 3q11.1 |
| Physiological role | AT inactivates thrombin and Xa (↑ by heparin) | Activated protein C inactivates Va, VIIIa | Acts as co-factor of protein C |
| Prevalence | 0.02–0.2% | 0.2–0.5% | Low (unclear) |
| VTE risk (OR) | 16.3× (highest) | 7× | Similar to protein C |
| % with VTE by age 60 | ~70% | ~50% | ~50% |
| Heparin resistance | Yes (hallmark) | No | No |
| Warfarin-induced skin necrosis | No (AT is NOT vit K-dependent) | Yes (classic, in first few days) | Yes (can occur) |
| Vit K-dependent? | No | Yes | Yes |
| Treatment of acute VTE | Higher dose LMWH (≥100 U/kg/d); AT concentrate for refractory VTE | Anticoagulation (should continue indefinitely) | Anticoagulation (individualise indefinite Rx) |
| Prophylaxis | Pregnancy, surgery, post-VTE | Pregnancy, surgery, post-VTE | Pregnancy, surgery |
"Warfarin-induced skin necrosis in the first few days" occurs in protein C deficiency because protein C has the shortest half-life (~6–8 hours) of the vitamin K-dependent factors — it drops first when warfarin is started, creating a transient prothrombotic window → cutaneous microvascular thrombosis. AT deficiency does NOT cause this because AT is not vitamin K-dependent. [2][7]
B. Differential Diagnosis of a Low Antithrombin Level
This is the laboratory-focused question: you've measured AT activity and it comes back low. Before diagnosing hereditary AT deficiency, you must exclude acquired causes (which are far more common).
| Mechanism | Specific Causes | Why / How AT Is Reduced |
|---|---|---|
| ↓ Synthesis | Liver cirrhosis / hepatic failure | AT is synthesised in the liver [1]; hepatocyte loss → ↓ production of all liver-synthesised proteins |
| Acute hepatitis | Transient hepatocellular dysfunction | |
| Neonatal immaturity | Immature liver in premature neonates | |
| L-asparaginase therapy | Used in ALL treatment; inhibits hepatic protein synthesis globally | |
| ↑ Consumption | DIC | Widespread activation of coagulation → massive consumption of AT (AT is used up trying to neutralise the excess thrombin) [3][9] |
| Acute extensive thrombosis | Large thrombus burden consumes AT at the site of clotting | |
| Major surgery / burns / trauma | Massive tissue factor release → coagulation activation → AT consumption | |
| Sepsis | Endothelial damage + DIC pathway | |
| Pre-eclampsia / eclampsia / HELLP | Endothelial dysfunction + microangiopathic consumption | |
| ↑ Loss | Nephrotic syndrome | Loss of antithrombin in urine [2][4] — AT (58 kDa) leaks through the damaged glomerular basement membrane alongside albumin |
| Protein-losing enteropathy | GI loss of plasma proteins including AT | |
| Drug-induced | Heparin therapy | Heparin accelerates AT–protease complex formation → faster clearance of AT. AT levels can drop 30–50% on UFH. Paradoxical but important. |
| Oestrogen (OCP, HRT) | Oestrogen modulates hepatic AT expression | |
| Dilutional | Massive transfusion / fluid resuscitation | Dilution of all plasma proteins |
Critical Exam Point – Timing of Testing
Do not test AT levels at the time of an acute VTE event, or while the patient is receiving anticoagulants. [3]
- Acute thrombosis consumes AT → falsely low
- *Heparin therapy reduces AT levels → falsely low
- Warfarin does NOT affect AT (AT is not vit K-dependent) but other thrombophilia tests (protein C, protein S) are affected
- DOACs can interfere with functional clotting-based AT assays
Recommended timing: withhold warfarin × 2 weeks, DOAC × at least 2 days, and test in a stable, non-acute state [3].
| Feature | Hereditary AT Deficiency | Acquired AT Deficiency |
|---|---|---|
| AT level persistently low (on repeat testing, off anticoagulants, no acute illness) | Yes | No — normalises once the underlying cause resolves |
| Family history of VTE or low AT | Yes (AD inheritance) | No |
| Young age at first VTE | Typical (< 40–50 years) | Depends on the acquired cause |
| Associated with liver disease, nephrotic syndrome, DIC, heparin use | No | Yes — identify the underlying condition |
| Genetic testing (SERPINC1) | Confirms pathogenic variant | Negative |
Heparin resistance is a clinical scenario where heparin fails to achieve the expected anticoagulant effect (target APTT or anti-Xa level). AT deficiency is the classic cause, but other possibilities exist:
| Cause of Heparin Resistance | Mechanism |
|---|---|
| Antithrombin deficiency (hereditary or acquired) | Heparin relies on AT to exert its activity — without AT, heparin has nothing to enhance [1] |
| Increased heparin clearance | Acute phase proteins bind heparin; elevated factor VIII (acute phase reactant) competes |
| Elevated factor VIII levels | Factor VIII is an acute phase reactant; very high levels can "overwhelm" the AT–heparin complex's ability to suppress thrombin generation |
| Heparin binding to non-specific proteins | Platelet factor 4 (PF4), histidine-rich glycoprotein, fibronectin, vitronectin — all bind and neutralise heparin |
| Increased heparin volume of distribution | Splenomegaly, large body habitus, massive oedema/ascites |
| Spurious (lab artefact) | Underfilled citrate tubes, heparin-contaminated samples |
"IMPORTANT CONCEPT → Heparin relies on antithrombin to exert its activities // hence, without antithrombin, heparin has nothing to enhance, and thus cannot inhibit anything" [1]
The following diagram shows the clinical thinking pathway when evaluating a patient with suspected AT deficiency in the context of thrombophilia workup:
Both are important causes of thrombophilia but differ fundamentally:
| Feature | Antithrombin Deficiency | Antiphospholipid Syndrome |
|---|---|---|
| Nature | Inherited (usually) or acquired | Acquired autoimmune |
| Mechanism | Loss of natural anticoagulant | Autoantibodies against phospholipids/β2-GPI → endothelial and platelet activation |
| Thrombosis pattern | Venous >> arterial | Both venous AND arterial |
| Pregnancy complications | VTE risk in pregnancy | Recurrent miscarriage, IUFD, pre-eclampsia (obstetric morbidity is a diagnostic criterion) |
| Effect on APTT | Normal | Prolonged (lupus anticoagulant interferes with phospholipid-dependent APTT reagent) — paradoxically causes thrombosis despite prolonged APTT in vitro [8][9] |
| Mixing study | N/A (APTT is normal) | APTT does NOT correct (antibodies in patient plasma continue to interfere even after mixing with normal plasma) [8] |
| Key lab tests | AT activity level | Lupus anticoagulant (DRVVT), anti-cardiolipin IgG/IgM, anti-β2-GPI IgG/IgM |
| Treatment | Heparin (higher dose if needed), AT concentrate, long-term anticoagulation | Warfarin (target INR 2–3; or 3–4 if recurrent), hydroxychloroquine adjunct |
"Lupus anticoagulant classically causes a prolonged APTT, and a normal PT — but if prolonged APTT [suggests] prone to bleeding, why will lupus anticoagulant cause thrombosis?" [8] The answer is that the APTT prolongation is an in vitro artefact (the antibodies interfere with the phospholipid in the APTT reagent), but in vivo, the antibodies cause endothelial activation and complement-mediated thrombosis.
| Clinical Scenario | Top Differentials to Consider |
|---|---|
| Young patient with unprovoked DVT/PE | AT deficiency, protein C deficiency, protein S deficiency, APS, occult malignancy [2], PNH (rare) |
| VTE at unusual site (portal, mesenteric, cerebral sinus) | MPN (especially PV — JAK2+), PNH, APS, AT/PC/PS deficiency, Budd-Chiari (hepatic vein) |
| Recurrent VTE despite anticoagulation | APS, hereditary thrombophilia (AT/PC/PS), occult malignancy, non-compliance |
| Heparin resistance | AT deficiency (first consideration) [1], elevated factor VIII, heparin-binding proteins, large body habitus |
| VTE + nephrotic syndrome | Acquired AT deficiency from urinary loss [2][4] |
| VTE + liver disease | Acquired AT deficiency (reduced synthesis), though liver disease also reduces procoagulant factors — the balance determines clinical phenotype |
| VTE in pregnancy | Physiological hypercoagulability + hereditary thrombophilia (especially AT deficiency carries highest pregnancy VTE risk), APS |
| Warfarin-induced skin necrosis | Protein C deficiency (classic), protein S deficiency — NOT AT deficiency [2][7] |
| VTE + thrombocytopenia + MAHA | DIC, TTP, HUS — AT is consumed in DIC |
High Yield Summary — DDx of AT Deficiency
- AT deficiency is the rarest but most thrombogenic inherited thrombophilia (16.3× VTE risk) — must be differentiated from protein C deficiency, protein S deficiency, and APS
- In Chinese/HK patients, do NOT include Factor V Leiden or prothrombin G20210A mutation in the DDx [2][7]
- Acquired causes of low AT (liver disease, DIC, nephrotic syndrome, heparin use) are far more common than hereditary deficiency and must be excluded first
- Heparin resistance is the hallmark clinical clue pointing to AT deficiency — always consider it when heparin fails to prolong APTT adequately [1]
- Warfarin-induced skin necrosis points to protein C/S deficiency, NOT AT deficiency
- Malignancy is the most important cause of unprovoked VTE — always consider occult cancer before inherited thrombophilia in older patients [2]
- Thrombophilia testing should NOT be done during acute VTE or on anticoagulants [3]
- APS causes both venous and arterial thrombosis with prolonged APTT (in vitro artefact) — AT deficiency causes venous thrombosis with normal APTT
Active Recall - DDx of Antithrombin Deficiency
References
[1] Lecture slides: Block A - Leg swelling and chest pain: deep vein thrombosis; pulmonary embolism; Thrombophilia.pdf (Inherited thrombophilia section) [2] Senior notes: Ryan Ho Haemtology.pdf (Section 4.5 - Other Thrombotic Disorders); Adrian Lui Pediatrics Notes.pdf (Section 10.2.4 - Other Thrombotic Disorders) [3] Senior notes: Maksim Medicine Notes.pdf (Haematology - Thrombophilia screening) [4] Senior notes: Block A - Hematology Data Interpretation.pdf (Nephrotic syndrome complications - loss of antithrombin) [7] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (DVT/PE - Risk factors); MBBS Final MB (Surgery) (Felix PY Lai).pdf (DVT/PE - Risk factors) [8] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (Mixing study, lupus anticoagulant differentiation) [9] Senior notes: Maksim Medicine Notes.pdf (Clotting cascade interpretation; DIC)
Diagnostic Criteria, Diagnostic Algorithm and Investigations for Antithrombin Deficiency
1. Diagnostic Criteria
Unlike many medical conditions (e.g. SLE with SLICC criteria, or rheumatic fever with Jones criteria), antithrombin deficiency does not have a formal set of consensus diagnostic criteria with a checklist. Instead, diagnosis is established through a combination of:
- Clinical suspicion (appropriate clinical context)
- Laboratory confirmation (AT activity level ± AT antigen level)
- Exclusion of acquired causes of low AT
- Confirmatory repeat testing and family screening
- Optional genetic confirmation (SERPINC1 mutation analysis)
Let's break down each of these systematically.
Indications for thrombophilia screening (which would detect AT deficiency) [3]:
- Young patients with idiopathic venous thrombosis
- Suspected APLS, e.g. recurrent miscarriage
- Unusual sites of thrombosis (mesenteric, renal, portal vein, cerebral venous sinus)
- Warfarin-induced skin necrosis (protein C/S deficiency, but screen full panel)
Additional clinical triggers specific to AT deficiency:
- Heparin resistance — heparin relies on antithrombin to exert its activities; hence, without antithrombin, heparin has nothing to enhance, and thus cannot inhibit anything [1] — this is a near-pathognomonic clue
- Recurrent VTE despite adequate anticoagulation
- Strong family history of VTE (especially if multiple first-degree relatives affected before age 50)
- VTE in pregnancy or puerperium without other obvious risk factors
When NOT to Test
Do not test at the time of an acute VTE event, or while patients are receiving anticoagulants. [3]
Why?
- Acute thrombosis consumes AT → falsely low result
- Heparin (UFH and LMWH) accelerates AT–protease complex formation → AT levels drop 30–50% on therapy
- Warfarin does NOT affect AT levels (AT is not vitamin K-dependent), BUT warfarin lowers protein C and protein S → if you're doing a thrombophilia panel, warfarin confounds the other tests
- DOACs can interfere with functional clotting-based AT assays
Recommended timing: withhold warfarin × 2 weeks, DOAC × at least 2 days [3], and test when the patient is in a stable, non-acute state (typically ≥ 3 months after the acute VTE event, or after completing an initial course of anticoagulation if time-limited therapy is planned).
The diagnosis of hereditary AT deficiency requires:
| Criterion | Detail |
|---|---|
| 1. AT activity level < 80% (or below local laboratory reference range) | Measured by a functional (chromogenic or clot-based) assay — this is the first-line screening test |
| 2. Persistent low AT activity on repeat testing | A single low value is insufficient — must be confirmed on at least one repeat occasion, separated by weeks to months, to exclude transient acquired causes |
| 3. Exclusion of acquired causes | No concurrent liver disease, DIC, nephrotic syndrome, heparin therapy, L-asparaginase, or oestrogen use that could explain the low level |
| 4. AT antigen level (immunological assay) | Used to classify: Type I (antigen AND activity both low) vs. Type II (antigen normal, activity low) |
| 5. Family screening | Testing first-degree relatives; finding concordant low AT levels in multiple family members strongly supports hereditary diagnosis (AD inheritance) |
| 6. Genetic testing (optional but confirmatory) | SERPINC1 gene sequencing — identifies the specific pathogenic variant. Not required for clinical diagnosis but useful for genetic counselling and family screening |
Classification scheme based on lab results:
| AT Activity | AT Antigen | Heparin Cofactor Activity | Interpretation | |
|---|---|---|---|---|
| Normal | 80–120% | Normal | Normal | No AT deficiency |
| Type I (Quantitative) | ↓ | ↓ (proportionally) | ↓ | Reduced production or increased clearance of a structurally normal AT molecule |
| Type II-RS (Reactive Site) | ↓ | Normal | ↓ | Dysfunctional AT at the reactive centre loop — cannot bind thrombin/Xa properly |
| Type II-HBS (Heparin-Binding Site) | Normal or mildly ↓ (progressive AT activity may be normal) | Normal | ↓↓ | AT cannot be activated by heparin; the "stand-alone" function may be partially preserved |
| Type II-PE (Pleiotropic) | ↓ | Normal or slightly ↓ | ↓ | Multiple functional defects |
Why do we need both an activity assay AND an antigen assay? Because the activity assay tells you "how well does the AT work?" while the antigen assay tells you "how much AT protein is present?" If both are low proportionally, you have a quantitative deficiency (Type I — not enough protein). If the antigen is normal but activity is low, you have a qualitative deficiency (Type II — enough protein but it doesn't work properly). This distinction matters because Type I and Type II-RS carry the highest thrombotic risk.
2. Investigation Modalities — Detailed Breakdown
When thrombophilia screening is indicated, the standard panel includes:
| Test | What It Measures | Expected Finding in AT Deficiency | Explanation |
|---|---|---|---|
| AT activity (functional assay) | How well AT inhibits thrombin or factor Xa | ↓ (< 80% or below local reference) | The key diagnostic test. Uses a chromogenic substrate: patient plasma is incubated with excess thrombin or Xa + heparin → residual thrombin/Xa measured → if AT is deficient, more residual thrombin/Xa remains (less inhibition occurred) |
| AT antigen (immunological assay) | Quantitative AT protein level (ELISA or immunoturbidimetric) | ↓ in Type I; Normal in Type II | Distinguishes quantitative from qualitative deficiency |
| Protein C activity | Functional protein C level | Normal (in isolated AT deficiency) | Part of the standard thrombophilia panel |
| Protein S activity + free protein S antigen | Functional protein S level | Normal (in isolated AT deficiency) | Part of the standard thrombophilia panel |
| APLS panel: Lupus anticoagulant (DRVVT), anti-cardiolipin IgG/IgM, anti-β2-glycoprotein I IgG/IgM | Antiphospholipid antibodies | Negative (in isolated AT deficiency) | Must exclude APS as a cause of thrombophilia [3] |
GC High Yield — Heparin Cofactor Assay vs. Progressive AT Activity
The AT functional assay can be performed in two ways:
- Heparin cofactor activity: measures AT's ability to inhibit thrombin/Xa in the presence of heparin. This detects ALL types of AT deficiency (Type I, IIa, IIb, IIc) because it requires both the reactive site and the heparin-binding site to work.
- Progressive AT activity: measures AT's ability to inhibit thrombin/Xa without added heparin. This may be NORMAL in Type II-HBS because the reactive site still works — the defect is only in heparin binding.
Therefore, a patient with Type II-HBS may have a normal progressive AT activity but a low heparin cofactor activity. If you only do one assay, always do the heparin cofactor activity first as it is more sensitive for detecting all subtypes.
These are done in any patient presenting with VTE, not specific to AT deficiency, but critical for context:
| Test | Expected in AT Deficiency | Rationale |
|---|---|---|
| CBC | Normal (unless complicating condition) | Excludes MPN (polycythaemia vera, essential thrombocythaemia), DIC (thrombocytopenia), malignancy |
| PT | Normal | AT is NOT part of the extrinsic pathway; PT tests factor VII → tissue factor pathway. A normal PT does not exclude a hypercoagulable state [5][8] |
| APTT | Normal | AT is NOT a clotting factor but an inhibitor. APTT tests intrinsic pathway factor levels (VIII, IX, XI, XII). A normal APTT does not exclude AT deficiency [5][8] |
| Fibrinogen | Normal | Excludes DIC (fibrinogen ↓ in DIC) and dysfibrinogenaemia |
| D-dimer | ↑ if acute VTE present; normal if tested remotely | D-dimer is a fibrin degradation product → elevated in active thrombosis. Highly sensitive, not specific — useful to rule out VTE when low in a low-risk patient [10] |
"You can do well only on platelet count, PT and APTT" [5] — these are the bread-and-butter first-line haemostasis tests. But remember: they are tests of FACTOR DEFICIENCY (bleeding tendency), NOT tests of INHIBITOR DEFICIENCY (thrombotic tendency). A completely normal PT, APTT, and platelet count can coexist with severe AT deficiency.
Critical Concept — Normal Clotting Profile ≠ No Hypercoagulable State
Standard coagulation tests (PT, APTT) detect deficiencies of clotting factors — they tell you about bleeding risk. They do NOT detect deficiencies of natural anticoagulants (AT, protein C, protein S). Specific functional assays are required.
The only indirect clue from standard tests is heparin resistance: if UFH is administered and the APTT fails to prolong despite dose escalation, suspect AT deficiency. "Antithrombin activated, inactivates thrombin-induced activation of factors V, VIII, and XI" — without AT, heparin cannot prolong the APTT [5].
Before labelling a patient as having hereditary AT deficiency, the following must be assessed:
| Investigation | Acquired Cause Being Excluded | Rationale |
|---|---|---|
| LFT (albumin, ALT, AST, bilirubin, PT) | Liver disease (cirrhosis, acute hepatitis) | AT is synthesised in the liver [1]; hepatic dysfunction → reduced AT synthesis. Albumin is another liver-synthesised protein — if both albumin and AT are low, think liver failure rather than inherited AT deficiency |
| Urinalysis (dipstick + spot urine protein:creatinine ratio) | Nephrotic syndrome | Loss of antithrombin in urine [2][4] — AT (58 kDa) leaks through damaged GBM. If heavy proteinuria ( > 3.5 g/day) is present with low AT, the AT deficiency is likely acquired |
| DIC screen (PT, APTT, fibrinogen, D-dimer, platelet count, PBS for schistocytes) | DIC | Consumptive coagulopathy → AT is consumed along with clotting factors. DIC shows ↑ PT, ↑ APTT, ↓ fibrinogen, ↑ D-dimer, ↓ platelets, schistocytes on PBS [9] |
| Drug history | Heparin, L-asparaginase, OCP/HRT, oestrogen | Heparin accelerates AT clearance; L-asparaginase inhibits hepatic protein synthesis; oestrogen modulates AT expression |
| Pregnancy test (in women of reproductive age) | Pregnancy | Pregnancy can lower AT levels due to haemodilution and increased consumption |
| Investigation | Purpose | Details |
|---|---|---|
| Repeat AT activity and antigen | Confirm persistence | Must be done ≥ 2 weeks apart, off anticoagulants, not during acute illness. Two consistently low readings confirm the diagnosis |
| Heparin cofactor activity | Classify Type II-HBS | Specifically measures AT's ability to be activated by heparin. Low in Type II-HBS (the AT can still inhibit thrombin slowly on its own, but heparin cannot speed it up because the heparin-binding domain is defective) |
| Crossed immunoelectrophoresis (CIE) | Subtype qualitative defects | Can detect abnormal AT variants with altered electrophoretic mobility — used in specialised labs |
| SERPINC1 gene sequencing | Definitive genetic confirmation | Identifies the specific pathogenic variant (missense, nonsense, splice site, deletion). Useful for: genetic counselling, predictive testing of asymptomatic family members, distinguishing ambiguous cases |
| Family screening | Support AD inheritance pattern | Test AT activity in first-degree relatives. If ~50% of family members have low AT → consistent with AD inheritance |
Since AT deficiency presents through its thrombotic complications, you also need to diagnose and characterise the VTE:
| Investigation | Purpose | Key Findings |
|---|---|---|
| Compression ultrasonography (CUS) of lower limbs | Diagnose DVT | Non-compressibility of deep vein segment is diagnostic; Doppler shows absent or reduced flow |
| CT pulmonary angiography (CTPA) | Diagnose PE | Intraluminal filling defect in pulmonary arteries |
| D-dimer | Rule out VTE in low pre-test probability | Highly sensitive, not specific [10] — negative D-dimer effectively excludes VTE in low/intermediate pre-test probability patients |
| ECG | Assess for PE | Sinus tachycardia; S1Q3T3 pattern; T-wave inversion V1–V4; RBBB; right axis deviation [11] — these are signs of right heart strain |
| Echocardiography | Assess RV function in PE | RV dilatation, RV hypokinesis, tricuspid regurgitation, McConnell's sign — prognostic, not diagnostic |
| V/Q scan | Alternative to CTPA (e.g. CKD, contrast allergy) | Ventilation-perfusion mismatch |
The following algorithm integrates the clinical and laboratory approach:
4. Interpretation Framework — Putting It All Together
| Scenario | Interpretation | Next Step |
|---|---|---|
| Low AT activity, low AT antigen, no acquired cause, persistent on repeat | Hereditary Type I AT deficiency | SERPINC1 sequencing, family screening, long-term anticoagulation planning |
| Low AT activity, normal AT antigen, no acquired cause, persistent on repeat | Hereditary Type II AT deficiency → subtype with heparin cofactor assay | Determine IIa vs IIb vs IIc |
| Low AT activity in context of liver cirrhosis / low albumin | Acquired AT deficiency from reduced hepatic synthesis | Treat underlying liver disease; AT level should improve with liver recovery |
| Low AT activity + heavy proteinuria + low albumin | Acquired AT deficiency from nephrotic syndrome [4] | Treat nephrotic syndrome; consider prophylactic anticoagulation if very low AT |
| Low AT activity + ↑PT + ↑APTT + ↓fibrinogen + ↑D-dimer + ↓platelets | Acquired AT deficiency from DIC [9] | Treat underlying DIC trigger |
| Low AT activity while on heparin | Likely artefact — heparin consumes AT | Retest after stopping heparin |
| Normal AT activity + APTT fails to prolong on UFH | Consider: elevated factor VIII, heparin-binding proteins, non-compliance. If AT activity later found low on dedicated testing → AT deficiency | Measure AT activity specifically; measure anti-Xa level |
Interpretation of clotting profile [8][9]:
| PT | APTT | Interpretations |
|---|---|---|
| ↑ | N | Extrinsic pathway defect (factor 7): Vit K deficiency / warfarin / liver disease |
| N | ↑ | Intrinsic pathway defect (8, 9, 11, 12): Haemophilia, vWD (mixing study corrects); Heparin use, lupus anticoagulant (mixing study does not correct) |
| ↑ | ↑ | Common pathway defect (5, 10, 2) or multiple factor deficiency: NOAC, severe vit K deficiency, liver failure, DIC |
| N | N | Does NOT exclude hypercoagulable states (AT/PC/PS deficiency); Does NOT exclude platelet disorders |
"Can see that you can do well only on platelet count, PT and APTT" [5] — but for thrombophilia, these three tests are all NORMAL. You need the specific inhibitor assays.
Patient on unfractionated heparin — how is PT/APTT affected? [5]:
- No change in PT
- Increase in APTT
- Antithrombin activated, inactivates thrombin-induced activation of factors V, VIII, and XI
This tells you: if you give UFH and the APTT does NOT rise, the AT that heparin needs to activate is not there → think AT deficiency.
5. Special Considerations in Testing
| Confounder | Effect on AT Level | Mechanism |
|---|---|---|
| Acute VTE | ↓ (falsely low) | AT consumed at thrombosis site |
| UFH/LMWH therapy | ↓ (falsely low) | Heparin accelerates AT–protease complex clearance |
| Warfarin | No effect on AT itself | AT is NOT vitamin K-dependent; but warfarin ↓ protein C/S (confounds the rest of the panel) |
| DOACs | May interfere with functional assay | Factor Xa inhibitors (rivaroxaban, apixaban, edoxaban) can falsely elevate anti-Xa-based AT activity assays; dabigatran can interfere with thrombin-based assays |
| Oral contraceptives / oestrogen | Mildly ↓ | Oestrogen modulates hepatic AT expression |
| Pregnancy | ↓ in third trimester | Haemodilution + increased consumption |
| Liver disease | ↓ | Reduced synthesis |
| Nephrotic syndrome | ↓ | Urinary loss |
| L-asparaginase | ↓ | Inhibits hepatic protein synthesis |
| Fresh frozen plasma (FFP) transfusion | Transiently ↑ | FFP contains AT — falsely normalises level |
Exam Tip — Don't Be Fooled by FFP
If a patient received FFP before AT testing (e.g. in the context of DIC management), the AT level may be falsely normal. Always check the transfusion history before interpreting AT results.
The recommended approach to avoid false positives and false negatives:
- First measurement: AT activity (heparin cofactor assay) in a stable patient, off anticoagulants
- If low: Repeat AT activity + add AT antigen level, at a separate time point ≥ 2–4 weeks later
- If persistently low: Exclude all acquired causes (LFT, urinalysis, DIC screen, drug history)
- If no acquired cause: Screen first-degree family members
- If family members concordantly affected: Hereditary AT deficiency confirmed clinically
- Optional: SERPINC1 gene sequencing for definitive molecular diagnosis
| Investigation | Type | Key Finding | Interpretation |
|---|---|---|---|
| AT activity (heparin cofactor) | Functional/chromogenic | < 80% | Screening test — detected all AT deficiency types |
| AT antigen | Immunological (ELISA) | Low in Type I, Normal in Type II | Classifies quantitative vs qualitative |
| Progressive AT activity | Functional (without heparin) | Low in Type I and II-RS; Normal in II-HBS | Distinguishes HBS subtype |
| SERPINC1 sequencing | Genetic | Pathogenic variant identified | Definitive confirmation |
| PT | Coagulation | Normal | AT deficiency does not affect extrinsic pathway |
| APTT | Coagulation | Normal (unless heparin given → fails to prolong in AT deficiency) | AT deficiency does not affect intrinsic pathway factor levels |
| LFT | Biochemical | Abnormal if acquired hepatic cause | Exclude liver disease |
| Urinalysis/UPCR | Biochemical | Proteinuria if nephrotic cause | Exclude nephrotic syndrome |
| DIC screen | Coagulation + haematology | ↑PT, ↑APTT, ↓fibrinogen, ↑D-dimer, ↓platelets, schistocytes [9] | Exclude consumptive coagulopathy |
| Family member AT levels | Functional | ~50% of first-degree relatives affected | Supports AD inheritance |
High Yield Summary — Diagnosis of AT Deficiency
- No formal diagnostic criteria exist — diagnosis is based on persistently low AT activity, exclusion of acquired causes, and supportive family history/genetics
- AT activity (heparin cofactor assay) is the first-line screening test — low level triggers further workup
- AT antigen level classifies Type I (both low) vs Type II (antigen normal, activity low)
- Heparin cofactor activity vs progressive AT activity distinguishes Type II-HBS (heparin cofactor low, progressive normal) from Type II-RS (both low)
- Standard coagulation tests (PT, APTT) are NORMAL in AT deficiency — they test factor levels, not inhibitor levels [5][8]
- The only indirect clue on standard tests is heparin resistance — UFH fails to prolong APTT [1][5]
- Must exclude acquired causes before diagnosing hereditary AT deficiency: liver disease, nephrotic syndrome, DIC, heparin use, L-asparaginase, OCP
- Timing matters: do not test during acute VTE or on anticoagulants [3] — withhold warfarin ≥ 2 weeks, DOAC ≥ 2 days
- SERPINC1 gene sequencing provides definitive molecular confirmation but is not required for clinical diagnosis
- Family screening of first-degree relatives is essential for an autosomal dominant condition
Active Recall - Diagnosis of AT Deficiency
References
[1] Lecture slides: Block A - Leg swelling and chest pain: deep vein thrombosis; pulmonary embolism; Thrombophilia.pdf (Inherited thrombophilia section) [2] Senior notes: Ryan Ho Haemtology.pdf (Section 4.5 - Other Thrombotic Disorders); Adrian Lui Pediatrics Notes.pdf (Section 10.2.4 - Other Thrombotic Disorders) [3] Senior notes: Maksim Medicine Notes.pdf (Haematology - Thrombophilia screening) [4] Senior notes: Block A - Hematology Data Interpretation.pdf (Nephrotic syndrome complications — loss of antithrombin) [5] Senior notes: Block A - Abnormal bleeding after tooth extraction: bleeding tendency; thrombocytopenia.pdf (Coagulation tests in various disorders); Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (Coagulation cascade interpretation) [8] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (Mixing study, APTT interpretation) [9] Senior notes: Maksim Medicine Notes.pdf (Clotting cascade interpretation; DIC) [10] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (D-dimer — causes of raised D-dimer) [11] Senior notes: Ryan Ho Respiratory.pdf (PE — ECG findings, S1Q3T3)
Management of Antithrombin Deficiency
The management of antithrombin (AT) deficiency is organised around three clinical scenarios, each requiring a distinct approach:
- Acute management — treatment of active VTE in a patient with known or newly discovered AT deficiency
- Long-term management — secondary prevention after a thrombotic event
- Prophylactic management — prevention of first or recurrent VTE during high-risk situations (surgery, pregnancy, puerperium)
The overarching principle is simple: AT deficiency removes the body's primary brake on thrombin. Management therefore aims to (a) provide exogenous anticoagulation to compensate for the missing brake, and (b) in certain situations, replenish the missing AT itself.
1. Acute Management — Treatment of Active VTE
When a patient with AT deficiency presents with an acute DVT or PE, the immediate priority is the same as for any VTE: anticoagulation. However, AT deficiency introduces a unique wrinkle — heparin resistance.
Heparin relies on antithrombin to exert its activities — hence, without antithrombin, heparin has nothing to enhance, and thus cannot inhibit anything. [1]
This means standard-dose heparin may not achieve therapeutic anticoagulation. The approach differs depending on severity:
| Scenario | First-Line Approach | Rationale |
|---|---|---|
| Mild AT deficiency (AT 50–70%) with adequate heparin response | Standard-dose LMWH or UFH, titrated to target anti-Xa or APTT | If AT levels are only mildly reduced, there may be enough residual AT to achieve adequate heparin effect. Always monitor. |
| Moderate-to-severe AT deficiency (AT < 50%) or documented heparin resistance | Higher dose of LMWH (≥ 100 U/kg/day) [2] | Higher heparin doses can partially overcome the reduced AT by driving more heparin–AT complex formation from the limited AT available |
| Refractory heparin resistance despite dose escalation | Antithrombin concentrate [2] + heparin (see below) | If there simply isn't enough AT to work with, you must replenish the "army" before the "king" (heparin) can be effective |
| Alternative: Direct thrombin inhibitors or direct Xa inhibitors | Argatroban (IV DTI), bivalirudin (IV DTI), or DOACs once stable | These drugs do NOT require AT to function — they directly inhibit thrombin or factor Xa. Bypass the AT-dependent mechanism entirely |
"Basically anti-thrombin is the army, heparin is the king → without an army, no matter how smart the king is, cannot defeat the enemy" [1]
How to Recognise and Manage Heparin Resistance in Practice
- Start UFH infusion → target APTT 1.5–2.5× baseline [12][13]
- Monitor APTT at 6 hours after initiation or dose change
- If APTT fails to reach target despite dose escalation (e.g. > 35,000 U/day):
- Check anti-Xa level (more reliable than APTT in this context) — if anti-Xa is subtherapeutic despite high UFH dose, confirms true heparin resistance
- Measure AT activity — if low, this is the cause
- Administer AT concentrate to raise AT level to > 80% → then re-attempt heparin dosing
- OR switch to a direct thrombin inhibitor (argatroban, bivalirudin) which does not require AT
GC High Yield — Heparin Mechanism Recap
"Antithrombin activated, inactivates thrombin-induced activation of factors V, VIII, and XI" [5]
Heparin / UFH → activate antithrombin Enoxaparin / LMWH → activate antithrombin Fondaparinux / ULMWH → activate antithrombin [14]
All three of these drug classes are AT-dependent. If AT is deficient, ALL of them are less effective. This is why AT concentrate or direct inhibitors are needed as rescue.
This is the specific treatment for AT deficiency — it directly replaces the missing protein.
| Feature | Detail |
|---|---|
| Product types | Plasma-derived AT concentrate (e.g. Thrombate III, ATryn [recombinant AT from transgenic goats]) |
| Indication | Antithrombin concentrate in refractory VTE [2] — when heparin cannot achieve therapeutic effect due to AT deficiency |
| Mechanism | Directly replenishes circulating AT → restores heparin's ability to function → allows therapeutic anticoagulation |
| Dosing | Target AT activity level > 80–120%. Dose calculated: Dose (IU) = (Target AT% − Baseline AT%) × body weight (kg) / 1.4. Typically 50–100 IU/kg as initial bolus, then maintenance to keep AT > 80% |
| Half-life | ~2.5–3 days for plasma-derived concentrate (shorter during acute thrombosis due to increased consumption) |
| When to use with heparin | Administer AT concentrate first, then initiate/resume heparin — the AT concentrate "reloads the army" so heparin can now "command" it effectively |
| Monitoring | Measure AT activity 30 minutes after infusion, then daily. Adjust dose to maintain AT > 80% |
| Risks | Allergic/anaphylactic reactions (rare), theoretical viral transmission risk with plasma-derived products (mitigated by viral inactivation steps), heparin-related bleeding once AT is replenished (now heparin works "too well" relative to earlier doses) |
Important Dosing Consideration
When you administer AT concentrate and then give heparin, the heparin that was previously "ineffective" will suddenly become potent. You may need to reduce the heparin dose after AT repletion to avoid over-anticoagulation and bleeding. Always recheck APTT/anti-Xa after AT concentrate is given.
These bypass the AT pathway entirely and are increasingly used:
| Drug | Class | Mechanism | AT-Dependent? | Route | Notes |
|---|---|---|---|---|---|
| Argatroban | Direct thrombin inhibitor (DTI) | Directly binds and inhibits thrombin active site | No | IV infusion | Also used in HIT. Hepatically cleared — caution in liver disease. Monitored by APTT |
| Bivalirudin | Direct thrombin inhibitor | Direct, reversible thrombin inhibitor | No | IV infusion | Short half-life (~25 min). Used mainly in PCI setting [14] |
| Dabigatran | Oral direct thrombin inhibitor | Directly inhibits thrombin | No | Oral | Not used in acute phase; for transition to long-term Rx |
| Rivaroxaban, Apixaban, Edoxaban | Oral direct factor Xa inhibitors | Directly inhibit factor Xa | No | Oral | DOACs — increasingly used for long-term management after initial parenteral anticoagulation |
Why are direct inhibitors attractive in AT deficiency? Because the entire problem is that AT-dependent drugs (heparin, LMWH, fondaparinux) cannot work without their cofactor. Direct inhibitors sidestep this completely — they grab thrombin or Xa directly without needing any intermediary.
2. Long-Term Management — Secondary Prevention
After the acute VTE is treated, the question becomes: how long should anticoagulation continue?
| Clinical Scenario | Recommended Duration | Rationale |
|---|---|---|
| First provoked VTE (clear transient trigger: surgery, immobilisation, OCP) | Minimum 3–6 months, then reassess | The trigger has resolved; AT deficiency alone may not be sufficient to sustain thrombosis without a second hit. However, the underlying AT deficiency persists, so many experts still favour extended therapy |
| First unprovoked VTE | Indefinite anticoagulation strongly recommended | AT deficiency is a permanent prothrombotic state. Recurrence risk is high (~10–15%/year off anticoagulation) — the benefits of continued anticoagulation generally outweigh bleeding risk |
| Recurrent VTE | Indefinite anticoagulation | Clearly demonstrates ongoing thrombotic risk; should not be stopped |
| Life-threatening VTE (massive PE, cerebral venous sinus thrombosis) | Indefinite anticoagulation | Risk of recurrence with potentially fatal outcome justifies lifelong therapy |
For protein C deficiency, the notes explicitly state "anticoagulation in VTE → should continue indefinitely" [2]. The same principle applies even more strongly to AT deficiency, which carries a higher thrombotic risk (16.3× vs 7×).
| Option | Considerations |
|---|---|
| Warfarin (VKA) | Traditional choice. Target INR 2.0–3.0. AT is NOT vitamin K-dependent, so warfarin does not worsen AT deficiency (unlike its effect on protein C/S). However, requires INR monitoring, has food/drug interactions, and carries teratogenic risk. No warfarin-induced skin necrosis risk in AT deficiency (that's protein C/S deficiency) |
| DOACs (rivaroxaban, apixaban, edoxaban, dabigatran) | Oral DOACs preferred in VTE management in many current guidelines [15]. DOACs do NOT require AT to function (they are direct inhibitors). Fixed dosing, no routine monitoring, fewer drug/food interactions. Increasingly used as first-line long-term anticoagulation in AT deficiency |
| LMWH | Preferred in cancer-associated VTE and pregnancy [13]. In AT deficiency, may need higher doses. Self-administered SC injections |
Why DOACs May Be Ideal for AT Deficiency
Think about it from first principles: the entire problem in AT deficiency is that AT-dependent anticoagulants (heparin, LMWH, fondaparinux) don't work well. DOACs bypass this problem completely because they directly inhibit thrombin (dabigatran) or factor Xa (rivaroxaban, apixaban, edoxaban) without requiring AT as a cofactor. This makes them theoretically ideal for long-term management of AT-deficient patients.
However, large randomised trials specifically in AT deficiency are lacking (it's too rare). Current evidence is extrapolated from general VTE trials + expert consensus + mechanistic reasoning.
DOAC Dosing for VTE (for reference) [15]:
| DOAC | Initial Phase | Maintenance Phase |
|---|---|---|
| Rivaroxaban | 15 mg BD for 21 days | then 20 mg daily |
| Apixaban | 10 mg BD for 7 days | then 5 mg BD |
| Dabigatran | At least 5 days of parenteral anticoagulant first | then 150 mg BD |
| Edoxaban | At least 5 days of parenteral anticoagulant first | then 60 mg daily |
Conditions not suitable for DOAC [12]:
- Valvular AF / mechanical prosthetic heart valves — must use warfarin
- Severe renal impairment (CrCl < 15 mL/min for most DOACs; < 30 mL/min for dabigatran)
- Pregnancy and breastfeeding — use LMWH instead
- Antiphospholipid syndrome (APS) — DOACs shown to be inferior to warfarin in triple-positive APS (TRAPS trial)
- Active pathological bleeding
Contraindications to anticoagulation [12]:
- Recent major surgery
- Uncontrolled hypertension / haemorrhagic stroke
- Major bleeding diathesis
- Active bleeding
- Severe thrombocytopenia (platelet < 50 × 10⁹/L)
3. Prophylactic Management — Prevention in High-Risk Situations
Prophylaxis in pregnancy, surgery or post-VTE [2]
AT-deficient patients who have never had a VTE may still need prophylactic anticoagulation during periods of increased thrombotic risk:
| Phase | Management | Rationale |
|---|---|---|
| Pre-operative | Start prophylactic-dose LMWH (or higher dose if previous VTE history); consider AT concentrate peri-operatively if AT level very low ( < 50%) | Surgery = endothelial injury + immobilisation = Virchow's triad activated. AT-deficient patients need pharmacological prophylaxis |
| Intra-operative | AT concentrate infusion to maintain AT > 80% during major surgery | Ensures heparin given intra-operatively (e.g. for vascular procedures) actually works |
| Post-operative | Continue LMWH prophylaxis until fully mobile; transition to oral anticoagulant (warfarin or DOAC) if long-term therapy indicated | Early mobilisation alone is insufficient in AT deficiency |
If the patient is already on long-term anticoagulation:
- Stop warfarin 5 days before elective surgery (or stop 2 days + oral vitamin K) [16]
- Bridging with LMWH when INR becomes subtherapeutic → discontinue LMWH 12 hours before surgery [16]
- Indications for bridging: known hypercoagulability e.g. protein C/S deficiency [16] — AT deficiency falls into this same high-risk category requiring bridging
Peri-Operative Bridging in AT Deficiency
Bridging anticoagulation with LMWH is indicated when stopping long-term anticoagulation for surgery in patients with AT deficiency because:
- The underlying prothrombotic state persists
- The peri-operative period adds further thrombotic risk (stasis, tissue injury)
- The window without anticoagulation must be minimised
Higher LMWH doses may be needed due to relative heparin resistance, and AT concentrate should be considered for major procedures.
Pregnancy in AT-deficient women is high-risk — VTE risk estimated at 30–50% per pregnancy without prophylaxis.
| Phase | Management | Rationale |
|---|---|---|
| Pre-conception | Genetic counselling; plan pregnancy with haematologist; discuss risks | AD inheritance → 50% chance of passing to offspring |
| Antepartum | Prophylactic LMWH throughout pregnancy (therapeutic dose if prior VTE history); ± AT concentrate if AT level very low | Warfarin crosses placenta → risk of fetal ICH and teratogenicity [13] — MUST use LMWH in pregnancy. DOACs are also contraindicated in pregnancy |
| Peripartum | Switch to UFH or stop LMWH ≥ 12–24 hours before delivery to allow regional anaesthesia | Short UFH half-life allows safe epidural; AT concentrate standby for emergencies |
| Postpartum | Continue anticoagulation for ≥ 6 weeks postpartum [13]; can switch back to LMWH, warfarin, or DOAC | Postpartum period carries the highest thrombotic risk as blood returns from the placental bed |
"Pregnancy: switch to LMWH when 1st trimester (↓ teratogenicity) and > 36 weeks (avoid PPH); cover up to 6 weeks postpartum (highest risk as blood returns from uterus)" [13]
In AT deficiency specifically, LMWH doses may need to be higher than standard prophylactic doses (e.g. ≥ 100 U/kg/day [2]), and AT concentrate should be readily available for delivery.
| Situation | Prophylactic Approach |
|---|---|
| Long-haul travel ( > 4–6 hours) | Graduated compression stockings; hydration; consider single prophylactic LMWH dose if prior VTE history |
| Hospitalisation for medical illness | Pharmacological VTE prophylaxis with LMWH (may need higher dose) |
| Oral contraceptives / HRT | Avoid oestrogen-containing OCP — oestrogen increases procoagulant factor production and further reduces AT levels. Use progesterone-only contraception or non-hormonal methods instead |
| IMiD therapy (thalidomide/lenalidomide) in myeloma patients with concurrent AT deficiency | VTE prophylaxis mandatory (aspirin or LMWH or warfarin) [17] |
5. Special Management Considerations
Not all individuals with hereditary AT deficiency will develop VTE. For asymptomatic carriers (identified through family screening) who have never had a thrombotic event:
- No routine long-term anticoagulation is recommended — the bleeding risk of lifelong anticoagulation outweighs the benefit in someone who has never clotted
- Lifestyle counselling: avoid prolonged immobilisation, maintain hydration during travel, avoid oestrogen-containing contraceptives
- Situational prophylaxis: LMWH for surgery, pregnancy, prolonged immobilisation
- Education: recognise symptoms of DVT/PE and seek immediate medical attention
- Genetic counselling: explain AD inheritance, 50% transmission risk to offspring
| Point | Explanation |
|---|---|
| No warfarin-induced skin necrosis risk | AT is NOT vitamin K-dependent. Warfarin does not deplete AT. Skin necrosis from early protein C depletion does NOT occur in AT deficiency [2] |
| Bridging when starting warfarin | Still recommended to bridge with heparin when initiating warfarin (standard practice for all VTE), though the specific risk of skin necrosis is absent |
| INR target | Standard 2.0–3.0 for VTE |
| Drug interactions | Standard warfarin precautions apply (CYP2C9, vitamin K intake) |
| Feature | AT Deficiency | Protein C Deficiency | Protein S Deficiency |
|---|---|---|---|
| Acute VTE treatment | Higher dose LMWH ≥ 100 U/kg/day; AT concentrate for refractory VTE [2] | Standard anticoagulation | Standard anticoagulation |
| Long-term Rx | Indefinite anticoagulation recommended (especially if unprovoked) | Should continue indefinitely [2] | Individualise decision for indefinite anticoagulation [2] |
| Prophylaxis | Pregnancy, surgery, post-VTE [2] | Pregnancy, surgery, post-VTE [2] | Pregnancy, surgery [2] |
| Special precaution with warfarin | None specific (AT not vit K-dependent) | Must bridge with heparin when starting warfarin (risk of skin necrosis in first few days) [2] | Must bridge (lower but present risk of skin necrosis) |
| Specific replacement therapy | AT concentrate available | Protein C concentrate available (rare use) | No specific replacement |
The management differs fundamentally from hereditary AT deficiency because the primary goal is to treat the underlying cause:
| Acquired Cause | Management |
|---|---|
| Liver disease | Treat underlying liver disease; FFP contains AT and can be used as a bridge; AT levels improve with liver recovery |
| DIC | Treat the triggering condition (sepsis, malignancy, obstetric emergency); supportive therapy with platelets, FFP, cryoprecipitate [9]; AVOID tranexamic acid / PCC in DIC (promote thrombosis) [9]; AT concentrate has been studied but is not standard therapy |
| Nephrotic syndrome | Treat the glomerular disease; immunosuppression if appropriate; consider prophylactic anticoagulation if AT severely low ( < 50%) or albumin < 20 g/L |
| Heparin-induced AT depletion | Usually self-corrects after heparin is stopped; if AT level is needed urgently, AT concentrate can be given |
| L-asparaginase therapy | Monitor AT levels during treatment; AT concentrate if levels drop dangerously low; antithrombin is one of the proteins whose synthesis is inhibited |
| Drug | Mechanism | AT-Dependent? | Route | Monitoring | Key Points for AT Deficiency |
|---|---|---|---|---|---|
| UFH | Complex with antithrombin → inhibit Xa and thrombin [12] | Yes | IV | APTT Q6h (target 1.5–2.5× baseline) [12] | May need higher doses; if ineffective → AT concentrate or switch to DTI |
| LMWH | Inhibit Xa via AT [12] | Yes | SC | Anti-Xa level (if monitoring needed) | Higher dose ≥ 100 U/kg/day in VTE [2] |
| Fondaparinux | Selective Xa inhibition via AT (pentasaccharide) | Yes | SC | Anti-Xa (rarely needed) | Also may be less effective in severe AT deficiency |
| Warfarin | Vitamin K epoxide reductase inhibitor → reduce factor II, VII, IX, X activation [12] | No | Oral | INR (target 2–3) [12] | Safe in AT deficiency (AT not vit K-dependent). No skin necrosis risk. Drug/food interactions |
| Rivaroxaban | Direct Xa inhibitor | No | Oral | No routine monitoring | 15 mg BD × 21 days, then 20 mg daily [15] |
| Apixaban | Direct Xa inhibitor | No | Oral | No routine monitoring | 10 mg BD × 7 days, then 5 mg BD [15] |
| Dabigatran | Direct thrombin inhibitor | No | Oral | No routine monitoring | ≥ 5 days parenteral first, then 150 mg BD [15]. Antidote: idarucizumab |
| Edoxaban | Direct Xa inhibitor | No | Oral | No routine monitoring | ≥ 5 days parenteral first, then 60 mg daily [15] |
| Argatroban | Direct thrombin inhibitor | No | IV | APTT | Used in heparin resistance / HIT |
| AT concentrate | Replaces deficient AT protein | N/A (is AT itself) | IV | AT activity level | For refractory VTE [2]; dose to target AT > 80% |
| Complication | Drug(s) | Mechanism | Management |
|---|---|---|---|
| Bleeding | All anticoagulants | Over-anticoagulation; especially after AT concentrate given (heparin now "works too well") | Reduce dose; protamine (for UFH/LMWH); idarucizumab (for dabigatran); andexanet alfa (for Xa inhibitors — limited availability) [12]; vitamin K / PCC (for warfarin) |
| Heparin-induced thrombocytopenia (HIT) | UFH > LMWH [12] | Antibodies to PF4–heparin complex → platelet activation → paradoxical thrombosis | Stop ALL heparin; switch to non-heparin anticoagulant (argatroban, bivalirudin, fondaparinux); do NOT give warfarin until platelets recover |
| Warfarin skin necrosis | Warfarin | Local thrombosis from protein C/S deficiency [12] | NOT a risk in AT deficiency (AT not vit K-dependent). Risk in protein C deficiency. Bridge with heparin when initiating warfarin |
| Osteoporosis | Long-term UFH, warfarin | Heparin: inhibits osteoblast function; Warfarin: interferes with vitamin K-dependent bone proteins | Prefer LMWH or DOAC for long-term use; monitor bone density |
| Teratogenicity | Warfarin, DOACs | Warfarin crosses placenta → nasal hypoplasia, stippled epiphyses, CNS defects | Use LMWH in pregnancy [13] |
| Warfarin — purple toes syndrome | Warfarin | Cholesterol microembolisation as healing of ulcerated atheromatous plaque is interfered [12] | Rare; stop warfarin |
High Yield Summary — Management of AT Deficiency
- Acute VTE: Higher dose LMWH (≥ 100 U/kg/day) [2]; if heparin resistance → AT concentrate [2] or direct thrombin inhibitor (argatroban/bivalirudin)
- Heparin resistance is the hallmark clinical challenge — heparin needs AT to work; without AT, heparin is ineffective [1]
- AT concentrate replenishes the missing protein → restores heparin effectiveness. Target AT > 80%. Caution: previously ineffective heparin may now over-anticoagulate once AT is replenished
- DOACs (rivaroxaban, apixaban, dabigatran, edoxaban) are AT-independent and increasingly preferred for long-term management. They bypass the AT-dependent pathway entirely
- Duration: unprovoked or recurrent VTE → indefinite anticoagulation. Even provoked VTE → consider extended therapy given permanent AT deficiency
- Prophylaxis: required for pregnancy, surgery, post-VTE situations [2]
- Pregnancy: LMWH throughout (warfarin contraindicated — teratogenic); ± AT concentrate; cover ≥ 6 weeks postpartum [13]
- Avoid oestrogen-containing OCP — worsens hypercoagulable state
- No warfarin-induced skin necrosis risk in AT deficiency (AT not vitamin K-dependent; that's protein C/S)
- Contraindications to DOACs: mechanical valves, severe CKD, pregnancy, APS [12]
- Asymptomatic carriers: no routine anticoagulation; situational prophylaxis + lifestyle counselling + genetic counselling
Active Recall - Management of AT Deficiency
References
[1] Lecture slides: Block A - Leg swelling and chest pain: deep vein thrombosis; pulmonary embolism; Thrombophilia.pdf (Inherited thrombophilia section) [2] Senior notes: Ryan Ho Haemtology.pdf (Section 4.5 - Other Thrombotic Disorders); Adrian Lui Pediatrics Notes.pdf (Section 10.2.4 - Other Thrombotic Disorders) [3] Senior notes: Maksim Medicine Notes.pdf (Haematology - Thrombophilia screening) [5] Senior notes: Block A - Abnormal bleeding after tooth extraction: bleeding tendency; thrombocytopenia.pdf (UFH mechanism, coagulation tests) [9] Senior notes: Maksim Medicine Notes.pdf (DIC management) [12] Senior notes: Maksim Medicine Notes.pdf (Clinical pharmacology — antiplatelets and anticoagulants); Block A - Clinical pharmacology of antiplatelets and anticoagulation.pdf [13] Senior notes: Ryan Ho Haemtology.pdf (DVT/PE management — pregnancy, cancer, anticoagulation duration) [14] Senior notes: Block A - Sudden severe chest pain: acute myocardial infarction; aortic dissection.pdf (Antithrombin therapy — heparin, LMWH, fondaparinux, bivalirudin) [15] Senior notes: Handbook of Internal Medicine 2024.pdf (DOAC dosing regimens for VTE) [16] Senior notes: Maksim Surgery Notes.pdf (Peri-operative warfarin management, bridging anticoagulation) [17] Senior notes: Block A - An old man with bone pain and anaemia: multiple myeloma; monoclonal gammopathy.pdf (IMiD and VTE prophylaxis)
Complications of Antithrombin Deficiency
The complications of antithrombin (AT) deficiency can be divided into two major categories:
- Complications of the disease itself — i.e., the thrombotic consequences of unopposed thrombin generation
- Complications of treatment — i.e., the adverse effects of the anticoagulants used to manage the condition
Both are examinable. Let's walk through each systematically, always linking back to "why" the complication occurs.
1. Complications of AT Deficiency Itself (Thrombotic Complications)
The fundamental pathology is simple: without adequate AT, thrombin and factor Xa accumulate → unopposed coagulation → clots form where they shouldn't [1]. The clinical consequences depend on where the clot forms and how big it is.
| Feature | Detail |
|---|---|
| Site | Lower extremity deep veins (ilio-femoral > popliteal > calf); less commonly upper extremity (especially with central venous catheters) |
| Why AT deficiency causes DVT | Venous flow is slow (relative stasis) → coagulation factors accumulate locally → without AT to neutralise them, thrombin generation proceeds unchecked → fibrin mesh traps RBCs and platelets → organised venous thrombus |
| Frequency | ~70% of individuals with hereditary AT deficiency will experience at least one VTE event before age 60 [2] |
| Clinical presentation | Unilateral leg swelling, pain, warmth, erythema; may be asymptomatic and found incidentally |
| Feature | Detail |
|---|---|
| Mechanism | Thrombus from DVT (usually proximal ilio-femoral) detaches → travels via IVC → right heart → lodges in pulmonary vasculature → obstructs blood flow |
| Why PE is dangerous | Acute obstruction of pulmonary arterial bed → ↑ pulmonary vascular resistance → acute right ventricular pressure overload → RV dilatation and failure → ↓ LV filling → cardiogenic shock. Patients with PE usually die from right heart failure (cardiogenic shock) rather than hypoxaemia [7] |
| Spectrum | Small PE: pleuritic chest pain, dyspnoea, tachycardia. Submassive PE: RV strain on echo, elevated troponin/BNP. Massive PE: haemodynamic collapse, sBP < 90 mmHg, obstructive shock |
High Yield — PE Mortality
Massive PE with cardiogenic shock carries ~25–50% mortality. AT deficiency patients are at particular risk of large-burden PE because their thrombi tend to be extensive (ongoing thrombin generation promotes clot propagation). The combination of AT deficiency + heparin resistance makes acute management especially challenging — clinicians may need AT concentrate or direct thrombin inhibitors while simultaneously considering thrombolysis.
| Feature | Detail |
|---|---|
| Why recurrence is common | AT deficiency is a permanent prothrombotic state (in hereditary cases). Unlike a provoked VTE where the trigger resolves, the underlying AT deficiency persists for life. Without lifelong anticoagulation, each new "second hit" (surgery, immobilisation, illness, pregnancy) re-exposes the patient to thrombotic risk |
| Recurrence rate off anticoagulation | Estimated ~10–15% per year for unprovoked VTE in thrombophilia patients — significantly higher than the general population (~3–5%/year) |
| Implication | This is why indefinite anticoagulation is strongly recommended for unprovoked or recurrent VTE in AT deficiency [2] |
This is the chronic sequela of DVT and affects up to 20–50% of DVT patients, including those with AT deficiency.
| Feature | Detail |
|---|---|
| Pathophysiology | Acute DVT damages venous valves → chronic venous reflux (valvular incompetence) + residual venous obstruction from incomplete thrombus resolution → chronic venous hypertension → tissue damage |
| Clinical features | Chronic leg swelling, heaviness/aching (worse with prolonged standing, better with elevation), skin changes: haemosiderin deposition (brown discolouration), lipodermatosclerosis (woody induration of subcutaneous tissue), stasis dermatitis (eczematous changes), venous ulceration (typically medial malleolus) |
| Why AT deficiency patients are at high risk | Recurrent DVTs in the same limb cause cumulative valve damage; extensive proximal DVTs (common in AT deficiency due to ongoing thrombin generation) cause more severe PTS than distal DVTs |
| Prevention | Adequate acute anticoagulation (reduce residual thrombus), graduated compression stockings (debated — ATTRACT trial showed limited benefit for routine use), early mobilisation |
| Management | Compression therapy, elevation, exercise, wound care for ulcers. No specific treatment reverses valve damage |
AT deficiency can cause thrombosis in sites less commonly seen with typical provoked VTE:
| Site | Clinical Consequence | Why It Matters |
|---|---|---|
| Cerebral venous sinus thrombosis (CVST) | Headache, papilloedema, focal neurological deficits, seizures, reduced consciousness | Raised intracranial pressure from impaired venous drainage; can lead to venous infarction ± haemorrhagic transformation |
| Portal vein thrombosis | Abdominal pain, ascites, variceal bleeding, hepatic dysfunction | Portal vein thrombosis causes pre-hepatic portal hypertension → increased variceal bleeding, increased ascites [18]. In cirrhotic patients with concurrent AT deficiency, this is a double hit |
| Mesenteric vein thrombosis | Acute abdominal pain (out of proportion to examination), bloody diarrhoea, bowel infarction | Venous congestion of bowel wall → ischaemia → infarction if untreated. Surgical emergency if bowel necrosis |
| Renal vein thrombosis | Flank pain, haematuria, acute kidney injury, proteinuria | Particularly relevant in nephrotic syndrome where AT is lost in urine → loss of antithrombin → thrombosis [4][19] |
| Hepatic vein thrombosis (Budd-Chiari syndrome) | Hepatomegaly, ascites, abdominal pain, liver failure | Obstruction of hepatic venous outflow → post-sinusoidal portal hypertension |
Unusual-Site Thrombosis = Think Thrombophilia
Any thrombosis at an unusual site (especially in a young patient) should trigger a thrombophilia workup, including AT activity measurement. This is a key exam teaching point: unusual sites of thrombosis (mesenteric, renal, portal vein, cerebral venous sinus) are an indication for thrombophilia screening [3].
AT deficiency in pregnancy carries the highest VTE risk among the inherited thrombophilias:
| Complication | Mechanism |
|---|---|
| VTE during pregnancy | Pregnancy = physiological hypercoagulable state (↑ factors II, VII, VIII, X, fibrinogen; ↓ protein S) + mechanical IVC compression by gravid uterus + reduced mobility. AT deficiency amplifies this already prothrombotic milieu → estimated VTE risk of 30–50% per pregnancy without prophylaxis |
| VTE in puerperium | Postpartum period carries the highest thrombotic risk as blood returns from the placental bed [13] + endothelial injury from delivery + fluid shifts |
| Recurrent pregnancy loss | Although more strongly associated with APS, AT deficiency may contribute to placental microvascular thrombosis → placental insufficiency → miscarriage, IUGR, stillbirth |
| Pre-eclampsia / HELLP | Thrombo-inflammatory cascade in placental vasculature; AT deficiency may exacerbate endothelial dysfunction |
While not a "complication" in the traditional sense, heparin resistance is arguably the most clinically important consequence of AT deficiency because it directly impacts acute treatment:
"Heparin relies on antithrombin to exert its activities — hence, without antithrombin, heparin has nothing to enhance, and thus cannot inhibit anything" [1]
| Aspect | Detail |
|---|---|
| Clinical scenario | Patient with acute VTE is started on UFH → APTT fails to prolong despite dose escalation → patient remains under-anticoagulated → clot extends or embolises |
| Why this is dangerous | The very drug you're relying on to treat the acute thrombosis doesn't work. This creates a "treatment failure" scenario where the thrombus can propagate or embolise despite apparent adequate therapy |
| Management | AT concentrate, direct thrombin inhibitors, or dose-escalated LMWH (≥ 100 U/kg/day) [2] |
2. Complications of Treatment (Anticoagulant-Related)
Since AT deficiency requires lifelong or prolonged anticoagulation, the complications of anticoagulant therapy are a significant part of the disease burden.
The universal risk of all anticoagulants. The trade-off is always: preventing thrombosis vs. causing haemorrhage.
| Type of Bleeding | Examples | Risk Factors | Management |
|---|---|---|---|
| Minor bleeding | Gum bleeding, epistaxis, easy bruising, menorrhagia | Over-anticoagulation, concurrent antiplatelets, elderly | Dose reduction; local measures |
| Major bleeding | GI haemorrhage (upper > lower), retroperitoneal haemorrhage, haemarthrosis | Supratherapeutic INR (warfarin), renal impairment (DOAC accumulation), concurrent NSAID/antiplatelet use | Stop anticoagulant; reversal agents (see below); supportive care (IV fluids, transfusion, endoscopy for GI bleed) |
| Life-threatening bleeding | Intracranial haemorrhage, massive GI bleed with haemodynamic compromise | Same as above; anticoagulation intensity, advanced age, prior stroke | Emergency reversal + neurosurgical/surgical intervention |
Reversal agents [12]:
| Anticoagulant | Reversal Agent |
|---|---|
| UFH | Protamine sulphate (1 mg neutralises ~100 U heparin) |
| LMWH | Protamine sulphate (partially effective, ~60% reversal of anti-Xa activity) |
| Warfarin | Vitamin K (oral or IV) + PCC (prothrombin complex concentrate) for urgent reversal; FFP if PCC unavailable |
| Dabigatran | Idarucizumab ("idarucizumab" — think "I dare you" + "zumab") — monoclonal antibody fragment that specifically binds dabigatran [12] |
| Rivaroxaban / Apixaban / Edoxaban | Andexanet alfa (recombinant modified factor Xa decoy — binds and sequesters Xa inhibitors). Limited availability. PCC or aPCC as alternatives [12] |
AT Concentrate + Heparin = Beware Over-Anticoagulation
A specific complication unique to AT deficiency management: when AT concentrate is given to a patient already on high-dose heparin (for heparin resistance), the heparin that was previously ineffective suddenly becomes fully potent. This can result in acute over-anticoagulation and bleeding. Always reduce the heparin dose or recheck APTT/anti-Xa promptly after AT concentrate infusion.
| Feature | Detail |
|---|---|
| Mechanism | Antibodies form against complexes of platelet factor 4 (PF4) + heparin → these IgG antibodies bind to FcγRIIa receptors on platelets → massive platelet activation → thrombocytopenia (from consumption) AND paradoxical thrombosis (from activated platelet aggregation) |
| Why AT deficiency patients are at risk | They often require higher heparin doses and prolonged heparin exposure (due to heparin resistance), increasing cumulative heparin exposure and thus HIT risk. UFH carries higher HIT risk than LMWH [12] |
| Timing | Typically days 5–14 after starting heparin (or earlier if prior heparin exposure within 30 days — "rapid-onset HIT") |
| Clinical | Platelet count falls > 50% from baseline; new thrombosis (arterial or venous) despite being on heparin; skin necrosis at heparin injection sites |
| Diagnosis | Clinical probability: 4Ts score (Thrombocytopenia, Timing, Thrombosis, oTher causes) [20]; Confirmatory: HIT antibody ELISA + serotonin release assay |
| Management | STOP ALL heparin immediately (including heparin flushes and heparin-coated lines); Switch to non-heparin anticoagulant (argatroban IV, bivalirudin IV, fondaparinux SC); Do NOT give warfarin until platelets recover (risk of venous limb gangrene); Do NOT give platelet transfusions (fuels the fire) |
HIT is doubly dangerous in AT deficiency: the patient already has a prothrombotic state from AT deficiency, and now HIT adds a second prothrombotic mechanism on top. These patients can develop catastrophic thrombosis.
| Complication | Mechanism | Relevance to AT Deficiency |
|---|---|---|
| Warfarin-induced skin necrosis | Local thrombosis from protein C/S depletion in first few days of warfarin use [2][10] | NOT a risk in AT deficiency — AT is NOT vitamin K-dependent. This is a complication of protein C deficiency. However, if a patient has combined AT + protein C deficiency (rare), the risk exists. Always bridge with heparin when starting warfarin regardless |
| Purple toes syndrome | Cholesterol microembolisation as healing of ulcerated atheromatous plaque is interfered [12] | Non-specific warfarin complication; not specific to AT deficiency |
| Teratogenicity | Warfarin crosses placenta → nasal hypoplasia, stippled chondral calcification (first trimester), CNS defects | Warfarin contraindicated in pregnancy — use LMWH [13] |
| Osteoporosis | Chronic warfarin use interferes with vitamin K-dependent bone proteins (osteocalcin, matrix Gla protein) | Relevant for young AT-deficient patients on lifelong warfarin; consider DOAC switch |
| Drug/food interactions | CYP2C9/VKORC1 polymorphisms; vitamin K-rich foods; drug interactions (antibiotics, amiodarone, etc.) | Requires regular INR monitoring and patient education |
| Complication | Detail |
|---|---|
| GI bleeding | All DOACs carry a small increased risk of GI bleeding compared to warfarin (rivaroxaban and dabigatran particularly). Mechanism: local anticoagulant effect on GI mucosa + topical mucosal injury (dabigatran) |
| Renal accumulation | Dabigatran is 80% renally cleared — contraindicated if CrCl < 30 mL/min. Xa inhibitors require dose adjustment at CrCl 15–50 mL/min |
| No reliable routine monitoring | Unlike warfarin (INR), DOACs lack a widely available simple monitoring test. This is usually an advantage (no monitoring needed) but becomes a disadvantage in emergencies (difficult to quantify anticoagulant effect) |
| Limited reversal options | Idarucizumab for dabigatran is effective and available; andexanet alfa for Xa inhibitors has limited availability and uncertain clinical benefit [12] |
| Anticoagulant | Mechanism | Relevance |
|---|---|---|
| Long-term UFH | Directly inhibits osteoblast function and promotes osteoclast activity | Relevant if AT-deficient patient requires prolonged heparin (e.g. pregnancy where LMWH is used for months) |
| LMWH | Lower osteoporosis risk than UFH but still present with prolonged use | Monitor bone density in patients on LMWH for extended periods (e.g. multiple pregnancies) |
| Warfarin | Inhibits vitamin K-dependent bone proteins (osteocalcin) | Cumulative risk over decades of use in young AT-deficient patients |
| DOACs | No known direct bone effect | Preferred for long-term use in part for this reason |
3. Complications Related to Specific Clinical Situations
| Complication | Mechanism | Prevention/Management |
|---|---|---|
| Maternal VTE | Pregnancy + AT deficiency = severely prothrombotic | LMWH throughout pregnancy + ≥ 6 weeks postpartum |
| Pregnancy loss | Placental microvascular thrombosis → insufficiency | Anticoagulation may reduce risk; limited evidence |
| Bleeding at delivery | Anticoagulation near term | Time LMWH cessation (≥ 12–24h before delivery); AT concentrate standby |
| Fetal AT deficiency | 50% chance (AD inheritance) | Genetic counselling; neonatal AT level testing if indicated |
| Neonatal purpura fulminans | NOT expected in AT deficiency (homozygous Type I is lethal in utero). This is a complication of homozygous protein C deficiency | Distinguish from protein C deficiency |
| Complication | Mechanism | Prevention |
|---|---|---|
| Peri-operative VTE | Surgery = tissue injury (endothelial damage) + immobilisation (stasis) + inflammatory response (hypercoagulability) in a patient already lacking AT | Pre-operative AT concentrate ± LMWH prophylaxis; early mobilisation; mechanical prophylaxis (compression devices) |
| Peri-operative bleeding | From anticoagulation used for prophylaxis/treatment | Careful timing of LMWH cessation (≥ 12h before surgery); bridging strategy; surgical haemostasis |
Often overlooked but clinically important:
| Issue | Detail |
|---|---|
| Lifelong anticoagulation burden | Regular INR monitoring (if on warfarin), dietary restrictions, drug interaction awareness, bleeding anxiety |
| Restriction of activities | Contact sports discouraged while on anticoagulation; travel precautions needed |
| Reproductive counselling | Complex pregnancy management; contraceptive limitations (avoid oestrogen-containing OCP); genetic implications for offspring |
| Psychological impact | Anxiety about recurrent VTE, bleeding complications, passing condition to children |
| Cost | AT concentrate is expensive; DOACs are more costly than warfarin; regular haematology follow-up |
When AT is low due to an acquired cause, the complications are those of the underlying disease PLUS the additive thrombotic risk:
| Underlying Disease | AT-Related Complication |
|---|---|
| Nephrotic syndrome | Urinary loss of antithrombin III and Protein S [19] → hypercoagulability → renal vein thrombosis, cerebral vein thrombosis, PE |
| Liver cirrhosis | Reduced AT synthesis → but also reduced procoagulant factor synthesis → "rebalanced haemostasis" that can tip either way. Portal vein thrombosis develops in > 10% of cirrhotic patients (25% when decompensated) [18] |
| DIC | AT is consumed along with clotting factors → vicious cycle: less AT → more thrombin → more clot → more AT consumption. DIC complications include both bleeding AND thrombosis (organ ischaemia, MAHA, purpura fulminans) [9] |
| L-asparaginase therapy (ALL) | Reduced AT synthesis → VTE during chemotherapy (especially cerebral venous sinus thrombosis in paediatric ALL patients) |
Nephrotic Syndrome and AT Loss
Nephrotic syndrome complications include: loss of antithrombin → thrombosis [4][19]
The mechanism deserves emphasis: AT (58 kDa) is similar in size to albumin (67 kDa). When the glomerular basement membrane is damaged enough to leak albumin at nephrotic-range ( > 3.5 g/day), it also leaks AT. Simultaneously, the liver ramps up synthesis of procoagulant factors (fibrinogen, factors V and VIII) as part of the "overshoot" response to protein loss. The net result: more clotting factors + fewer anticoagulant brakes = hypercoagulable state.
"Urinary loss of antithrombotic factors such as antithrombin III and Protein S" + "Increased synthesis of clotting factors in liver" + "Increased viscosity from vascular stasis due to hemoconcentration and intravascular volume depletion" [19] — a perfect storm for thrombosis.
| Category | Complication | Mechanism | Key Points |
|---|---|---|---|
| Thrombotic | DVT | Unopposed thrombin generation → venous clot | Most common manifestation; ~70% by age 60 |
| PE | DVT embolisation to pulmonary vasculature | Can be fatal (RV failure); heparin resistance complicates Rx | |
| Recurrent VTE | Persistent AT deficiency + new triggers | Justifies indefinite anticoagulation | |
| Post-thrombotic syndrome | Venous valve damage from DVT → chronic venous HTN | Chronic leg swelling, skin changes, ulceration | |
| Unusual-site thrombosis | CVST, portal/mesenteric/renal/hepatic vein thrombosis | Should trigger thrombophilia screening | |
| Pregnancy VTE | Physiological hypercoagulability + AT deficiency | 30–50% VTE risk/pregnancy without prophylaxis | |
| Treatment | Bleeding | Over-anticoagulation | Universal risk; reversal agents available |
| HIT | Anti-PF4/heparin antibodies → platelet activation | Higher risk due to prolonged/high-dose heparin exposure | |
| Warfarin teratogenicity | Crosses placenta | Use LMWH in pregnancy | |
| Osteoporosis | Long-term heparin/warfarin | Monitor BMD; prefer DOACs for long-term Rx | |
| Over-anticoagulation post-AT concentrate | Heparin suddenly effective | Reduce heparin dose after AT repletion | |
| Acquired AT depletion | Nephrotic → VTE | Urinary AT loss | Renal vein thrombosis classic |
| Cirrhosis → portal vein thrombosis | Reduced AT synthesis + stasis | > 10% of cirrhotics | |
| DIC → organ failure | AT consumption in vicious cycle | Treat underlying cause |
High Yield Summary — Complications of AT Deficiency
- DVT and PE are the cardinal complications — ~70% develop VTE before age 60; PE kills by RV failure, not hypoxaemia [2][7]
- Recurrent VTE is common due to the permanent nature of the defect → justifies indefinite anticoagulation [2]
- Post-thrombotic syndrome affects 20–50% of DVT patients — chronic venous hypertension → skin changes → ulceration
- Unusual-site thrombosis (CVST, portal, mesenteric, renal vein, Budd-Chiari) should always prompt thrombophilia screening [3]
- Heparin resistance is both a complication and a diagnostic clue — heparin needs AT to work [1]; requires AT concentrate or direct thrombin inhibitors
- Pregnancy carries 30–50% VTE risk per pregnancy without prophylaxis; LMWH throughout + ≥ 6 weeks postpartum [13]
- Treatment complications: bleeding (all anticoagulants), HIT (heparin), warfarin teratogenicity, osteoporosis (long-term heparin/warfarin), over-anticoagulation after AT concentrate
- Warfarin-induced skin necrosis does NOT occur in AT deficiency (AT is not vitamin K-dependent) — that's protein C/S deficiency [2]
- In nephrotic syndrome: urinary loss of AT → thrombosis (renal vein, CVST, PE) [4][19]
- In cirrhosis: portal vein thrombosis in > 10% of patients; > 25% when decompensated [18]
- In DIC: AT is consumed → vicious cycle of thrombin generation → both bleeding and thrombosis [9]
Active Recall - Complications of AT Deficiency
References
[1] Lecture slides: Block A - Leg swelling and chest pain: deep vein thrombosis; pulmonary embolism; Thrombophilia.pdf (Inherited thrombophilia section) [2] Senior notes: Ryan Ho Haemtology.pdf (Section 4.5 - Other Thrombotic Disorders); Adrian Lui Pediatrics Notes.pdf (Section 10.2.4 - Other Thrombotic Disorders) [3] Senior notes: Maksim Medicine Notes.pdf (Haematology - Thrombophilia screening) [4] Senior notes: Block A - Hematology Data Interpretation.pdf (Nephrotic syndrome complications — loss of antithrombin) [7] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (DVT/PE — risk factors and clinical features) [9] Senior notes: Maksim Medicine Notes.pdf (DIC section) [10] Senior notes: Block A - Leg swelling and chest pain: deep vein thrombosis; pulmonary embolism; Thrombophilia.pdf (Protein C deficiency — warfarin-induced skin necrosis) [12] Senior notes: Maksim Medicine Notes.pdf (Clinical pharmacology — antiplatelets and anticoagulants); Block A - Clinical pharmacology of antiplatelets and anticoagulation.pdf [13] Senior notes: Ryan Ho Haemtology.pdf (DVT/PE management — pregnancy, anticoagulation) [18] Senior notes: Block A - Abdominal distension: ascites and cirrhosis.pdf (Portal vein thrombosis in cirrhosis) [19] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Nephrotic syndrome complications — hypercoagulability, urinary AT III loss); MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (same section) [20] Senior notes: Handbook of Internal Medicine 2024.pdf (HIT — 4Ts scoring system)
High Yield Summary
Antithrombin Deficiency — Key Points for Exams:
- Autosomal dominant inheritance, SERPINC1 gene on chromosome 1q25.1 [1][2]
- AT is synthesised in the liver; it is a serine protease inhibitor (serpin) that neutralises thrombin (IIa), Xa, IXa, XIa, XIIa [1]
- Heparin works BY potentiating AT → without AT, heparin cannot work → heparin resistance is the hallmark clinical clue [1]
- Prevalence: 0.02–0.2%; confers the highest VTE risk (~16.3×) among inherited thrombophilias [2]
- ~70% of affected individuals develop VTE before age 60 [2]
- Type I (quantitative: ↓antigen and ↓activity) vs Type II (qualitative: normal antigen, ↓activity) [1]
- NOT vitamin K-dependent → NOT associated with warfarin-induced skin necrosis (that's protein C/S)
- Normal PT and APTT — standard clotting tests do NOT detect natural anticoagulant deficiencies
- Acquired causes: liver disease, DIC, nephrotic syndrome (urinary loss), heparin therapy, L-asparaginase
- Treatment: higher-dose LMWH (≥100 U/kg/day), AT concentrate for refractory VTE, long-term anticoagulation, prophylaxis in high-risk situations [2]
- Factor V Leiden is NOT found in Chinese — do not include in differential for Chinese patients [1][2]
- Thrombophilia screening should NOT be done during acute VTE or while on anticoagulants [3]
High Yield Summary — DDx of AT Deficiency
- AT deficiency is the rarest but most thrombogenic inherited thrombophilia (16.3× VTE risk) — must be differentiated from protein C deficiency, protein S deficiency, and APS
- In Chinese/HK patients, do NOT include Factor V Leiden or prothrombin G20210A mutation in the DDx [2][7]
- Acquired causes of low AT (liver disease, DIC, nephrotic syndrome, heparin use) are far more common than hereditary deficiency and must be excluded first
- Heparin resistance is the hallmark clinical clue pointing to AT deficiency — always consider it when heparin fails to prolong APTT adequately [1]
- Warfarin-induced skin necrosis points to protein C/S deficiency, NOT AT deficiency
- Malignancy is the most important cause of unprovoked VTE — always consider occult cancer before inherited thrombophilia in older patients [2]
- Thrombophilia testing should NOT be done during acute VTE or on anticoagulants [3]
- APS causes both venous and arterial thrombosis with prolonged APTT (in vitro artefact) — AT deficiency causes venous thrombosis with normal APTT
High Yield Summary — Diagnosis of AT Deficiency
- No formal diagnostic criteria exist — diagnosis is based on persistently low AT activity, exclusion of acquired causes, and supportive family history/genetics
- AT activity (heparin cofactor assay) is the first-line screening test — low level triggers further workup
- AT antigen level classifies Type I (both low) vs Type II (antigen normal, activity low)
- Heparin cofactor activity vs progressive AT activity distinguishes Type II-HBS (heparin cofactor low, progressive normal) from Type II-RS (both low)
- Standard coagulation tests (PT, APTT) are NORMAL in AT deficiency — they test factor levels, not inhibitor levels [5][8]
- The only indirect clue on standard tests is heparin resistance — UFH fails to prolong APTT [1][5]
- Must exclude acquired causes before diagnosing hereditary AT deficiency: liver disease, nephrotic syndrome, DIC, heparin use, L-asparaginase, OCP
- Timing matters: do not test during acute VTE or on anticoagulants [3] — withhold warfarin ≥ 2 weeks, DOAC ≥ 2 days
- SERPINC1 gene sequencing provides definitive molecular confirmation but is not required for clinical diagnosis
- Family screening of first-degree relatives is essential for an autosomal dominant condition
High Yield Summary — Management of AT Deficiency
- Acute VTE: Higher dose LMWH (≥ 100 U/kg/day) [2]; if heparin resistance → AT concentrate [2] or direct thrombin inhibitor (argatroban/bivalirudin)
- Heparin resistance is the hallmark clinical challenge — heparin needs AT to work; without AT, heparin is ineffective [1]
- AT concentrate replenishes the missing protein → restores heparin effectiveness. Target AT > 80%. Caution: previously ineffective heparin may now over-anticoagulate once AT is replenished
- DOACs (rivaroxaban, apixaban, dabigatran, edoxaban) are AT-independent and increasingly preferred for long-term management. They bypass the AT-dependent pathway entirely
- Duration: unprovoked or recurrent VTE → indefinite anticoagulation. Even provoked VTE → consider extended therapy given permanent AT deficiency
- Prophylaxis: required for pregnancy, surgery, post-VTE situations [2]
- Pregnancy: LMWH throughout (warfarin contraindicated — teratogenic); ± AT concentrate; cover ≥ 6 weeks postpartum [13]
- Avoid oestrogen-containing OCP — worsens hypercoagulable state
- No warfarin-induced skin necrosis risk in AT deficiency (AT not vitamin K-dependent; that's protein C/S)
- Contraindications to DOACs: mechanical valves, severe CKD, pregnancy, APS [12]
- Asymptomatic carriers: no routine anticoagulation; situational prophylaxis + lifestyle counselling + genetic counselling
High Yield Summary — Complications of AT Deficiency
- DVT and PE are the cardinal complications — ~70% develop VTE before age 60; PE kills by RV failure, not hypoxaemia [2][7]
- Recurrent VTE is common due to the permanent nature of the defect → justifies indefinite anticoagulation [2]
- Post-thrombotic syndrome affects 20–50% of DVT patients — chronic venous hypertension → skin changes → ulceration
- Unusual-site thrombosis (CVST, portal, mesenteric, renal vein, Budd-Chiari) should always prompt thrombophilia screening [3]
- Heparin resistance is both a complication and a diagnostic clue — heparin needs AT to work [1]; requires AT concentrate or direct thrombin inhibitors
- Pregnancy carries 30–50% VTE risk per pregnancy without prophylaxis; LMWH throughout + ≥ 6 weeks postpartum [13]
- Treatment complications: bleeding (all anticoagulants), HIT (heparin), warfarin teratogenicity, osteoporosis (long-term heparin/warfarin), over-anticoagulation after AT concentrate
- Warfarin-induced skin necrosis does NOT occur in AT deficiency (AT is not vitamin K-dependent) — that's protein C/S deficiency [2]
- In nephrotic syndrome: urinary loss of AT → thrombosis (renal vein, CVST, PE) [4][19]
- In cirrhosis: portal vein thrombosis in > 10% of patients; > 25% when decompensated [18]
- In DIC: AT is consumed → vicious cycle of thrombin generation → both bleeding and thrombosis [9]
Prothrombin G20210A Mutation
Prothrombin G20210A mutation is a hereditary point mutation in the 3' untranslated region of the prothrombin gene that leads to elevated plasma prothrombin levels and an increased risk of venous thromboembolism.
Protein C Deficiency
Protein C deficiency is a hereditary or acquired thrombophilic disorder in which reduced levels or function of protein C impair the inactivation of factors Va and VIIIa, leading to an increased risk of venous thromboembolism.