Acute Promyelocytic Leukaemia

Acute promyelocytic leukemia is a subtype of acute myeloid leukemia characterized by the t(15;17) translocation producing the PML-RARα fusion protein, leading to accumulation of abnormal promyelocytes and a high risk of disseminated intravascular coagulation.

Exam domain	One-glance essentials
Definition / diagnosis	*AML subtype defined by PML::RARA, usually t(15;17), with promyelocyte maturation arrest* ^[1] The 20% blast threshold does not apply when the defining fusion is present ^[2]
Epidemiology / risks	Often affects younger adults than AML overall; therapy-related cases may follow topoisomerase-II inhibitors ^[2] High-risk APL = presenting WBC > 10 × 10⁹/L ^[1]
Core mechanism	PML-RARα recruits co-repressors → blocks retinoic-acid–driven granulocytic differentiation ^[2] Tissue factor + Annexin-II–mediated hyperfibrinolysis + granule proteases → severe DIC/hypofibrinogenaemia ^[3]
Clinical picture	*Pancytopenia with bleeding out of proportion to the platelet count = suspect APL* ^[1] Petechiae, ecchymoses, mucosal/line-site bleeding; intracranial haemorrhage is the leading early threat ^[2]
Investigations	PBS/BM: hypergranular abnormal promyelocytes and bundles of Auer rods; *faggot cells strongly suggest APL* ^[3] MPO+, CD13+, CD33+, HLA-DR−, usually CD34− ^[2] PT↑, fibrinogen↓↓, D-dimer↑; confirm urgently by FISH/RT-PCR for PML::RARA ^[2]^[3]
Management	*Start ATRA immediately on clinical/morphological suspicion—do not wait for genetic confirmation* ^[1] Correct coagulopathy aggressively with platelets, FFP and cryoprecipitate ^[2] Lower-risk: ATRA + ATO; high-risk: add cytoreduction/anthracycline-based therapy ^[2]
Complications	DIC → fatal intracranial or pulmonary haemorrhage ^[2] Differentiation syndrome: fever, dyspnoea, oedema, hypotension, effusions/AKI after ATRA/ATO → dexamethasone urgently ^[2] ATO may prolong QTc; ATRA is teratogenic ^[2]
Prevention / follow-up	Maintain platelets and fibrinogen with frequent coagulation monitoring during induction ^[2] Monitor ECG/electrolytes with ATO; provide TLS and infection prophylaxis according to risk ^[1]
Exam traps	*APL is both the most immediately dangerous and one of the most curable AML subtypes* if treated promptly ^[2] Microgranular APL may mimic monocytic AML: use the HLA-DR−/CD34− pattern and PML::RARA testing ^[2] Do not perform an invasive procedure during uncontrolled DIC unless essential ^[3]

References

^[1] Lecture slides: GC 060. High white cell count.pdf (acute leukaemia workup and supportive treatment) ^[2] Senior notes: Ryan Ho Haemtology.pdf (Acute Promyelocytic Leukaemia, p.52–59) ^[3] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (APL blood film and DIC, p.8, 19)

Acute promyelocytic leukaemia (APL) — let's break the name down:

"Acute" → ≥20% blasts in bone marrow or peripheral blood; rapidly fatal if untreated (as opposed to chronic leukaemias where maturation is preserved) ^[1]
"Promyelocytic" → the malignant clone is arrested at the promyelocyte stage of myeloid differentiation (the cell between myeloblast and myelocyte)
"Leukaemia" → clonal malignant disease of the haematopoietic system ^[1]

APL is a biologically and clinically distinct subtype of acute myeloid leukaemia (AML), historically classified as AML-M3 under the French-American-British (FAB) system. Under the current WHO 2022 (5th edition) / ICC 2022 classification, it falls under "AML with recurrent genetic abnormalities" — specifically AML with t(15;17)(q24.1;q21.2) / PML::RARA" ^[2]^[3].

Key Defining Feature

APL is defined by a specific balanced reciprocal translocation t(15;17) that creates the PML-RARα (PML::RARA) fusion oncoprotein. This fusion is present in > 95% of cases and is both the diagnostic hallmark and the therapeutic target ^[3]^[4].

Why is APL singled out from other AMLs?

It is a haematological emergency — untreated median survival is < 1 month ^[3]
It carries a uniquely high risk of early death from DIC and life-threatening bleeding (especially intracranial haemorrhage) ^[2]^[3]^[5]
Paradoxically, it is the most curable form of AML (> 90% cure rate) when treated with differentiation therapy (ATRA + ATO) ^[4]^[6]

This is the classic "oxymoron" of haematology — the most dangerous AML upfront is the most curable with the right treatment ^[1].

Feature	Detail
Proportion of AML	5–20% of all AML cases ^[3]
Incidence	~0.5–1.5 per 100,000 per year (lower than AML overall)
Age distribution	Younger median age than other AML subtypes — median age at diagnosis ~40 years (range: bimodal with a peak in young adults and another smaller peak in older adults). Relatively more common in younger adults and adolescents compared to other AML subtypes
Sex	Slight male predominance (~1.2–1.5:1 M:F)
Geography	Higher incidence reported in populations of Hispanic/Latino descent and in parts of Southern China, including Hong Kong ^[3]. Some registries in Southern Europe and Latin America also show higher rates
Paediatric	Accounts for ~5–10% of childhood AML; rare before age 3

Hong Kong Relevance

APL is proportionally more common among AML cases in Southern Chinese populations. Hong Kong has been a world leader in the development of oral arsenic trioxide (ATO) therapy for APL — the pioneering work by Prof. Yok-Lam Kwong at Queen Mary Hospital established the efficacy of oral ATO, now widely adopted ^[1]^[3].

Most APL cases arise de novo without identifiable risk factors. However, recognized risk factors include:

Prior chemotherapy exposure (therapy-related APL)
- Topoisomerase II inhibitors (e.g., etoposide, doxorubicin, mitoxantrone) — these cause DNA double-strand breaks at specific sites, increasing the risk of balanced translocations including t(15;17) ^[2]^[3]
- Latency: typically 1–5 years after exposure (shorter than alkylating-agent-related AML which takes 5–7 years)
Radiation exposure — ionising radiation ^[2]
Chemical exposure — benzene and related solvents ^[2]
Genetic predisposition — much less established than for other AMLs. Down syndrome, Fanconi anaemia, and Bloom syndrome are risk factors for AML in general, but APL specifically does not show strong association with these syndromes ^[2]
Obesity — emerging epidemiological data suggest a modest association

Unlike many other cancers, APL is not strongly associated with age-related accumulation of mutations — hence its younger age at diagnosis. The t(15;17) translocation is often the single dominant driver event.

To understand APL, you need to know where promyelocytes sit in normal myelopoiesis:

The Promyelocyte

Morphology: larger than a myeloblast, with abundant azurophilic (primary) granules that contain myeloperoxidase (MPO), elastase, lysozyme, and other proteases
Function: the granule-forming stage — this is where the cell loads up with its antimicrobial arsenal
Key surface markers: CD13+, CD33+, MPO+, HLA-DR usually negative (distinguishes from other AML subtypes)
Normal maturation: promyelocytes normally mature through myelocyte → metamyelocyte → band → segmented neutrophil under the influence of transcription factors including RARα (retinoic acid receptor alpha)

Why does the promyelocyte get "stuck" in APL? → Because the PML-RARα fusion protein blocks the very transcription factor (RARα) that drives further differentiation. The cell is locked at the promyelocyte stage.

Etiology and Pathophysiology

In > 95% of APL cases, there is a balanced reciprocal translocation between chromosomes 15 and 17 ^[3]^[4]:

Component	Chromosome	Normal Function
PML (promyelocytic leukaemia gene)	15q24.1	Tumour suppressor; forms nuclear bodies involved in apoptosis, DNA damage response, senescence
RARα (retinoic acid receptor alpha)	17q21.2	Nuclear receptor transcription factor; mediates granulocytic differentiation in response to retinoic acid (vitamin A derivative)

The translocation fuses PML to RARα, creating the PML-RARα (PML::RARA) fusion oncoprotein.

How PML-RARα Causes Leukaemia (Molecular Pathophysiology)

Let's build this from first principles:

RARα is a nuclear receptor that, when bound by its ligand (all-trans retinoic acid, the active form of vitamin A), activates transcription of genes necessary for granulocytic differentiation
In the absence of ligand, RARα recruits co-repressors (NCoR/SMRT) + histone deacetylases (HDACs), which compact chromatin → gene silencing (differentiation genes OFF)
When physiological concentrations of ATRA bind normal RARα → co-repressors are released → co-activators recruited → chromatin opens → differentiation genes turned ON → promyelocyte matures to neutrophil

The fusion protein is a "supercharged repressor":

PML-RARα binds co-repressors with much higher affinity than wild-type RARα
Physiological concentrations of retinoic acid are insufficient to dissociate these co-repressors
Therefore, even in the presence of normal vitamin A levels, differentiation genes remain silenced
Additionally, PML-RARα recruits DNA methyltransferases (DNMTs), causing epigenetic silencing of differentiation target genes
PML-RARα also disrupts normal PML nuclear bodies → loss of PML's tumour suppressor function (impaired apoptosis, impaired senescence, impaired DNA damage response)

Net result: Impaired maturation + uncontrolled proliferation + impaired apoptosis → accumulation of malignant promyelocytes ^[1]

At pharmacological doses (much higher than physiological), ATRA can overcome the enhanced affinity of PML-RARα for co-repressors
ATRA forces PML-RARα to release co-repressors → recruits co-activators → differentiation genes turned ON
Malignant promyelocytes undergo terminal differentiation into mature (non-dividing) neutrophils → and then undergo apoptosis
This is "differentiation therapy" — you don't kill the cancer cells directly; you force them to grow up and die naturally

ATO targets the PML moiety of the fusion protein
ATO induces degradation of PML-RARα via SUMOylation → ubiquitination → proteasomal degradation
ATO also induces apoptosis of APL cells
ATO and ATRA are synergistic: ATRA forces differentiation while ATO degrades the oncoprotein

The Two-Hit Hypothesis in AML — Applied to APL

The "two-hit hypothesis" for AML states that leukaemogenesis requires both:

Class I mutation: confers proliferative/survival advantage (e.g., FLT3-ITD, present in ~30–40% of APL)
Class II mutation: blocks differentiation (e.g., PML-RARα)

In APL, the PML-RARα fusion itself acts as the Class II mutation. Additional cooperating mutations (FLT3-ITD being the most common) act as Class I mutations and may worsen prognosis ^[3].

DIC (disseminated intravascular coagulation) is a hallmark complication of APL and is a major cause of early mortality ^[1]^[2]^[3]^[5].

The coagulopathy in APL is multifactorial — it is not just classic DIC but a combination of three simultaneous mechanisms:

Mechanism	How It Happens	Consequence
1. Procoagulant activity	APL promyelocytes express abundant tissue factor (TF / Factor III) on their surface AND within their granules. When cells lyse (spontaneously or from chemotherapy), TF is released → activates the extrinsic coagulation pathway	Widespread fibrin deposition; consumption of clotting factors
2. Primary hyperfibrinolysis	APL cells express high levels of Annexin II (a receptor for plasminogen and tPA) → enhanced plasmin generation → excessive fibrinolysis	Breaks down clots AND fibrinogen → profound hypofibrinogenaemia
3. Proteolytic activity	APL granules release elastase and other proteases that directly degrade clotting factors (fibrinogen, vWF, etc.)	Further depletes clotting factors

Net result: a unique "triple coagulopathy" — simultaneous thrombosis AND bleeding, with profound hypofibrinogenaemia as a hallmark laboratory finding ^[5].

The clotting profile classically shows: ^[5]

↑ PT (prolonged) — because tissue factor activates the extrinsic pathway, Factor VII (shortest half-life of all clotting factors, ~6 hours) is consumed first
APTT may be relatively preserved initially — because Factor VIII (acute phase reactant) is elevated and buffers the intrinsic pathway
↓↓ Fibrinogen — consumed by DIC AND destroyed by hyperfibrinolysis
↑↑ D-dimer — from fibrin degradation
↓ Platelets — consumed by microthrombi

If left untreated over a longer period, all clotting factors eventually deplete, and APTT will also rise ^[5]

Variant	Translocation	Fusion Gene	ATRA Sensitivity
Classic	t(15;17)	PML-RARα	Sensitive
PLZF-RARα	t(11;17)	PLZF-RARα	Resistant to ATRA
NPM-RARα	t(5;17)	NPM-RARα	Sensitive
NuMA-RARα	t(11;17)	NuMA-RARα	Sensitive
STAT5b-RARα	der(17)	STAT5b-RARα	Resistant

Exam Pearl

The t(11;17) PLZF-RARα variant is resistant to ATRA — if a question mentions APL not responding to ATRA, think of this variant. However, this is exceedingly rare (< 1% of APL).

Classification

FAB Subtype	Name	Key Features
M3	Acute promyelocytic leukaemia — hypergranular (classical) variant	Heavily granulated promyelocytes with numerous Auer rods; "faggot cells" (bundles of Auer rods)
M3v	Acute promyelocytic leukaemia — microgranular (hypogranular) variant	Bilobed/reniform nucleus; fine granules (may be missed on light microscopy); often presents with higher WBC count and may be mistaken for monocytic leukaemia

APL is classified under "AML with recurrent genetic abnormalities" specifically:

APL with PML::RARA fusion

Key points from WHO 2022:

The diagnosis requires demonstration of PML::RARA fusion (by FISH, RT-PCR, or cytogenetics)
The 20% blast threshold does NOT apply — if t(15;17)/PML-RARα is present, the diagnosis of APL is made regardless of blast count ^[3]

This is the most commonly used prognostic risk stratification for APL:

Risk Category	WBC (×10⁹/L)	Platelet Count (×10⁹/L)	Relapse Risk
Low	≤ 10	> 40	~10%
Intermediate	≤ 10	≤ 40	~15%
High	> 10	—	~25–30%

High-risk APL (WBC > 10 × 10⁹/L) has a significantly higher risk of early death and relapse and may require the addition of chemotherapy (anthracycline) to ATRA + ATO ^[3]

High Yield: Risk Stratification Determines Treatment

Low/intermediate risk: ATRA + ATO (chemotherapy-free regimen) → cure rate > 95%
High risk: ATRA + ATO + chemotherapy (anthracycline-based, e.g., idarubicin) → or ATRA + conventional chemotherapy

This risk stratification is based on WBC and platelet count at presentation — simple bedside parameters.

Clinical Features

Symptom	Pathophysiological Basis	Details
Fatigue, lethargy, exercise intolerance	Anaemia — malignant promyelocytes crowd out normal erythroid precursors → ↓ RBC production	Often the earliest and most common symptom; may precede diagnosis by weeks
Easy bruising, petechiae, mucosal bleeding (epistaxis, gingival bleeding, menorrhagia)	Thrombocytopenia (BM failure) + DIC/coagulopathy (consumption of clotting factors + hyperfibrinolysis) → dual mechanism of bleeding	Bleeding is the most prominent presenting feature of APL — more severe than in other AML subtypes because of the superimposed DIC ^[1]^[2]^[3]
Fever	(1) Infection from neutropenia — functional neutrophil count is low despite total WBC which may be normal or even elevated (the promyelocytes are non-functional); (2) Tumour fever from cytokine release by leukaemic cells	Present in ~15–30% at diagnosis
Headache, visual disturbance, altered consciousness	Intracranial haemorrhage (ICH) — the most feared and lethal complication of APL-associated DIC; also leucostasis if WBC very high	ICH is the leading cause of early death in APL ^[3]
Bone pain	Expansion of leukaemic cells within the marrow cavity → periosteal stretching	Less common in APL than in ALL
Weight loss, night sweats	Hypermetabolic state from rapid cell turnover + cytokine release (IL-1, IL-6, TNF-α)	Constitutional "B symptoms"
Dyspnoea	Severe anaemia; pulmonary haemorrhage from coagulopathy; or leucostasis with pulmonary infiltration	May be multifactorial

Sign	Pathophysiological Basis	Details
Pallor	Anaemia (↓ RBC production)	Check conjunctivae, palmar creases, nail beds
Petechiae, purpura, ecchymoses	Thrombocytopenia + DIC → loss of primary haemostasis (platelet plug) + loss of secondary haemostasis (coagulation cascade)	Widespread petechiae + large ecchymoses in a young adult = think APL until proven otherwise
Mucosal bleeding (gingival oozing, epistaxis)	Same dual mechanism (thrombocytopenia + coagulopathy)	May be the presenting complaint
Retinal haemorrhages, Roth spots, cotton wool spots	Thrombocytopenia + coagulopathy → retinal vessel bleeding; Roth spots = fibrin-platelet aggregates in retinal haemorrhages	Fundoscopy important — may indicate risk of ICH
Fever	Neutropenia → infection; or tumour-related pyrexia	Must assume infection and treat empirically if neutropenic
Hepatosplenomegaly	Uncommon in APL (unlike ALL or other AML subtypes) — APL tends to have less extramedullary infiltration	When present, usually mild
Lymphadenopathy	Rare in APL	Prominent lymphadenopathy should prompt consideration of ALL or lymphoma
Gum hypertrophy	Not characteristic of APL — this is a feature of AML-M5 (acute monoblastic/monocytic leukaemia) ^[1]	Important negative to note
DIC-related signs	See DIC section above: (1) Bleeding — oozing from venepuncture sites, line sites, mucosal surfaces; (2) Thrombosis — less common but can manifest as DVT, PE, digital ischaemia, organ infarction	The combination of a young patient with pancytopenia + DIC (bleeding out of proportion) = APL until proven otherwise
Signs of leucostasis (rare, only in high WBC)	Very high WBC → hyperviscosity → microvascular occlusion especially in the lungs (dyspnoea, hypoxia) and CNS (confusion, headache, visual changes)	Less common in APL than in other AML subtypes because APL typically presents with leukopenia or normal WBC ^[3]

Clinical Pearl: APL vs Other AML Subtypes

What makes APL clinically distinct from other AMLs:

Younger age (median ~40 vs ~65 for AML overall)
DIC/coagulopathy dominates the presentation — bleeding is often severe and out of proportion to the platelet count alone
Less extramedullary disease — minimal hepatosplenomegaly, lymphadenopathy, or gum hypertrophy
Often presents with leukopenia (low WBC) rather than leukocytosis — the peripheral blood may show very few circulating leukaemic cells ^[3]
Rapidly fatal if not treated immediately — but most curable if treated correctly

Feature	Description	Why It's Important
Hypergranular promyelocytes	Large cells with abundant azurophilic granules that may obscure the nucleus	Classic appearance of APL (M3)
Auer rods	Rod-shaped crystalline inclusions of fused azurophilic granules (contain MPO)	Present in AML in general, but multiple Auer rods are highly characteristic of APL
"Faggot cells"	Promyelocytes containing bundles of Auer rods (the word "faggot" means a bundle of sticks)	Virtually pathognomonic of APL ^[3]
Bilobed / reniform nuclei	Seen in the microgranular variant (M3v) — nuclei have a folded, kidney-bean shape	Can be mistaken for monocytes — important to do immunophenotyping and molecular testing

Immunophenotype of APL: MPO+, CD13+, CD33+, HLA-DR negative, CD34 usually negative ^[3] The HLA-DR negativity and CD34 negativity help distinguish APL from other AML subtypes (most other AMLs are HLA-DR+ and CD34+).

High Yield Summary

Acute Promyelocytic Leukaemia (APL) — Pre-Diagnostic Summary:

Definition: AML subtype defined by t(15;17)(q24.1;q21.2) / PML-RARα fusion, arrested at promyelocyte stage
Epidemiology: 5–20% of AML; younger median age (~40y); relatively more common in Hispanic and Southern Chinese populations including Hong Kong
Pathophysiology: PML-RARα acts as a "super-repressor" of differentiation → maturation arrest + impaired apoptosis. Tissue factor expression + Annexin II-mediated hyperfibrinolysis + granule protease release → unique "triple coagulopathy" causing DIC
Risk stratification: Based on WBC and platelet count (Sanz score) — WBC > 10 = high risk
Clinical hallmarks:
- Young patient + pancytopenia + bleeding out of proportion (DIC) = APL until proven otherwise
- Faggot cells (bundles of Auer rods) on blood film
- HLA-DR negative, CD34 negative immunophenotype
- Less extramedullary disease than other AMLs
Most important teaching point: APL is a haematological EMERGENCY — untreated median survival < 1 month. Start ATRA immediately on clinical/morphological suspicion — do NOT wait for genetic confirmation ^[3]^[5]^[6]
Clotting profile: PT ↑, APTT may be initially preserved (Factor VIII buffers), Fibrinogen ↓↓, D-dimer ↑↑, Platelets ↓ ^[5]
The paradox: Most dangerous AML at presentation → most curable AML with treatment (> 90% cure with ATRA + ATO) ^[4]

Active Recall - APL: Definition, Epidemiology, Pathophysiology & Clinical Features

^[1] Senior notes: Block A - High white cell count: acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf ^[2] Senior notes: Maksim Medicine Notes.pdf (Haematology, p.171–173) ^[3] Senior notes: Ryan Ho Haemtology.pdf (p.53, p.59 — Acute Promyelocytic Leukaemia section) ^[4] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.738 — APL treatment and prognosis) ^[5] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (p.19 — DIC clotting profile in APL) ^[6] Senior notes: Learning_Points_All_Lectures.txt (Learning Point 2 — APL as haematological emergency)

Differential Diagnosis of Acute Promyelocytic Leukaemia (APL)

The differential diagnosis of APL is critically important for one overriding reason: APL is a haematological emergency with a median survival of < 1 month if untreated ^[3]^[6]. Misdiagnosing APL as another condition delays ATRA initiation and risks death from intracranial haemorrhage. Conversely, misdiagnosing a non-APL condition as APL exposes the patient to unnecessary differentiation therapy and its side effects (including differentiation syndrome). The differential therefore operates in two directions:

Conditions that mimic APL — i.e. you think it's APL, but it's actually something else
Conditions that APL can be mistaken for — i.e. the patient has APL, but you miss it because you think it's another diagnosis

The practical approach is: when you see a young patient with pancytopenia + severe bleeding/DIC, you must rapidly distinguish APL from its mimics using the 5-step MCICM approach: Morphology → Cytochemistry → Immunophenotyping → Cytogenetics → Molecular genetics ^[7]^[8]^[9].

Let's think about this logically. A patient presenting with suspected APL will typically show some combination of:

Pancytopenia (or leukopenia with circulating abnormal cells)
Bleeding diathesis / DIC
Abnormal promyelocyte-like cells on blood film

The differential therefore includes conditions that share one or more of these features:

Condition	Why It Mimics APL	How to Distinguish from APL
AML-M5 (Acute monocytic/monoblastic leukaemia)	Can have DIC (though less common than APL); monocytic blasts can occasionally be confused with the microgranular variant (M3v) of APL because of bilobed nuclei	Gum hypertrophy is characteristic of AML-M5 (not APL) ^[1]^[8]. Immunophenotype: AML-M5 is HLA-DR positive, CD34 positive, CD14+; APL is HLA-DR negative, CD34 negative ^[3]. No t(15;17). MPO may be weak/negative in monocytic variants
AML-NOS (other subtypes)	All AMLs share features of bone marrow failure (anaemia, neutropenia, thrombocytopenia) and may have Auer rods	Single Auer rods can be seen in any AML; bundles of Auer rods ("faggot cells") are virtually pathognomonic of APL ^[3]^[8]^[9]. Other AMLs are usually HLA-DR+ and CD34+. Molecular testing: no PML-RARα
AML with t(8;21) or inv(16) — "Core Binding Factor AML"	Also classified under "AML with recurrent genetic abnormalities"; relatively favourable prognosis like APL	Distinguished by their specific cytogenetic abnormalities: t(8;21)/RUNX1-RUNX1T1 or inv(16)/CBFB-MYH11 ^[3]^[10]. No DIC. Different morphology (M2 features for t(8;21), eosinophilia for inv(16))

High Yield GC Lecture Point

Severe bleeding is suggestive of APML → should look for lab evidence of DIC ^[8]^[9]. When you see disproportionate bleeding in any acute leukaemia, your reflex should be to check clotting profile, D-dimer, fibrinogen to screen for DIC — if present, APL jumps to the top of the differential ^[7].

Feature	Why It Mimics APL	How to Distinguish
Shared features	Both present with pancytopenia, fatigue, bleeding, fever. In paediatrics, ALL is far more common (80% of childhood leukaemia vs 15% AML) ^[2]^[8]	Lymphoblasts do NOT have Auer rods — Auer rods confirm myeloid lineage ^[9]. ALL is MPO negative (lineage-defining), whereas APL is strongly MPO positive ^[8]. Flow cytometry: ALL expresses lymphoid markers (CD10, CD19, CD20 for B-ALL; CD2, CD3, CD7 for T-ALL) and TdT positive. APL is TdT negative
Atypical lymphocytes vs blasts	On a hurried blood film review, lymphoblasts can be mistaken for atypical lymphocytes (reactive cells seen in infectious mononucleosis) — and vice versa ^[9]^[11]. However, this is a different error from confusing ALL with APL	Atypical lymphocytes are NOT neoplastic — they are reactive, activated lymphocytes from viral infections. Not to be mistaken as blasts ^[9]

Key distinguishing features of ALL vs APL:

ALL has more extramedullary disease: hepatosplenomegaly, lymphadenopathy, mediastinal mass (T-ALL), CNS involvement, testicular involvement ^[1]^[8]
APL has less extramedullary disease and more prominent DIC
Some ALL cells may co-express myeloid markers or even be mixed phenotype acute leukaemia ^[3] — immunophenotyping and molecular genetics are essential

Feature	Why It Mimics APL	How to Distinguish
Shared features	MDS causes pancytopenia with dysmorphic haematopoiesis; can have increased blasts (up to 19%); may present with bleeding	Blast count in MDS must be < 20% (otherwise it's AML by definition) ^[2]^[3]. MDS typically affects older patients (median ~65 years) vs APL (median ~40). MDS shows dysplasia in ≥1 lineage on morphology. No faggot cells. No t(15;17). MDS has NO hepatosplenomegaly ^[2]
MDS transforming to AML	MDS can transform to AML (including rarely APL) — so a patient with known MDS developing worsening cytopenias and DIC should be re-evaluated	Repeat bone marrow with cytogenetics/molecular studies. If blasts ≥ 20% or AML-defining genetics found, reclassify as AML

Blast count may be marginal (approaching 20%) especially with BM regeneration after chemotherapy or G-CSF injection, but does not always imply AML transformation ^[3]

Feature	Why It Mimics APL	How to Distinguish
Shared features	CML in blast crisis has ≥ 20% blasts → meets criteria for acute leukaemia. If myeloid blast crisis, it is essentially AML arising from CML	CML has the BCR-ABL1 fusion / t(9;22) Philadelphia chromosome — this is the defining feature ^[3]. CML typically has a preceding chronic phase with massive splenomegaly and basophilia (neither typical of APL). Essential to identify underlying CML because BCR-ABL TKIs (imatinib etc.) may still have a role ^[3]

Since DIC is the most dramatic presenting feature of APL, it's important to consider other causes of DIC in a patient who doesn't have APL:

DIC Cause	Why It Mimics APL	How to Distinguish
Sepsis / severe infection	DIC with pancytopenia (from sepsis-related marrow suppression) can closely resemble APL	Blood cultures positive; no circulating promyelocytes on PBS; no Auer rods; respond to antibiotics + source control. Bone marrow not replaced by blasts
Obstetric emergencies (placental abruption, amniotic fluid embolism, HELLP)	DIC in a young woman — same age demographic as APL	Clinical context is obvious (pregnant/peripartum). No leukaemic cells
Solid organ malignancies (especially adenocarcinomas — prostate, pancreas, gastric)	Can cause DIC with pancytopenia from bone marrow infiltration	PBS may show leukoerythroblastic picture (nucleated RBCs + tear drop cells) ^[9]. Bone marrow shows metastatic solid tumour, not blasts
Other haematological malignancies	Some aggressive lymphomas and other AML subtypes can cause DIC, though less frequently than APL	Morphology, immunophenotype, cytogenetics differentiate. Faggot cells absent

Feature	Why It Mimics APL	How to Distinguish
Shared features	Both cause severe pancytopenia with bleeding and infections	Aplastic anaemia has a hypocellular bone marrow (fat-filled) with NO blasts, NO malignant infiltration ^[2]^[12]^[13]. No circulating promyelocytes. No Auer rods. Reticulocyte count is low in both. Key: aplastic anaemia does NOT cause DIC

Feature	Why It Mimics APL	How to Distinguish
Shared features	Pancytopenia with prominent erythroid precursors in marrow — may mimic erythroleukaemia ^[3]. Megaloblastic changes can cause hypersegmented neutrophils and large, bizarre-looking cells that might be mistaken for blasts	Check serum B12 and folate levels (will be low). Bone marrow shows megaloblastic erythropoiesis (large erythroid precursors with open, immature-looking chromatin) but NO Auer rods, NO increased blasts. No t(15;17). Responds to B12/folate replacement

This deserves special mention because M3v is actually APL but is frequently misdiagnosed as something else:

Feature	The Diagnostic Trap	How to Avoid It
M3v morphology	Bilobed/reniform nuclei with fine or absent granules on light microscopy — can be mistaken for acute monocytic leukaemia (AML-M5) or even ALL	Always do immunophenotyping: M3v is HLA-DR negative, CD34 negative (unlike AML-M5). Always do FISH or RT-PCR for PML-RARα when clinical suspicion exists. M3v often presents with higher WBC count than classical APL

Exam Trap: M3v Misdiagnosis

The microgranular variant (M3v) of APL is a classic exam pitfall. It looks nothing like classical APL on morphology — no obvious granules, bilobed nuclei mimicking monocytes, and often higher WBC. If you rely on morphology alone, you will miss it. The saving grace is the immunophenotype (HLA-DR neg, CD34 neg) and molecular testing (PML-RARα positive). Always send for molecular/FISH testing when DIC is present in any acute leukaemia ^[3].

Feature	APL	Other AML	ALL	MDS	Aplastic Anaemia
Age	Younger (~40)	Older (~65)	Children/young adults	Older (~65)	Any age
WBC	Usually low	Variable	Variable	Low-normal	Low
DIC	Hallmark	Uncommon	Rare	No	No
Auer rods	Bundles (faggot cells)	Single possible	None	Rare	None
Extramedullary disease	Minimal	Variable	Prominent	Absent	Absent
MPO	Strongly positive	Usually positive	Negative	Variable	N/A
HLA-DR	Negative	Usually positive	Positive	Variable	N/A
CD34	Usually negative	Usually positive	Positive	Variable	N/A
Defining genetics	t(15;17) / PML-RARα	Various	Various	Various	None
BM cellularity	Hypercellular	Hypercellular	Hypercellular	Variable	Hypocellular
Gum hypertrophy	No	Yes (AML-M5) ^[1]	No	No	No
Prognosis with Rx	Best (> 90% cure)	Variable	Good in children	Variable	Variable

High Yield: The 3 Critical Differentiators for APL

When you're trying to distinguish APL from its mimics at the bedside and under time pressure, focus on these three:

Morphology: Faggot cells (bundles of Auer rods) = APL until proven otherwise ^[3]^[8]^[9]
Immunophenotype: HLA-DR negative + CD34 negative separates APL from virtually all other AML subtypes and ALL ^[3]
Coagulopathy pattern: DIC with disproportionately low fibrinogen in a young patient with leukaemia = APL ^[5]^[7]

Do NOT wait for cytogenetics/molecular confirmation before starting ATRA — start ATRA on clinical + morphological suspicion. The genetic confirmation comes later ^[3]^[6].

Since a significant proportion of APL patients present primarily with a bleeding emergency rather than with an obvious "leukaemia presentation," you may encounter APL through the DIC differential pathway:

Category	Examples	Clue That It's Not APL
Infection	Gram-negative sepsis, meningococcaemia	Positive cultures, no blasts on PBS
Obstetric	Abruption, eclampsia, amniotic fluid embolism	Pregnant/peripartum context
Malignancy	Mucin-secreting adenocarcinoma, other AMLs	Solid tumour history; non-APL morphology
Trauma/Surgery	Massive tissue injury, burns	Obvious clinical context
Vascular	Giant haemangioma (Kasabach-Merritt), aortic aneurysm	Imaging findings

The combination of pancytopenia with circulating promyelocytes, DIC with low fibrinogen, and t(15;17) translocation mandates urgent ATRA therapy even before formal diagnosis to prevent fatal bleeding complications ^[6]

High Yield Summary — Differential Diagnosis of APL

Primary mimics of APL: Other AML subtypes (especially AML-M5 and M3v pitfall), ALL, MDS, CML in blast crisis, aplastic anaemia, megaloblastic anaemia
Key distinguishing feature of APL: Faggot cells + HLA-DR neg/CD34 neg + t(15;17)/PML-RARα + DIC
DIC differential: Always consider sepsis, obstetric causes, solid tumours, trauma — but in a young patient with unexplained DIC + pancytopenia, APL must be at the top of the list
M3v trap: Microgranular variant looks like AML-M5 or even ALL morphologically — only immunophenotype and molecular testing will save you
Action principle: When clinical + morphological criteria suggest APL → start ATRA immediately → confirm with FISH/RT-PCR later ^[3]^[6]
The 5-step MCICM diagnostic approach (Morphology, Cytochemistry, Immunophenotyping, Cytogenetics, Molecular genetics) resolves virtually all the differentials listed above ^[8]^[9]

Active Recall - Differential Diagnosis of APL

References

^[1] Senior notes: Block A - High white cell count: acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf ^[2] Senior notes: Maksim Medicine Notes.pdf (Haematology section) ^[3] Senior notes: Ryan Ho Haemtology.pdf (p.53–54, p.59) ^[5] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (p.19) ^[6] Senior notes: Learning_Points_All_Lectures.txt (Learning Point 2) ^[7] Lecture slides: GC 060. High white cell count.pdf (Workup for suspected acute leukaemia) ^[8] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.418–421) ^[9] Senior notes: Ryan Ho Fundamentals.pdf (p.390 — MCICM approach, PBS interpretation) ^[10] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.734) ^[11] Senior notes: Block A - Generalised Lymphadenopathy: Differential diagnosis and principle of management.pdf ^[12] Senior notes: Block A - Family history of anaemia: inherited causes of anaemia; haemolytic anaemia; aplastic anaemia.pdf ^[13] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1468)

Diagnostic Criteria for APL

APL diagnosis is unique among acute leukaemias because you must act before the diagnosis is confirmed. Let me explain why this paradox exists and how the diagnostic framework is structured.

In most of medicine, we diagnose first and then treat. In APL, we must start treatment (ATRA) on clinical and morphological suspicion alone, BEFORE cytogenetic/molecular confirmation arrives ^[3]^[6]. Why? Because:

Cytogenetic results (karyotyping) take 3–7 days
FISH results take 24–48 hours
RT-PCR results take 24–48 hours
The patient can die from intracranial haemorrhage within hours ^[3]

This creates a two-tier diagnostic structure:

Presumptive (working) diagnosis → morphology + clinical features → START ATRA
Definitive (confirmed) diagnosis → genetic confirmation of PML-RARα

The diagnosis of APL requires:

Criterion	Details
1. Identification of PML::RARA fusion	By cytogenetics (karyotyping showing t(15;17)), FISH, or RT-PCR demonstrating PML-RARα transcript
2. The 20% blast threshold does NOT apply	Leukaemia with t(15;17) / PML-RARA is diagnostic of AML (APL) without regard to the blast count ^[3]^[10] — this is a crucial exam point
3. Myeloid lineage confirmation	Morphology (Auer rods), cytochemistry (MPO+/SBB+), or immunophenotyping (CD13+, CD33+)

High Yield Exam Point

For AML in general, diagnosis requires ≥ 20% blasts in PB or BM. However, three AML-defining genetic abnormalities bypass this threshold ^[3]^[10]:

t(15;17) / PML-RARA → APL
t(8;21) / RUNX1-RUNX1T1
inv(16) or t(16;16) / CBFB-MYH11

These are diagnosed as AML even if blast count is < 20%. The rationale is that these translocations define a distinct biological entity regardless of blast percentage.

Urgent consult HAEMAT (for urgent ATRA) when cytologic + clinical criteria are met ^[3]:

Feature	What Triggers Presumptive Diagnosis
Clinical	Young patient + pancytopenia + bleeding disproportionate to platelet count ± DIC
Morphological	Abnormal promyelocytes with numerous violet granules ± bundles of Auer rods ("faggot cells") on PBS or BM ^[3]^[5]
Immunophenotypic	MPO+, CD13+, CD33+, HLA-DR negative, CD34 usually negative ^[3]
Coagulation	DIC pattern: PT ↑, fibrinogen ↓↓, D-dimer ↑↑, platelets ↓ ^[5]^[14]

DO NOT wait for genetics for confirmation! ^[3]

The 5-Step MCICM Diagnostic Approach

This is the systematic framework for diagnosing all haematological malignancies, and it applies perfectly to APL. Think of it as building layers of diagnostic certainty, each step adding more specific information ^[8]^[9].

MCICM = Morphology → Cytochemistry → Immunophenotyping → Cytogenetics → Molecular genetics ^[8]^[9]

Step 1: Morphology (PBS + Bone Marrow)

Parameter	Expected Finding in APL	Explanation
Haemoglobin	Normochromic normocytic anaemia (of variable severity) ^[3]^[10]	Malignant promyelocytes crowd out erythroid precursors → ↓ RBC production. It's normochromic normocytic because the problem is production failure, not a maturation defect
WBC	Usually leukopenia ^[3]^[14] — this is characteristic of APL. Peripheral blood: usually leukopenia with rare leukemic cells ^[3]	Unlike other AMLs where WBC is often elevated, APL promyelocytes tend to stay in the marrow. However, the microgranular variant (M3v) may present with leukocytosis
Platelets	Thrombocytopenia ^[3]^[5]^[10]	(1) BM failure — megakaryopoiesis suppressed; (2) DIC — platelets consumed in microthrombi
Differential	May show few circulating abnormal promyelocytes; neutropenia	Functional neutrophil count is low because normal myeloid maturation is arrested
Reticulocyte count	Low/inappropriately normal	Marrow failure → cannot mount a reticulocyte response

GC Lecture High Yield

From GC 060 (High white cell count): CBC + differential (WBC high/normal/low) + manual count is listed as the first step in the workup for suspected acute leukaemia ^[7]. In APL specifically, the WBC is characteristically low — this is a distinguishing feature from other AMLs.

This is where the first morphological clue hits you. The PBS findings in APL are distinctive ^[5]:

Finding	Description	Significance
Abnormal promyelocytes	Large cells with abundant cytoplasm packed with numerous violet/azurophilic granules that may obscure the nucleus	The hallmark cell of classical APL (M3)
Auer rods	Rod-shaped crystalline inclusions of fused primary granules	Confirms myeloid lineage — Auer rods essentially represent fused granules and will stain positive for MPO/SBB ^[5]
Faggot cells	Promyelocytes containing bundles of Auer rods (≥ 3 Auer rods) ^[5]	Virtually pathognomonic of APL ^[3]^[5]^[9]
Bilobed (reniform/kidney-shaped) nucleus	Seen in the microgranular variant (M3v)	Can be mistaken for monocytes — important diagnostic pitfall
Giant platelets + schistocytes	Due to DIC — schistocytes are RBC fragments sheared by fibrin strands in microvasculature ^[5]	Evidence of microangiopathic haemolytic anaemia (MAHA) secondary to DIC
Absolute thrombocytopenia	Reduced platelet numbers	Confirms both BM failure and consumptive process
+ve for cytochemical staining (MPO / SBB)	See Step 2 below	Confirms myeloid lineage

"Blasts: always abnormal → if ≥ 20%, diagnostic of acute leukaemia. Auer rod: presence confirms myeloblastic nature, i.e. a diagnosis of AML. Faggot cell: abnormal promyelocytes with numerous Auer rods → APML (haematological emergency)" ^[9]

Finding	Details
Cellularity	Hypercellular marrow, replaced by abnormal promyelocytes ^[3]^[8]
Blast/promyelocyte count	Usually > 20% (but remember, the 20% threshold is not required if PML-RARα is demonstrated) ^[3]^[10]
Morphology	Sheets of abnormal promyelocytes with heavy granulation, Auer rods, faggot cells
Erythroid and megakaryocytic series	Markedly suppressed
Trephine	Shows hypercellularity, diffuse infiltration pattern, architecture effaced

Important practical point: consult HAEM for cytogenetic and molecular studies BEFORE performing BM ^[8]. Why? Because samples for FISH, RT-PCR, and flow cytometry must be collected fresh from the aspirate — if you don't send the right tubes at the time of aspiration, you may need to repeat the procedure.

Contraindication to BM biopsy: severe DIC is a relative contraindication ^[9]. In practice, for APL patients with active DIC, correct the coagulopathy first with FFP and platelets (aim platelets > 20 × 10⁹/L), then proceed ^[9].

Stain	Result in APL	Interpretation
Myeloperoxidase (MPO)	Strongly positive ^[3]^[5]^[8] — typically shows intense dark staining	MPO is present in the azurophilic granules of the promyelocytes. Strong MPO positivity confirms myeloid lineage. APL has the strongest MPO positivity of all AML subtypes because promyelocytes are the granule-loaded stage of myeloid differentiation
Sudan Black B (SBB)	Positive ^[5]^[8]	SBB stains lipids in granules — same principle as MPO. Positive = myeloid
PAS (Periodic Acid-Schiff)	Variable, usually negative or weakly positive	PAS positivity is more characteristic of ALL (block positivity) and erythroleukaemia
Non-specific esterase (NSE)	Negative	NSE positivity is characteristic of monocytic lineage (AML-M5). APL is not monocytic

"Myeloperoxidase, Sudan Black B staining indicates myeloid lineage. Usually present as 'rim of positivity' due to staining of granular contents in the periphery" ^[9]

Why MPO is lineage-defining

MPO ("myelo-peroxidase") — the name tells you: it's a peroxidase enzyme found in myeloid granules. It catalyses the formation of hypochlorous acid (bleach) from hydrogen peroxide and chloride ions, which is used to kill phagocytosed bacteria. Its presence confirms a cell is of myeloid (granulocytic/monocytic) origin. Lymphoid cells do NOT contain MPO — hence MPO negativity is lineage-defining for lymphoid origin ^[8].

Flow cytometry uses fluorescent antibodies against cell surface (and intracellular) markers to determine the lineage and maturation stage of the malignant cells. In APL, the immunophenotype is distinctive:

Marker	Result in APL	Why / Significance
MPO	Positive	Myeloid lineage confirmation (intracellular marker)
CD13	Positive	Pan-myeloid marker (present on granulocytes/monocytes at various stages)
CD33	Positive	Pan-myeloid marker
CD117 (c-Kit)	Variable (often positive)	Marker of immature myeloid cells
HLA-DR	NEGATIVE ← Key distinguishing feature ^[3]	Most other AML subtypes and ALL are HLA-DR positive. APL promyelocytes have lost HLA-DR expression during their differentiation to the promyelocyte stage. This is the single most useful flow cytometry marker for rapid identification of APL
CD34	Usually NEGATIVE ^[3]	CD34 marks very immature progenitors (haematopoietic stem cells/early blasts). Promyelocytes are past the CD34+ stage. Most other AML subtypes retain CD34
TdT	Negative	TdT (terminal deoxynucleotidyl transferase) is a marker of lymphoid precursors and is positive in ALL. Negative in APL
CD15	Variable	May be weakly positive
CD2	Sometimes positive (especially in M3v)	Aberrant CD2 expression can be seen — does not change diagnosis but is associated with M3v morphology

Flow cytometry: expression of CD34, HLA-DR, CD117, CD13, CD33 (varies with AML subtypes) ^[3]

The HLA-DR Negative + CD34 Negative Pattern

In the world of acute leukaemia immunophenotyping, most blasts are HLA-DR+ and CD34+. When you encounter a myeloid leukaemia (MPO+, CD13+, CD33+) that is HLA-DR negative and CD34 negative, APL should be your immediate thought. This pattern reflects the fact that APL cells are arrested at the promyelocyte stage — they have matured past the HLA-DR+/CD34+ stem cell/blast stage but are blocked from further differentiation by PML-RARα.

This is where the definitive diagnosis is made ^[14]:

Method	What It Detects	Turnaround Time	Result in APL
Conventional karyotyping	Whole-genome chromosomal abnormalities by growing cells in culture and examining metaphase chromosomes	3–7 days (requires cell division in culture)	t(15;17)(q24.1;q21.2) — balanced reciprocal translocation between chromosomes 15 and 17
FISH (Fluorescence In Situ Hybridization)	Targeted detection of specific translocations using fluorescent probes — can be done on non-dividing cells (interphase nuclei)	24–48 hours (faster than karyotyping)	Positive for PML-RARA fusion signal

"Cytogenetics: Characterized by t(15;17)(q22;q21). Resultant PML:RARA fusion protein creates a differentiation block" ^[14]

Why do we need both karyotyping AND FISH?

Karyotyping gives you the whole picture — it may reveal additional chromosomal abnormalities (e.g., trisomy 8, del(7q)) that affect prognosis or indicate complex karyotype
FISH is faster and more sensitive for the specific t(15;17) — can detect the fusion even in cells that aren't dividing (important when blast percentage is low)
In rare cryptic translocations where t(15;17) is not visible on karyotyping (submicroscopic rearrangement), FISH or RT-PCR will still detect PML-RARα

Method	What It Detects	Turnaround Time	Result in APL
RT-PCR (Reverse Transcription PCR)	Detects the PML-RARα mRNA transcript at the molecular level — identifies the specific breakpoint variant (bcr1, bcr2, bcr3)	24–48 hours	Positive for PML-RARA fusion transcript
Real-time quantitative RT-PCR (RQ-PCR)	Quantifies the level of PML-RARα transcript — used for monitoring measurable residual disease (MRD)	Same	Gives a quantitative measure — baseline level established at diagnosis, then monitored during treatment
Next-generation sequencing (NGS)	Detects cooperating mutations (FLT3, WT1, etc.) that influence prognosis	Days–weeks	FLT3-ITD present in ~30–40% of APL — associated with higher WBC count and historically poorer prognosis (less relevant with ATRA+ATO regimens)

Why molecular genetics matters:

Confirms the diagnosis when cytogenetics are equivocal or cryptic
Identifies the PML breakpoint variant — this determines which PCR primers to use for MRD monitoring during follow-up
Detects cooperating mutations (especially FLT3-ITD) for risk stratification
Establishes baseline for MRD monitoring — achieving molecular remission (PCR-negative for PML-RARα) after consolidation is the goal; persistent PCR positivity or molecular relapse triggers intervention before frank haematological relapse

GC Lecture Slide — Must Know

From GC 060 (High white cell count) — Workup for suspected acute leukaemia ^[7]:

Clotting profile, D-dimer, fibrinogen (DIC in APL)

This is listed as a priority investigation alongside CBC. In APL specifically, the coagulation profile is not just "nice to have" — it is essential for immediate management and is often the investigation that prompts you to suspect APL in the first place.

Parameter	Expected in APL-DIC	Pathophysiological Basis
PT (Prothrombin Time)	Prolonged	Tissue factor from APL cells activates extrinsic pathway → Factor VII consumed first (shortest half-life ~6h) ^[5]
APTT	May be initially preserved	Factor VIII is an acute phase reactant → elevated during acute illness → buffers intrinsic pathway. If untreated, APTT eventually rises too ^[5]
Fibrinogen	Markedly decreased (↓↓)	Triple mechanism: consumed by DIC, destroyed by hyperfibrinolysis (Annexin II), degraded by granule proteases (elastase). Low fibrinogen is the hallmark of APL-DIC
D-dimer	Markedly elevated (↑↑)	Fibrin degradation products from widespread fibrinolysis
Platelet count	Low	Consumed in DIC microthrombi + BM failure
Thrombin time	Prolonged	Low fibrinogen + FDPs interfere with fibrin polymerisation

"Bleeding symptoms are out of proportion to the platelet amounts" ^[14] — this is the clinical clue. A platelet count of 50 × 10⁹/L shouldn't cause severe mucosal bleeding and ICH in typical thrombocytopenia. When bleeding is disproportionate, think DIC → think APL.

These are required to prepare for treatment and detect complications:

Investigation	Purpose	Expected Finding
Biochemistry: RFT, K⁺, Ca²⁺, PO₄³⁻, urate, LDH	Screen for tumour lysis syndrome (TLS) ^[7]	May show ↑K⁺, ↑PO₄³⁻, ↓Ca²⁺, ↑urate, ↑LDH. LDH also serves as a tumour burden marker
CXR	Disease-related complications, infections ^[7]	May show pulmonary infiltrates (infection, haemorrhage, or later differentiation syndrome). Mediastinal mass is NOT typical of APL (more characteristic of T-ALL)
ECG + Echocardiogram	Before anthracycline therapy ^[7]	Baseline cardiac function — anthracyclines (daunorubicin/idarubicin) are cardiotoxic (cause dose-dependent dilated cardiomyopathy). Need LVEF > 50% before starting
Hepatitis serology (HBV, HCV) + HIV	Baseline infection screen ^[7]	HBV reactivation risk with immunosuppression; affects choice of chemotherapy
G6PD testing	Risk of oxidative haemolysis with co-trimoxazole ^[7]	Co-trimoxazole is used for PCP prophylaxis during treatment. If G6PD deficient → use alternative (e.g., dapsone, or pentamidine)
HLA typing	For patients with high-risk disease and candidates for HSCT ^[7]	APL rarely requires HSCT (> 90% cure with ATRA+ATO), but HLA typing is done early for patients who fail induction or have high-risk features
Central venous catheter arrangement	Vascular access for chemotherapy ^[7]	Usually PICC line or Hickman catheter — but must correct coagulopathy first in APL!
Blood group and crossmatch	Anticipated transfusion needs	APL patients often need multiple platelet and FFP transfusions
Lumbar puncture	CNS involvement assessment	NOT routinely done at diagnosis in APL (unlike ALL). Only performed if CNS symptoms present. Contraindicated during active DIC

GC 060 — Complete Pre-Treatment Workup

From the GC lecture slide, the full workup for suspected acute leukaemia includes ^[7]:

CBP + differential + manual count
Clotting profile, D-dimer, fibrinogen
Biochemistry — RFT, K⁺, Ca²⁺, PO₄³⁻, urate, LDH
Bone marrow examination + cytogenetics + molecular/NGS
CXR
ECG, echocardiogram (before anthracycline)
Hepatitis serology, HIV
G6PD
Central venous catheter insertion
HLA typing for high-risk disease/HSCT candidates

This is a complete, exam-ready checklist — memorise it.

The following algorithm captures the real-world diagnostic flow for APL, emphasising the dual-track approach of treating while confirming:

Pattern Recognition Table

Scenario	CBC	PBS	Coagulation	Immunophenotype	Genetics	Diagnosis
Classical APL	Pancytopenia, WBC low	Hypergranular promyelocytes, faggot cells, Auer rod bundles	DIC: PT↑, fibrinogen↓↓, D-dimer↑↑	MPO+, CD13+, CD33+, HLA-DR−, CD34−	t(15;17), PML-RARα+	APL (M3)
M3v APL	Pancytopenia but WBC may be HIGH	Bilobed nuclei, fine/absent granules — mimics monocytes	DIC pattern (same)	Same as above (HLA-DR−, CD34−)	t(15;17), PML-RARα+	APL microgranular variant
Other AML	Variable WBC	Myeloblasts, ± single Auer rods	Usually no DIC (except rare cases)	MPO+, CD13+, CD33+, HLA-DR+, CD34+	Various (t(8;21), inv(16), etc.)	AML (non-APL)
ALL	Variable WBC	Lymphoblasts, NO Auer rods	No DIC	MPO−, TdT+, lymphoid markers	Various	ALL
Aplastic anaemia	Pancytopenia	No blasts, no abnormal cells	Normal coagulation	N/A	Normal	Aplastic anaemia

High Yield Summary — Diagnosis of APL

Diagnostic criteria: t(15;17)/PML-RARα fusion is the gold standard. The 20% blast threshold does NOT apply when this translocation is present ^[3]^[10]^[14]
The MCICM 5-step approach resolves the diagnosis systematically: Morphology → Cytochemistry → Immunophenotyping → Cytogenetics → Molecular genetics ^[8]^[9]
Morphological hallmarks: Faggot cells (bundles of Auer rods), hypergranular promyelocytes, MPO/SBB strongly positive ^[5]
Immunophenotypic hallmark: HLA-DR negative + CD34 negative (distinguishes from virtually all other AMLs) ^[3]
Coagulation hallmark: DIC with disproportionately low fibrinogen — bleeding out of proportion to platelet count ^[5]^[14]
Action principle: START ATRA on morphological/clinical suspicion → confirm with FISH/RT-PCR → do NOT wait ^[3]^[6]
Pre-treatment workup (GC 060): CBP, clotting profile, biochemistry (TLS screen), BM + cytogenetics + molecular, CXR, ECG/echo, hepatitis/HIV, G6PD, HLA typing, CVC ^[7]
MRD monitoring: RQ-PCR for PML-RARα transcript is used post-treatment to confirm molecular remission and detect early relapse

Active Recall - Diagnostic Criteria, Algorithm & Investigations in APL

^[3] Senior notes: Ryan Ho Haemtology.pdf (p.53–54, p.59) ^[5] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (p.8) ^[6] Senior notes: Learning_Points_All_Lectures.txt (Learning Point 2) ^[7] Lecture slides: GC 060. High white cell count.pdf (Workup for suspected acute leukaemia, p.6) ^[8] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.418–421) ^[9] Senior notes: Ryan Ho Fundamentals.pdf (p.390–391) ^[10] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.734) ^[14] Senior notes: Block A - High white cell count: acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (DIC in APL section)

Management of Acute Promyelocytic Leukaemia (APL)

APL management is fundamentally different from other AML subtypes for three reasons:

The fusion oncoprotein (PML-RARα) is a druggable target — you can force differentiation with ATRA and degrade the protein with ATO, rather than relying solely on cytotoxic chemotherapy to kill cells
The immediate threat is not the leukaemia burden per se, but the DIC — the patient is most likely to die from haemorrhage (especially ICH) in the first hours to days, not from long-term marrow failure
Cure rates exceed 90% with appropriate therapy — this is the best prognosis of any AML, but only if you get through the dangerous induction period ^[3]^[4]

Therefore, APL management has two simultaneous tracks running in parallel:

Track 1: Definitive anti-leukaemia therapy (differentiation therapy ± chemotherapy)
Track 2: Aggressive supportive care (coagulopathy management, infection prevention, complication monitoring)

Phase	Goal	Duration
1. Emergency management	Prevent early death from DIC/ICH; start ATRA immediately	Hours (first contact)
2. Induction	Achieve complete remission (CR)	~4–6 weeks
3. Consolidation	Eliminate residual disease, achieve molecular remission	~3–4 cycles over 6–9 months
4. Maintenance (if indicated)	Prevent relapse	~1–2 years (only for ATRA+chemo regimens)
5. MRD monitoring	Detect molecular relapse early	Ongoing for 2–3 years post-consolidation
6. Relapse management	Re-induce remission → consider HSCT	If applicable

Phase 0: Immediate Emergency Management (First Minutes to Hours)

This is the most critical phase. The patient is bleeding. Time is life.

Urgent consult HAEMAT (for urgent ATRA) when cytologic + clinical criteria met. DO NOT wait for genetics for confirmation! ^[3]

Drug	ATRA — All-Trans Retinoic Acid
Full name breakdown	"All-trans" = the geometric isomer with all double bonds in trans configuration; "retinoic acid" = the active metabolic derivative of vitamin A (retinol → retinal → retinoic acid)
Trade name	Vesanoid (tretinoin)
Dose	45 mg/m²/day given orally in 2 divided doses
Route	Oral (PO) — capsules
When to start	The moment APL is clinically or morphologically suspected — before genetic confirmation ^[3]^[6]
Mechanism	At pharmacological doses, ATRA overcomes the enhanced co-repressor binding of PML-RARα → forces release of co-repressors → recruits co-activators → turns on differentiation genes → malignant promyelocytes undergo terminal differentiation into mature neutrophils → then undergo apoptosis. This is "differentiation therapy" — you don't kill the cells, you force them to grow up

Hong Kong Pioneering Role

Hong Kong is a world leader in APL treatment — Prof. Yok-Lam Kwong at Queen Mary Hospital pioneered the use of oral arsenic trioxide (ATO) ^[1]. The availability of oral ATO has made chemotherapy-free regimens practical and has been adopted internationally.

Supportive treatment: Coagulopathy controlled by platelet + FFP transfusion ^[3]

Intervention	Target	Rationale
Platelet transfusion	Maintain platelets > 30–50 × 10⁹/L (some guidelines say > 50 during active DIC/bleeding)	Platelets are consumed by DIC microthrombi AND reduced by BM failure — need aggressive replacement to prevent ICH
Fresh frozen plasma (FFP)	Replenish consumed clotting factors	DIC consumes all coagulation factors. FFP provides all factors in physiological ratios
Cryoprecipitate	Maintain fibrinogen > 1.5 g/L (some centres target > 1.0 g/L)	Cryoprecipitate is enriched in fibrinogen, Factor VIII, vWF, Factor XIII, and fibronectin. Fibrinogen is the most critically depleted factor in APL-DIC ^[14]
Tranexamic acid	Cautious use — consider if hyperfibrinolysis dominant	Anti-fibrinolytic; blocks plasmin-mediated fibrinolysis. Must use with caution in DIC as it may worsen thrombosis

Management of DIC in APL: Specific treatment → ATRA + Arsenic trioxide. Supportive treatment → Transfusion with platelet, FFP, cryoprecipitate. If you tide them over the acute period, no ICH, then prognosis is good ^[14]

GC Lecture High Yield — Supportive Treatment for Acute Leukaemia

From GC 060 (High white cell count) ^[7]:

Resuscitation if in septic shock
Immediate empirical broad spectrum antibiotics for neutropenic sepsis
Hydration and urate lowering agents (allopurinol, febuxostat, rasburicase) to prevent tumour lysis syndrome
Urgent leukapheresis if signs of leukostasis
Disseminated intravascular haemolysis (DIC) in acute promyelocytic leukaemia (APL)
RBC and platelet transfusions
Antifungal prophylaxis in patients with prolonged and profound neutropenia
Proper nursing care — reverse isolation, face mask, hand hygiene, low bacteria diet

Phase 1: Induction Therapy — Risk-Stratified Approach

APL induction is stratified by the Sanz risk classification based on WBC and platelet count at presentation:

ATRA + ATO (chemotherapy-free regimen) ^[3]^[4]

Agent	Dose	Duration	Mechanism
ATRA	45 mg/m²/day PO in 2 divided doses	Until haematological CR (typically 4–6 weeks)	Forces terminal differentiation of malignant promyelocytes → mature neutrophils
ATO (Arsenic trioxide)	0.15 mg/kg/day IV (or oral formulation in HK)	Until haematological CR	Targets the PML moiety of PML-RARα → induces SUMOylation → ubiquitination → proteasomal degradation of the fusion protein. Also induces apoptosis directly. At low doses, promotes differentiation; at high doses, induces apoptosis

Combined ATRA/ATO without cytotoxic drugs as initial therapy for APL is promising with complete remission (CR) rate ~100% and increased 2-year event-free survival ^[4]

Why is chemotherapy-free possible in low/intermediate risk APL? Because ATRA + ATO together address both the differentiation block AND the survival of the malignant clone. ATRA forces the cells to differentiate (Class II mutation reversed), while ATO degrades the driving oncoprotein AND induces apoptosis (eliminates the cell). Together, they achieve synergistic killing without the toxicity of conventional cytotoxics.

ATRA + ATO + chemotherapy (anthracycline-based) ^[3]

Agent	Dose	Rationale
ATRA	45 mg/m²/day PO	Same as above
ATO	0.15 mg/kg/day IV	Same as above
Idarubicin (or daunorubicin)	Standard AML doses × 3–4 doses during induction	Anthracycline adds cytotoxic killing to debulk the higher tumour burden. High WBC means more leukaemic cells → greater DIC risk → need faster cell reduction

OR the alternative (older regimen still used in some centres):

ATRA + conventional chemotherapy (cytarabine + daunorubicin) without ATO during induction ^[3]

ATRA + conventional chemo (cytarabine + daunorubicin) for high risk ^[3]

Why Does High WBC Need Chemotherapy?

At WBC > 10 × 10⁹/L, the tumour burden is higher → more tissue factor release → more severe DIC → higher risk of early death. Additionally, ATRA+ATO alone may not be fast enough to reduce the cell count before differentiation syndrome develops (see complications below). The anthracycline provides rapid cytoreduction to complement the differentiation effect.

Agent	Target	Primary Mechanism	Secondary Effects
ATRA ("all-trans retinoic acid")	RARα portion of PML-RARα	Forces co-repressor release at pharmacological doses → chromatin remodelling → turns on differentiation genes → terminal differentiation of promyelocytes into mature neutrophils	Degrades PML-RARα (partially); induces cell cycle arrest
ATO ("arsenic trioxide"; As₂O₃)	PML portion of PML-RARα	Induces SUMOylation of PML-RARα → polyubiquitination → proteasomal degradation of the fusion protein	At low concentrations: promotes differentiation. At high concentrations: induces apoptosis via ROS generation, mitochondrial pathway. Restores normal PML nuclear body formation

The combination is synergistic because they attack the fusion protein from both ends — ATRA from the RARα side (functional) and ATO from the PML side (structural) ^[3]^[4]

After achieving haematological CR (typically ~4–6 weeks), consolidation aims to eliminate residual leukaemic cells and achieve molecular remission (PCR-negative for PML-RARα).

If Induction Was	Consolidation Regimen	Details
ATRA + ATO (low/intermediate risk)	ATO + ATRA — multiple cycles ^[3]	Typically 4 cycles of ATO (e.g., 0.15 mg/kg/day × 5 days/week × 4 weeks, repeated every 8 weeks) + ATRA (45 mg/m² × 2 weeks on / 2 weeks off). Entirely chemotherapy-free
ATRA + chemo (high risk or older regimen)	Anthracycline + ATRA — 2–3 cycles ^[3]	E.g., idarubicin/daunorubicin + ATRA for 2–3 consolidation cycles. Some protocols add cytarabine

Post-remission therapy: remission achieved in 90% of patients ^[3]

End-of-consolidation assessment:

BM aspirate for morphology + RT-qPCR for PML-RARα
Goal: molecular remission = PCR-negative for PML-RARα transcript
If PCR-positive after consolidation → consider additional therapy or HSCT

Regimen	Maintenance?	Details
ATRA + ATO based	Generally NO maintenance required	The ATRA+ATO regimen achieves such deep molecular remissions that maintenance adds no benefit — this is a major advantage over older chemo-based regimens
ATRA + chemo based	Yes — typically ATRA + low-dose chemotherapy for 1–2 years	E.g., ATRA (15 days every 3 months) + 6-mercaptopurine + methotrexate for 1–2 years

This is one of the major advantages of the ATRA+ATO regimen — not only is it chemotherapy-free (avoiding toxicity), but it also eliminates the need for prolonged maintenance.

Parameter	Method	Schedule	Action
PML-RARα transcript	RT-qPCR (real-time quantitative PCR)	Every 3 months for the first 2–3 years after consolidation	If transcript turns positive → molecular relapse → intervene before haematological relapse

Why MRD monitoring matters in APL:

APL is unique in having a single, trackable molecular target (PML-RARα transcript)
Molecular relapse (PCR turns positive) precedes haematological relapse by 2–3 months on average
Early intervention at the molecular relapse stage has much better outcomes than waiting for full haematological relapse

APL relapse is uncommon with modern therapy (< 10% for low/intermediate risk) but when it occurs:

Situation	Management
Molecular relapse (PCR positive, no haematological relapse)	Re-induce with ATO ± ATRA → if molecular remission achieved → consider autologous HSCT
Haematological relapse	ATO-based re-induction → if CR2 achieved → allogeneic HSCT in suitable candidates (age < 60, fit, HLA-matched donor)

APL rarely requires HSCT upfront (unlike high-risk non-APL AML) because first-line cure rates are so high. HSCT is reserved for relapsed/refractory disease.

GC 060 — Supportive Treatment Checklist

Watch out for haematological emergencies ^[7]^[14]:

Resuscitation if in septic shock
Immediate empirical broad spectrum antibiotics for neutropenic sepsis
Hydration and urate lowering agents (allopurinol, febuxostat, rasburicase) to prevent tumour lysis syndrome
Urgent leukapheresis if signs of leukostasis
DIC in APL
RBC and platelet transfusions
Antifungal prophylaxis in patients with prolonged and profound neutropenia
Proper nursing care — reverse isolation, face mask, hand hygiene, low bacteria diet

Supportive Measure	Details	Rationale
Blood product support	RBC transfusion if symptomatic anaemia; PLT transfusion if PLT ≤ 10 (or ≤ 20 if fever/bleeding); FFP/cryoprecipitate for DIC ^[3]^[15]	Maintain oxygen delivery, prevent bleeding
TLS prevention	Hydration + urate lowering agents (allopurinol or febuxostat) ^[7]^[15]	Rapid cell lysis releases intracellular contents (K⁺, PO₄³⁻, uric acid, nucleic acids) → hyperuricaemia, hyperkalaemia, hyperphosphataemia, hypocalcaemia, AKI
Allopurinol precaution	Check HLA-B5801 before starting allopurinol → risk of SJS/TEN* ^[14]	HLA-B*5801 is more common in Southern Chinese/Southeast Asian populations — particularly relevant in Hong Kong
Rasburicase	For established or high-risk TLS	Recombinant urate oxidase — converts uric acid to allantoin (soluble, easily excreted). Contraindicated in G6PD deficiency (generates H₂O₂ during reaction → oxidative haemolysis) ^[3]^[15]
Infection prevention	Reverse isolation, face mask, hand hygiene, low-bacteria diet, HEPA filter ^[7]^[14]	Neutropenia makes patients susceptible to bacterial and fungal infections
Antimicrobial prophylaxis	Fluoroquinolone (antibacterial), antifungal prophylaxis (fluconazole or posaconazole), acyclovir (HSV prophylaxis), co-trimoxazole (PCP prophylaxis — check G6PD first) ^[15]	Prolonged neutropenia → opportunistic infections
Anti-emetics, analgesia, nutrition	Standard supportive measures ^[15]	Chemotherapy-induced nausea, bone pain, poor appetite

This is the most important treatment-related complication specific to APL therapy.

Feature	Details
What is it?	A clinical syndrome caused by ATRA and/or ATO treatment, historically called "retinoic acid syndrome"
Pathogenesis	Due to production of inflammatory cytokines by large burden of maturing myeloid cells ^[3]. As ATRA/ATO forces promyelocytes to differentiate, the maturing cells release IL-1β, TNF-α, and other cytokines → systemic inflammatory response. Additionally, differentiating cells express adhesion molecules and infiltrate tissues
Clinical features	Dyspnoea, fever, peripheral oedema, hypotension, pleuro-pericardial effusion, acute renal failure, jaundice ^[3] — essentially a capillary leak / SIRS-like picture
Incidence	~10–25% of patients receiving ATRA ± ATO
Timing	Usually within the first 2–3 weeks of ATRA initiation, often coinciding with rising WBC (as differentiating promyelocytes release into the peripheral blood)
Risk factors	High WBC at presentation, rising WBC during treatment
Management	Dexamethasone 10 mg IV Q12h × ≥ 3 days — as prophylaxis in high-risk patients or as treatment once suspected ^[3]. Some protocols give prophylactic dexamethasone to all patients. Do NOT stop ATRA unless life-threatening — continue if tolerable, as interruption risks DIC recurrence

Differentiation Syndrome — Why It Happens

Think of it this way: ATRA is forcing millions of malignant promyelocytes to mature all at once. These newly-maturing cells are loaded with granules and adhesion molecules. As they mature, they (1) release inflammatory cytokines en masse, (2) upregulate adhesion molecules and stick to vascular endothelium causing infiltration, and (3) cause capillary leak. It's essentially an iatrogenic cytokine storm from successful treatment. The treatment is dexamethasone — a potent anti-inflammatory steroid that suppresses the cytokine response.

Drug	Key Precautions / Contraindications
ATRA	Teratogenic — absolutely contraindicated in pregnancy (Category X). Pregnancy test before starting. Headache and pseudotumour cerebri (from ↑ intracranial pressure) can occur — monitor for papilloedema. Hepatotoxicity (monitor LFT). Hypertriglyceridaemia. Differentiation syndrome (see above)
ATO	QTc prolongation — monitor ECG; correct K⁺ and Mg²⁺ before starting; avoid concurrent QTc-prolonging drugs. Hepatotoxicity. Peripheral neuropathy (dose-dependent, usually reversible). Differentiation syndrome. Keep electrolytes (K⁺ > 4.0, Mg²⁺ > 0.8) to reduce arrhythmia risk
Anthracyclines (idarubicin, daunorubicin)	Cardiotoxicity — dose-dependent dilated cardiomyopathy; cumulative dose limit (e.g., daunorubicin ~550 mg/m² lifetime). Requires baseline ECG + echocardiogram before starting ^[7]. Severe myelosuppression. Vesicant — extravasation causes tissue necrosis
Allopurinol	Check HLA-B5801* before starting — risk of SJS/TEN ^[14]. Dose-adjust for renal impairment
Rasburicase	Contraindicated in G6PD deficiency — generates H₂O₂ as a byproduct ^[7]^[15]
Co-trimoxazole (PCP prophylaxis)	Check G6PD — risk of oxidative haemolysis in deficient patients ^[7]

Outcome	ATRA + ATO Regimen	ATRA + Chemo Regimen
Complete remission rate	~100% ^[4]	~90–95%
2-year event-free survival	> 95% ^[4]	~85–90%
Overall cure rate	> 90% ^[3]^[4]	~85%
Early death rate (within 30 days)	~3–5% (mainly DIC/ICH)	~5–10%
Treatment-related mortality	Very low (chemotherapy-free)	Higher (anthracycline toxicity, infections)
Maintenance needed	No	Yes (1–2 years)
Late effects	Minimal	Cardiotoxicity, secondary malignancies

APL is associated with a good prognosis — best prognosis among all AMLs with > 90% cure ^[4]

High Yield Summary — Management of APL

EMERGENCY: Start ATRA immediately on clinical/morphological suspicion — do NOT wait for genetic confirmation ^[3]^[6]
Coagulopathy correction: Platelet transfusion + FFP + cryoprecipitate. Targets: PLT > 30–50, Fibrinogen > 1.5 g/L ^[3]^[14]
Risk stratification by WBC: Low/intermediate (WBC ≤ 10) → ATRA + ATO. High (WBC > 10) → ATRA + ATO + anthracycline ^[3]
Differentiation syndrome: Caused by cytokines from maturing myeloid cells. S/S: dyspnoea, fever, oedema, hypotension, effusions. Rx: Dexamethasone 10 mg IV Q12h ≥ 3 days ^[3]
Consolidation: ATRA + ATO regimen → ATO + ATRA cycles (chemo-free). ATRA + chemo regimen → anthracycline + ATRA cycles
MRD monitoring: RT-qPCR for PML-RARα every 3 months for 2–3 years. Molecular relapse → ATO re-induction ± HSCT
Hong Kong pioneered oral ATO for APL ^[1]
Key drug precautions: ATRA = teratogenic + differentiation syndrome. ATO = QTc prolongation. Anthracycline = cardiotoxicity (need baseline ECG/echo). Allopurinol = check HLA-B5801. Rasburicase = contraindicated in G6PD deficiency* ^[7]^[14]
GC 060 supportive care checklist: RBC/PLT transfusion, antibiotics for neutropenic sepsis, TLS prevention (hydration + allopurinol/febuxostat/rasburicase), antifungal prophylaxis, reverse isolation, face mask, hand hygiene, low-bacteria diet ^[7]
Prognosis: > 90% cure — the most curable form of AML ^[3]^[4]

Active Recall - Management of APL

References

^[1] Senior notes: Block A - High white cell count: acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (p.9, p.20) ^[3] Senior notes: Ryan Ho Haemtology.pdf (p.52, p.56–57, p.59) ^[4] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.738 — APL treatment and prognosis) ^[6] Senior notes: Learning_Points_All_Lectures.txt (Learning Point 2) ^[7] Lecture slides: GC 060. High white cell count.pdf (p.11 — Supportive treatment) ^[14] Senior notes: Block A - High white cell count: acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (p.9, p.20 — DIC in APL, haematological emergencies, HLA-B*5801) ^[15] Senior notes: Ryan Ho Haemtology.pdf (p.52 — Ix at diagnosis, general management)

Complications in APL can be organised into three temporal categories:

Disease-related complications (at presentation / early) — arising from the leukaemia itself
Treatment-related complications — arising from ATRA, ATO, chemotherapy, or supportive care
Late complications — arising after remission/cure

This distinction matters because the management approach is completely different for each. Let's work through them systematically.

1A. Disseminated Intravascular Coagulation (DIC) and Life-Threatening Haemorrhage

This is the single most important complication of APL and the leading cause of early death.

Feature	Details
Incidence	Present in 60–90% of APL patients at diagnosis
Leading cause of death	Intracranial haemorrhage (ICH) — accounts for the majority of early mortality (within first 7–30 days) ^[3]^[4]^[6]
Other bleeding sites	Pulmonary haemorrhage, GI bleeding, mucosal bleeding, oozing from venepuncture/line sites

The DIC in APL is due to the release of procoagulants from the leukemic blasts ^[14] — but the reality is more complex than typical DIC. Three simultaneous mechanisms create a unique and particularly dangerous coagulopathy:

Mechanism	Process	Complication
1. Procoagulant activity	APL promyelocytes express tissue factor (TF / Factor III) both on their surface and within granules. Cell lysis (spontaneous or chemotherapy-induced) releases TF → activates extrinsic pathway → widespread fibrin deposition	Microvascular thrombosis — fibrin strands occlude small vessels → end-organ ischaemia (uncommon clinically but occurs). Consumption of clotting factors → paradoxical bleeding
2. Primary hyperfibrinolysis	APL cells express Annexin II (a plasminogen/tPA receptor) at high levels → enhanced plasmin generation → excessive fibrinolysis beyond what is needed to break down thrombi	Profound hypofibrinogenaemia — fibrinogen levels drop dramatically. Fibrin clots that form are rapidly dissolved → bleeding persists. This is why fibrinogen is the most critically depleted factor in APL-DIC
3. Proteolytic enzyme release	Azurophilic granules of promyelocytes contain elastase and other proteases → directly degrade fibrinogen, von Willebrand factor, and other clotting factors	Further depletes haemostatic proteins independently of the coagulation cascade

Laboratory profile ^[5]:

PT prolonged → prothrombin time reflecting extrinsic pathway, solely by Factor 7, which has the shortest half-life of all factors ^[5]
APTT may be initially preserved → in the acute phase of APL, the acute phase reactant Factor 8 is in high supply, allowing for buffering on the intrinsic pathway ^[5]
If left untreated over a longer period, all clotting factors will eventually be used up, causing a rise in APTT as well ^[5]
Fibrinogen markedly decreased
D-dimer markedly elevated
Platelets decreased — consumed by microthrombi + BM failure

Microangiopathic haemolytic anaemia (MAHA):

These fibrin clots line the blood vessels and will shear RBCs that are passing through → microangiopathic hemolytic anemia (MAHA) ^[5]

On PBS, this manifests as schistocytes (fragmented RBCs) + giant platelets ^[5]. The MAHA is a secondary consequence of DIC — the fibrin strands act like a cheese grater inside blood vessels, slicing RBCs as they squeeze through.

Why APL-DIC Is More Dangerous Than Typical DIC

In most causes of DIC (e.g., sepsis), the primary mechanism is procoagulant activation with secondary fibrinolysis. In APL, there is simultaneous primary hyperfibrinolysis on top of DIC — this means the body's own clot-forming ability is doubly impaired. The result is that bleeding in APL is more severe, more refractory to replacement therapy, and more likely to cause fatal ICH than DIC from other causes.

Although bleeding dominates, thrombosis can also occur ^[3]:

DVT, PE, arterial thrombosis, digital ischaemia
Pathophysiology: the procoagulant arm of DIC (tissue factor-driven thrombin generation) causes microvascular and occasionally macrovascular thrombosis
Less common than bleeding but can be fatal (massive PE, stroke)

Feature	Details
Why it occurs	Neutropenia — functional neutrophil count is low because (1) BM is replaced by malignant promyelocytes → no space for normal granulopoiesis, (2) the promyelocytes themselves are non-functional (arrested at an immature stage, cannot perform phagocytosis properly)
Timing	At presentation (disease-related neutropenia) AND during treatment (chemotherapy-induced marrow aplasia)
Common organisms	Gram-negative bacteria (E. coli, Pseudomonas, Klebsiella), Gram-positive (Staph aureus, coagulase-negative Staph from line infections), fungi (Aspergillus, Candida) in prolonged neutropenia
Clinical importance	Febrile neutropenia is a medical emergency requiring blood cultures and broad-spectrum antibiotics within one hour ^[6]

GC Lecture High Yield — Haematological Emergencies

From GC 060 (High white cell count) ^[7]:

Resuscitation if in septic shock
Immediate empirical broad spectrum antibiotics for neutropenic sepsis
Antifungal prophylaxis in patients with prolonged and profound neutropenia
Proper nursing care — reverse isolation, face mask, hand hygiene, low bacteria diet

ANC below 0.5 × 10⁹/L combined with fever necessitates urgent empirical therapy, as delayed treatment significantly increases mortality in immunocompromised patients ^[6]

Feature	Details
What it is	Microvascular occlusion by large, rigid leukaemic cells → end-organ ischaemia (primarily lungs and CNS)
When it occurs in APL	Rare — because APL typically presents with leukopenia rather than leukocytosis ^[3]. However, the microgranular variant (M3v) can present with high WBC and may develop leucostasis
Symptoms	Headache, vision change, SOB ^[2] — pulmonary infiltrates, hypoxia, altered consciousness
Management	Urgent leukapheresis if signs of leukostasis ^[7]^[14] — mechanically removes WBCs. Always combine with chemotherapy (hydroxyurea or induction) because leukaemia cells regrow rapidly

Feature	Details
What it is	Massive cell death (spontaneous or treatment-induced) releases intracellular contents into the blood → metabolic derangement
Pathophysiology	Cell lysis → release of K⁺ (hyperkalaemia), PO₄³⁻ (hyperphosphataemia → binds Ca²⁺ → hypocalcaemia), nucleic acids (metabolised to uric acid → hyperuricaemia → uric acid crystal deposition in renal tubules → AKI)
Lab findings	Biochemistry: renal function, potassium, calcium, phosphate, urate, LDH (features of tumour lysis syndrome) ^[7]
Risk in APL	Lower than high-WBC AMLs (because APL typically has low WBC). However, once ATRA/ATO initiates differentiation, there can be a wave of cell turnover that releases intracellular contents
Prevention	Hydration and urate lowering agents (allopurinol, febuxostat, rasburicase) to prevent tumour lysis syndrome ^[7]^[14]
Allopurinol precaution	Check HLA-B5801 before starting → risk of SJS/TEN* ^[14] — particularly relevant in Hong Kong (Southern Chinese have ~6–8% prevalence of HLA-B*5801)
Rasburicase precaution	Contraindicated in G6PD deficiency — generates H₂O₂ during conversion of uric acid to allantoin → oxidative haemolysis ^[7]

This is the most important treatment-specific complication of APL therapy.

Feature	Details
Incidence	~10–25% of patients receiving ATRA ± ATO
Timing	Usually within first 2–3 weeks of ATRA/ATO initiation, often coinciding with a rising WBC
Pathogenesis	Due to production of inflammatory cytokines by large burden of maturing myeloid cells ^[3]. As ATRA/ATO forces malignant promyelocytes to differentiate, the maturing cells: (1) release IL-1β, IL-6, IL-8, TNF-α → systemic inflammatory response; (2) upregulate adhesion molecules (ICAM-1, CD11b) → adhere to vascular endothelium → tissue infiltration; (3) cause capillary leak → third-spacing of fluid
Signs and symptoms	Dyspnoea, fever, peripheral oedema, hypotension, pleuro-pericardial effusion, acute renal failure, jaundice ^[3]
Severity	Ranges from mild (low-grade fever, peripheral oedema) to life-threatening (pulmonary infiltrates with respiratory failure, multiorgan dysfunction)
Management	Dexamethasone 10 mg IV Q12h × ≥ 3 days ^[3] — as prophylaxis in high-risk patients OR as treatment once suspected. Dexamethasone is chosen because it is a potent anti-inflammatory glucocorticoid that suppresses the cytokine storm. Do NOT stop ATRA unless life-threatening — stopping ATRA risks DIC recurrence from loss of differentiation pressure
Prevention	Prophylactic dexamethasone (especially if WBC > 10 at diagnosis or rising rapidly). Some protocols add hydroxyurea if WBC rises > 10 during induction

Understanding Differentiation Syndrome from First Principles

Think of it as the paradox of successful treatment. You've just told millions of abnormal promyelocytes to grow up — and they do, all at once. These newly-maturing granulocytes are biologically active: they produce cytokines, express sticky adhesion molecules, and infiltrate tissues just like neutrophils do during an infection. The result is a sterile inflammatory storm — capillary leak, tissue oedema, effusions, organ dysfunction — not because treatment failed, but because it worked too well, too fast. Dexamethasone dampens this inflammatory cascade without affecting the underlying differentiation process.

Feature	Details
When it occurs	During and after induction/consolidation (especially if anthracyclines or cytarabine are used in high-risk protocols)
Pathophysiology	Cytotoxic chemotherapy is non-selective — it kills leukaemic cells AND normal haematopoietic precursors → profound BM hypocellularity → pancytopenia
Duration	Typically 2–4 weeks of nadir before marrow recovery
Consequences	Anaemia (need RBC transfusion); Neutropenia (infection risk — neutropenic fever/sepsis); Thrombocytopenia (bleeding risk)
Management	Blood product support (RBC, platelets), prophylactic antimicrobials, reverse isolation ^[7]^[14]
Relevance to ATRA+ATO regimen	One of the major advantages of the chemotherapy-free ATRA+ATO regimen is that marrow aplasia is significantly less severe — ATRA and ATO are not conventional cytotoxics and cause much less myelosuppression

Feature	Details
When it applies	High-risk APL patients receiving idarubicin/daunorubicin as part of ATRA+chemo or ATRA+ATO+chemo regimens
Pathophysiology	Anthracyclines generate free radicals (via redox cycling) + intercalate DNA + inhibit topoisomerase II → direct myocardial injury. Cardiomyocytes are particularly vulnerable because they have limited antioxidant defences and cannot regenerate
Type 1 (Acute)	Arrhythmias, transient LV dysfunction — usually reversible
Type 2 (Chronic/Delayed)	Dose-dependent dilated cardiomyopathy — irreversible. Risk increases with cumulative dose (daunorubicin > 550 mg/m², idarubicin > 150 mg/m²)
Prevention	Baseline ECG + echocardiogram before anthracycline ^[7]. Monitor LVEF during and after treatment. Limit cumulative dose. Consider dexrazoxane (iron chelator that reduces free radical generation) in high cumulative dose situations

Toxicity	Mechanism	Management
QTc prolongation	ATO blocks the hERG potassium channel (IKr) → delayed cardiac repolarisation → prolonged QTc → risk of torsades de pointes (polymorphic VT)	Monitor ECG weekly during ATO therapy. Maintain K⁺ > 4.0 mmol/L, Mg²⁺ > 0.8 mmol/L. Avoid concomitant QTc-prolonging drugs (macrolides, fluoroquinolones, anti-emetics like ondansetron in high doses). Stop ATO if QTc > 500 ms
Hepatotoxicity	Direct hepatocellular injury from arsenic metabolites	Monitor LFT regularly. Hold ATO if transaminases > 5× ULN
Peripheral neuropathy	Arsenic damages peripheral nerve axons (dose-dependent, usually reversible)	Monitor for paraesthesias. Usually resolves after ATO discontinuation
Hyperglycaemia	Arsenic impairs insulin secretion/sensitivity	Monitor blood glucose

Toxicity	Mechanism	Management
Pseudotumour cerebri (benign intracranial hypertension)	Retinoids increase CSF production and/or decrease CSF absorption → raised intracranial pressure	Headache, papilloedema, visual disturbance. Treat with acetazolamide. Consider ATRA dose reduction
Hypertriglyceridaemia	Retinoids upregulate hepatic VLDL synthesis → elevated triglycerides	Monitor lipids. Fibrate therapy if severe
Hepatotoxicity	Direct hepatotoxic effect of retinoic acid metabolites	Monitor LFT
Teratogenicity	ATRA is a vitamin A derivative — retinoids are potent teratogens causing craniofacial, cardiac, and CNS malformations	Absolutely contraindicated in pregnancy (Category X). Pregnancy test before starting. Effective contraception during treatment
Skin and mucosal dryness	Retinoid effect on epithelial cell differentiation → desquamation	Emollients, lip balm. Usually tolerable

Feature	Details
Relevance	Primarily a concern for patients who received anthracycline-based chemotherapy. Less relevant for chemotherapy-free ATRA+ATO regimens
Types	Therapy-related MDS/AML (t-MDS/t-AML) — latency 3–7 years after alkylating agents, 1–5 years after topoisomerase II inhibitors. Solid tumours (less common)
Pathophysiology	Cytotoxic drugs cause DNA damage in normal stem cells → secondary mutations → clonal expansion → secondary malignancy
Practical relevance in APL	With the shift towards chemotherapy-free ATRA+ATO, this complication is becoming less relevant — another reason why the ATRA+ATO regimen is preferred

3. Late Complications (Post-Remission)

Feature	Details
Incidence	< 5–10% for low/intermediate risk with ATRA+ATO; ~15–25% for high-risk or chemo-based regimens
Types	Molecular relapse (PML-RARα transcript turns positive on RT-qPCR, no clinical/haematological evidence yet) vs Haematological relapse (recurrence of leukaemia with abnormal promyelocytes, cytopenia, DIC)
Timing	Most relapses occur within the first 2–3 years after completing consolidation
Detection	MRD monitoring by RT-qPCR for PML-RARα every 3 months for 2–3 years. Molecular relapse precedes haematological relapse by ~2–3 months — early detection allows pre-emptive treatment
Risk factors for relapse	High-risk disease (WBC > 10), FLT3-ITD mutation, failure to achieve molecular remission after consolidation, M3v subtype
Management	See Management section (ATO re-induction → HSCT if suitable)
CNS relapse	Rare in APL (unlike ALL). However, it has been reported, especially in patients with high WBC at presentation. Not routinely given CNS prophylaxis

Feature	Details
Fatigue	Persistent fatigue is common even after cure — multifactorial (anaemia recovery, deconditioning, psychological)
Psychological impact	Anxiety, depression, PTSD-like symptoms from the acute life-threatening presentation. Fear of relapse
Fertility	ATRA+ATO regimen: minimal impact on fertility (major advantage). Anthracycline-based regimens: moderate risk of gonadal toxicity. Consider fertility preservation (sperm banking, oocyte cryopreservation) before treatment, especially in young patients
Long-term monitoring	Cardiac follow-up if anthracyclines were used. Endocrine function. Secondary malignancy screening

Complication	Timing	Mechanism	Key Management
DIC / ICH	Presentation + early treatment	Tissue factor, Annexin II, elastase → triple coagulopathy	ATRA (reduce procoagulant load), PLT/FFP/cryo transfusion
Neutropenic sepsis	Presentation + post-chemo nadir	BM failure → neutropenia	Empirical broad-spectrum antibiotics within 1 hour
TLS	Early treatment	Cell lysis → K⁺↑, PO₄³⁻↑, Ca²⁺↓, urate↑	Hydration, allopurinol/febuxostat/rasburicase
Leucostasis	Presentation (rare in APL)	High WBC → microvascular occlusion	Leukapheresis + chemotherapy
Differentiation syndrome	1–3 weeks into ATRA/ATO	Cytokine storm from maturing cells	Dexamethasone 10 mg IV Q12h
Anthracycline cardiotoxicity	Acute or delayed	Free radical damage to cardiomyocytes	Baseline + serial echo; cumulative dose limit
ATO QTc prolongation	During ATO therapy	hERG K⁺ channel blockade	ECG monitoring; K⁺ > 4, Mg²⁺ > 0.8
ATRA pseudotumour cerebri	During ATRA therapy	↑ CSF production/↓ absorption	Acetazolamide; ATRA dose reduction
ATRA teratogenicity	During ATRA therapy	Retinoid embryopathy	Absolutely contraindicated in pregnancy
Relapse	Months–years post-treatment	Residual leukaemic clone	MRD monitoring → ATO re-induction → HSCT
Secondary MDS/AML	Years post-treatment	DNA damage from chemotherapy	Screening; less relevant with chemo-free regimens

High Yield Summary — Complications of APL

DIC is the #1 killer in APL — caused by the triple coagulopathy of tissue factor release + Annexin II-mediated hyperfibrinolysis + granule protease activity. ICH is the leading cause of early death ^[3]^[4]^[5]^[6]
Excessive activation of coagulation cascade leads to three processes: too much fibrin → vessel blockage; excessive consumption of clotting factors → bleeding; excess fibrin traps platelets → thrombocytopenia. Fibrin clots shear RBCs → MAHA ^[5]
PT elevated early (Factor VII consumed first — shortest half-life), APTT initially preserved (Factor VIII is acute phase reactant), fibrinogen markedly low, D-dimer markedly high ^[5]
Differentiation syndrome: cytokine storm from maturing myeloid cells → dyspnoea, fever, oedema, hypotension, effusions, AKI, jaundice → treat with dexamethasone 10 mg IV Q12h ≥ 3 days ^[3]
Febrile neutropenia = medical emergency — blood cultures + broad-spectrum antibiotics within 1 hour ^[6]^[7]
TLS prevention: hydration + allopurinol (check HLA-B5801) / febuxostat / rasburicase (C/I in G6PD deficiency)* ^[7]^[14]
ATO causes QTc prolongation — monitor ECG, keep K⁺ > 4, Mg²⁺ > 0.8. ATRA causes pseudotumour cerebri and is teratogenic (Category X)
Anthracycline cardiotoxicity — cumulative dose-dependent DCM; baseline ECG + echo before anthracycline ^[7]
Relapse is uncommon (< 10% with ATRA+ATO) but monitored by RT-qPCR for PML-RARα every 3 months for 2–3 years
The ATRA+ATO chemo-free regimen avoids many complications: less marrow aplasia, less cardiotoxicity, less secondary malignancy, no maintenance needed

Active Recall - Complications of APL

^[2] Senior notes: Maksim Medicine Notes.pdf (Haematology section, p.173) ^[3] Senior notes: Ryan Ho Haemtology.pdf (p.53, p.59) ^[4] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.738) ^[5] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (p.19) ^[6] Senior notes: Learning_Points_All_Lectures.txt (Learning Points 2, 3) ^[7] Lecture slides: GC 060. High white cell count.pdf (p.6 — workup; p.11 — supportive treatment) ^[14] Senior notes: Block A - High white cell count: acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (p.9, p.20)

High Yield Summary

Acute Promyelocytic Leukaemia (APL) — Pre-Diagnostic Summary:

Definition: AML subtype defined by t(15;17)(q24.1;q21.2) / PML-RARα fusion, arrested at promyelocyte stage
Epidemiology: 5–20% of AML; younger median age (~40y); relatively more common in Hispanic and Southern Chinese populations including Hong Kong
Pathophysiology: PML-RARα acts as a "super-repressor" of differentiation → maturation arrest + impaired apoptosis. Tissue factor expression + Annexin II-mediated hyperfibrinolysis + granule protease release → unique "triple coagulopathy" causing DIC
Risk stratification: Based on WBC and platelet count (Sanz score) — WBC > 10 = high risk
Clinical hallmarks:
- Young patient + pancytopenia + bleeding out of proportion (DIC) = APL until proven otherwise
- Faggot cells (bundles of Auer rods) on blood film
- HLA-DR negative, CD34 negative immunophenotype
- Less extramedullary disease than other AMLs
Most important teaching point: APL is a haematological EMERGENCY — untreated median survival < 1 month. Start ATRA immediately on clinical/morphological suspicion — do NOT wait for genetic confirmation ^[3]^[5]^[6]
Clotting profile: PT ↑, APTT may be initially preserved (Factor VIII buffers), Fibrinogen ↓↓, D-dimer ↑↑, Platelets ↓ ^[5]
The paradox: Most dangerous AML at presentation → most curable AML with treatment (> 90% cure with ATRA + ATO) ^[4]

High Yield Summary — Differential Diagnosis of APL

Primary mimics of APL: Other AML subtypes (especially AML-M5 and M3v pitfall), ALL, MDS, CML in blast crisis, aplastic anaemia, megaloblastic anaemia
Key distinguishing feature of APL: Faggot cells + HLA-DR neg/CD34 neg + t(15;17)/PML-RARα + DIC
DIC differential: Always consider sepsis, obstetric causes, solid tumours, trauma — but in a young patient with unexplained DIC + pancytopenia, APL must be at the top of the list
M3v trap: Microgranular variant looks like AML-M5 or even ALL morphologically — only immunophenotype and molecular testing will save you
Action principle: When clinical + morphological criteria suggest APL → start ATRA immediately → confirm with FISH/RT-PCR later ^[3]^[6]
The 5-step MCICM diagnostic approach (Morphology, Cytochemistry, Immunophenotyping, Cytogenetics, Molecular genetics) resolves virtually all the differentials listed above ^[8]^[9]

High Yield Summary — Diagnosis of APL

Diagnostic criteria: t(15;17)/PML-RARα fusion is the gold standard. The 20% blast threshold does NOT apply when this translocation is present ^[3]^[10]^[14]
The MCICM 5-step approach resolves the diagnosis systematically: Morphology → Cytochemistry → Immunophenotyping → Cytogenetics → Molecular genetics ^[8]^[9]
Morphological hallmarks: Faggot cells (bundles of Auer rods), hypergranular promyelocytes, MPO/SBB strongly positive ^[5]
Immunophenotypic hallmark: HLA-DR negative + CD34 negative (distinguishes from virtually all other AMLs) ^[3]
Coagulation hallmark: DIC with disproportionately low fibrinogen — bleeding out of proportion to platelet count ^[5]^[14]
Action principle: START ATRA on morphological/clinical suspicion → confirm with FISH/RT-PCR → do NOT wait ^[3]^[6]
Pre-treatment workup (GC 060): CBP, clotting profile, biochemistry (TLS screen), BM + cytogenetics + molecular, CXR, ECG/echo, hepatitis/HIV, G6PD, HLA typing, CVC ^[7]
MRD monitoring: RQ-PCR for PML-RARα transcript is used post-treatment to confirm molecular remission and detect early relapse

High Yield Summary — Management of APL

EMERGENCY: Start ATRA immediately on clinical/morphological suspicion — do NOT wait for genetic confirmation ^[3]^[6]
Coagulopathy correction: Platelet transfusion + FFP + cryoprecipitate. Targets: PLT > 30–50, Fibrinogen > 1.5 g/L ^[3]^[14]
Risk stratification by WBC: Low/intermediate (WBC ≤ 10) → ATRA + ATO. High (WBC > 10) → ATRA + ATO + anthracycline ^[3]
Differentiation syndrome: Caused by cytokines from maturing myeloid cells. S/S: dyspnoea, fever, oedema, hypotension, effusions. Rx: Dexamethasone 10 mg IV Q12h ≥ 3 days ^[3]
Consolidation: ATRA + ATO regimen → ATO + ATRA cycles (chemo-free). ATRA + chemo regimen → anthracycline + ATRA cycles
MRD monitoring: RT-qPCR for PML-RARα every 3 months for 2–3 years. Molecular relapse → ATO re-induction ± HSCT
Hong Kong pioneered oral ATO for APL ^[1]
Key drug precautions: ATRA = teratogenic + differentiation syndrome. ATO = QTc prolongation. Anthracycline = cardiotoxicity (need baseline ECG/echo). Allopurinol = check HLA-B5801. Rasburicase = contraindicated in G6PD deficiency* ^[7]^[14]
GC 060 supportive care checklist: RBC/PLT transfusion, antibiotics for neutropenic sepsis, TLS prevention (hydration + allopurinol/febuxostat/rasburicase), antifungal prophylaxis, reverse isolation, face mask, hand hygiene, low-bacteria diet ^[7]
Prognosis: > 90% cure — the most curable form of AML ^[3]^[4]

High Yield Summary — Complications of APL

DIC is the #1 killer in APL — caused by the triple coagulopathy of tissue factor release + Annexin II-mediated hyperfibrinolysis + granule protease activity. ICH is the leading cause of early death ^[3]^[4]^[5]^[6]
Excessive activation of coagulation cascade leads to three processes: too much fibrin → vessel blockage; excessive consumption of clotting factors → bleeding; excess fibrin traps platelets → thrombocytopenia. Fibrin clots shear RBCs → MAHA ^[5]
PT elevated early (Factor VII consumed first — shortest half-life), APTT initially preserved (Factor VIII is acute phase reactant), fibrinogen markedly low, D-dimer markedly high ^[5]
Differentiation syndrome: cytokine storm from maturing myeloid cells → dyspnoea, fever, oedema, hypotension, effusions, AKI, jaundice → treat with dexamethasone 10 mg IV Q12h ≥ 3 days ^[3]
Febrile neutropenia = medical emergency — blood cultures + broad-spectrum antibiotics within 1 hour ^[6]^[7]
TLS prevention: hydration + allopurinol (check HLA-B5801) / febuxostat / rasburicase (C/I in G6PD deficiency)* ^[7]^[14]
ATO causes QTc prolongation — monitor ECG, keep K⁺ > 4, Mg²⁺ > 0.8. ATRA causes pseudotumour cerebri and is teratogenic (Category X)
Anthracycline cardiotoxicity — cumulative dose-dependent DCM; baseline ECG + echo before anthracycline ^[7]
Relapse is uncommon (< 10% with ATRA+ATO) but monitored by RT-qPCR for PML-RARα every 3 months for 2–3 years
The ATRA+ATO chemo-free regimen avoids many complications: less marrow aplasia, less cardiotoxicity, less secondary malignancy, no maintenance needed

Acute Promyelocytic Leukaemia

References

Definition

Epidemiology

Risk Factors

Anatomy and Function: Normal Haematopoiesis Context

The Promyelocyte

Etiology and Pathophysiology

The Core Genetic Lesion: t(15;17)(q24.1;q21.2)

How PML-RARα Causes Leukaemia (Molecular Pathophysiology)

1. Normal RARα Function

2. What PML-RARα Does Differently

3. Why Pharmacological ATRA Works

4. Why Arsenic Trioxide (ATO) Works

Pathophysiology of DIC in APL

Variant Translocations (< 5% of Cases)

Classification

FAB Classification (Historical)

WHO 2022 / ICC 2022 Classification

Risk Stratification (Sanz/PETHEMA Risk Score)

Clinical Features

A. Symptoms (What the Patient Tells You)

B. Signs (What You Find on Examination)

C. Morphological Features on Blood Film / Bone Marrow

Key Pathophysiological Concepts Summary

Active Recall - APL: Definition, Epidemiology, Pathophysiology & Clinical Features

References

Differential Diagnosis of Acute Promyelocytic Leukaemia (APL)

Why Differential Diagnosis Matters in APL

Systematic Framework for the Differential

1. Other Subtypes of AML (Non-APL)

2. Acute Lymphoblastic Leukaemia (ALL)

3. Myelodysplastic Syndrome (MDS)

4. CML in Blast Crisis

5. Disseminated Intravascular Coagulation (DIC) from Other Causes

6. Aplastic Anaemia

7. Vitamin B12 / Folate Deficiency (Megaloblastic Anaemia)

8. Microgranular Variant APL (M3v) — A Diagnostic Pitfall Within APL

Differential Diagnosis Summary: Mermaid Diagram

Key Distinguishing Features: Quick Reference Table

Differential Diagnosis of DIC in APL Context

Active Recall - Differential Diagnosis of APL

References

Diagnostic Criteria for APL

The Central Diagnostic Principle

Formal Diagnostic Criteria (WHO 2022 / ICC 2022)

Presumptive Diagnostic Criteria (When to Start ATRA Empirically)

The 5-Step MCICM Diagnostic Approach

Step 1: Morphology (PBS + Bone Marrow)

A. Complete Blood Count (CBC) with Differential

B. Peripheral Blood Smear (PBS)

C. Bone Marrow Aspirate and Trephine Biopsy

Step 2: Cytochemistry

Step 3: Immunophenotyping (Flow Cytometry)

Step 4: Cytogenetics (Karyotyping + FISH)

Step 5: Molecular Genetics (RT-PCR + Sequencing)

Coagulation Workup (Critical in APL)

Additional Investigations (Pre-treatment Workup)

Diagnostic Algorithm

Key Diagnostic Interpretations: Putting It All Together

Pattern Recognition Table

Active Recall - Diagnostic Criteria, Algorithm & Investigations in APL

References

Management of Acute Promyelocytic Leukaemia (APL)

Management Philosophy: Why APL Is Treated Differently from Other AMLs

Management Overview: Phases of Treatment

Phase 0: Immediate Emergency Management (First Minutes to Hours)

A. Start ATRA Immediately

B. Aggressive Coagulopathy Correction

Phase 1: Induction Therapy — Risk-Stratified Approach

Low/Intermediate Risk (WBC ≤ 10 × 10⁹/L)

High Risk (WBC > 10 × 10⁹/L)

Differentiating Agents: Mechanism Summary

Phase 2: Consolidation Therapy

Phase 3: Maintenance Therapy

Phase 4: MRD Monitoring

Phase 5: Relapse Management

Supportive Care (Running Concurrently Throughout)

Differentiation Syndrome: Recognition and Management

Specific Drug Precautions and Contraindications