HaematologyAnaemiaNormocytic AnaemiaHaemolytic AnaemiaPredominantly Extravascular
Pyruvate Kinase Deficiency
Pyruvate kinase deficiency is an autosomal recessive enzymopathy of the glycolytic pathway in red blood cells, leading to chronic nonspherocytic hemolytic anemia due to impaired ATP generation.
Pyruvate Kinase Deficiency
Pyruvate kinase (PK) deficiency is an autosomal recessive inherited red cell enzymopathy caused by deficient or dysfunctional pyruvate kinase enzyme in erythrocytes, leading to impaired glycolysis, inadequate ATP production, and resultant chronic, non-spherocytic, Coombs-negative haemolytic anaemia [1][2][3].
Let's break down the name:
- Pyruvate = the end-product of glycolysis (from Greek pyr = fire, referring to its discovery in dry distillation)
- Kinase = an enzyme that transfers a phosphate group (from Greek kinein = to move)
- So "pyruvate kinase" literally is the enzyme that moves a phosphate group in the final step of glycolysis, converting phosphoenolpyruvate (PEP) → pyruvate + ATP
PK deficiency is the 2nd most common red cell enzymopathy (after G6PD deficiency) and the most common cause of hereditary non-spherocytic haemolytic anaemia (HNSHA) due to a glycolytic enzyme defect. [1][2][3]
2. Epidemiology
- Rare disease: estimated prevalence ~1 in 20,000 in the general Caucasian population, though likely underdiagnosed due to mild/compensated phenotypes
- Carrier frequency: ~1 in 100 in European populations (heterozygote carriers are clinically asymptomatic)
- More common in Northern European and Chinese ancestry [1][2]
- Worldwide, >300 pathogenic variants in the PKLR gene have been described
- While rarer than G6PD deficiency (which affects 4.5% males in HK), PK deficiency does occur in Chinese populations and should be considered in the differential of a Coombs-negative chronic haemolytic anaemia in a Hong Kong patient
- The most common PKLR mutation in East Asian populations is c.1529G>A (p.Arg510Gln), distinct from the common European variant (c.1456C>T, p.Arg486Trp)
- Similar to G6PD deficiency and thalassaemia, there is epidemiological evidence suggesting that PK-deficient heterozygotes may have a selective advantage against Plasmodium falciparum malaria, helping maintain the allele in malaria-endemic populations [4]
- Autosomal recessive (AR) — both parents must be carriers (heterozygotes) for a child to be affected [1][2][3]
- This distinguishes it from G6PD deficiency which is X-linked recessive
- Heterozygous carriers have ~50% PK activity — enough to prevent clinical haemolysis, though occasionally mild laboratory abnormalities can be detected
Key Distinction from G6PD Deficiency
G6PD deficiency = X-linked recessive (more common in males, female carriers have variable expression due to lyonization). PK deficiency = autosomal recessive (affects males and females equally). Both are red cell enzymopathies but affect different metabolic pathways.
3. Anatomy & Function: Red Cell Metabolism
To understand PK deficiency from first principles, you need to understand why RBCs need ATP and how they make it.
- Mature RBCs have no nucleus, no mitochondria, no endoplasmic reticulum
- Therefore, they cannot perform oxidative phosphorylation (the mitochondrial pathway that generates ~36 ATP per glucose)
- They rely entirely on anaerobic glycolysis (Embden-Meyerhof pathway) for ATP production — yielding only 2 ATP per glucose molecule
The Two Key Metabolic Pathways in RBCs
- Produces ATP to maintain internal environment and cell volume (by Na⁺/K⁺/ATPase) [1][2]
- Source: anaerobic breakdown of glucose via Embden-Meyerhof (glycolysis) pathway [1][2]
- Defect: most commonly pyruvate kinase deficiency → haemolysis [1][2]
- The pathway: Glucose → Glucose-6-phosphate → ... → Phosphoenolpyruvate (PEP) → Pyruvate + ATP (this final step is catalyzed by pyruvate kinase)
- ATP is essential for:
- Na⁺/K⁺-ATPase pump: maintains ionic gradient across the RBC membrane → controls cell volume and prevents osmotic lysis
- Ca²⁺-ATPase pump: keeps intracellular Ca²⁺ low → prevents membrane rigidity and echinocyte formation
- Membrane lipid repair and cytoskeletal maintenance
- Priming steps of glycolysis itself (hexokinase and phosphofructokinase steps require ATP)
- A side-pathway off glycolysis that converts 1,3-bisphosphoglycerate → 2,3-bisphosphoglycerate (2,3-BPG)
- 2,3-BPG binds deoxyhemoglobin and shifts the oxygen-dissociation curve to the RIGHT → promotes O₂ release to tissues
- This shunt is critically important in PK deficiency — see pathophysiology below
The glycolysis pathway is the SOLE source of ATP in mature RBCs. Pyruvate kinase catalyzes the final ATP-generating step. Without PK, RBCs cannot make sufficient ATP. [1][2]
4. Etiology
-
Gene: PKLR (Pyruvate Kinase, Liver and Red cell type) — located on chromosome 1q22
- PKLR encodes two tissue-specific isoforms via alternative splicing:
- PK-R: expressed in red blood cells (and to a lesser extent in other haematopoietic cells)
- PK-L: expressed in liver and kidney
- In PK deficiency, pathogenic variants affect the PK-R isoform — this is why haemolysis occurs but liver function is preserved (the liver uses PK-L and also has mitochondria for oxidative phosphorylation)
- PKLR encodes two tissue-specific isoforms via alternative splicing:
-
Mutation types: >300 variants described; most are missense mutations causing structural instability or reduced catalytic activity
- Common European variant: c.1456C>T (p.Arg486Trp) — "PK Amish" variant
- Common East Asian variant: c.1529G>A (p.Arg510Gln)
- Other types: nonsense mutations, splice-site variants, deletions (generally cause more severe phenotypes)
- Compound heterozygotes (two different mutant alleles) are common and explain the wide clinical spectrum
-
Autosomal recessive inheritance [1][2][3]:
- Homozygotes or compound heterozygotes = clinically affected
- Heterozygote carriers = ~50% enzyme activity, clinically well
- Parents are obligate carriers
- Mature RBCs lack mitochondria → 100% dependent on glycolysis for ATP
- Other cells (including reticulocytes!) have mitochondria and can use oxidative phosphorylation, so they are NOT affected
- The liver expresses a different isoform (PK-L) from the same gene → liver metabolism is preserved
High Yield: Gene and Isoform
The PKLR gene encodes both PK-R (red cells) and PK-L (liver) via alternative splicing. Mutations in PK deficiency specifically impair the PK-R isoform. Mature RBCs are uniquely vulnerable because they lack mitochondria and depend entirely on glycolytic ATP. The liver is spared because it expresses PK-L and has mitochondria.
5. Pathophysiology
This is the core of understanding PK deficiency. Let's build it step by step.
- PK catalyzes the final ATP-generating step of glycolysis: PEP → Pyruvate + ATP
- Deficient/dysfunctional PK → reduced ATP production in RBCs
- Consequences of ATP depletion:
- Na⁺/K⁺-ATPase dysfunction → Na⁺ and water influx into the cell → cell swelling and eventual lysis
- Ca²⁺-ATPase dysfunction → intracellular Ca²⁺ accumulation → membrane rigidity, loss of deformability → RBCs become stiff and are trapped/destroyed in the splenic cords (extravascular haemolysis)
- Loss of membrane integrity → progressive membrane loss → formation of echinocytes (spiculated/crenated cells) and acanthocytes rather than spherocytes (hence "non-spherocytic" haemolytic anaemia)
- Because the PK step is blocked, there is accumulation of upstream glycolytic intermediates, including 1,3-bisphosphoglycerate
- 1,3-BPG is shunted into the Rapoport-Luebering pathway → increased 2,3-BPG production
- 2,3-BPG shifts the oxygen-dissociation curve to the RIGHT → enhanced O₂ delivery to tissues [1][2]
- This is clinically significant: patients with PK deficiency tolerate their anaemia better than expected for their haemoglobin level
Patho: haemolysis due to PK deficiency, but usually better tolerated due to ↑O₂ delivery from ↑2,3-BPG [1][2]
Why PK-Deficient Patients Tolerate Anaemia Well
The metabolic block at the pyruvate kinase step causes accumulation of glycolytic intermediates proximal to the block. This feeds into the Rapoport-Luebering shunt, producing excess 2,3-BPG. Elevated 2,3-BPG shifts the O₂-dissociation curve rightward, meaning haemoglobin releases O₂ more readily to tissues. A patient with PK deficiency and Hb 7 g/dL may feel as well as someone with Hb 9-10 g/dL from another cause — this is a classic exam point!
- Predominantly extravascular haemolysis in the spleen (and to a lesser extent the liver)
- ATP-depleted, rigid RBCs are trapped in the splenic sinusoids and destroyed by splenic macrophages
- Reticulocytes (young RBCs) still have residual mitochondria → they can generate ATP via oxidative phosphorylation → they survive better in the circulation
- After splenectomy, reticulocyte counts often rise paradoxically (sometimes to 40-70%) because the spleen was the major site destroying even the reticulocytes
- This reticulocytosis post-splenectomy can cause a misleading MCV elevation (reticulocytes are larger than mature RBCs)
- In more severe PK deficiency, there is also a component of ineffective erythropoiesis — destruction of developing red cell precursors within the bone marrow before they even enter the circulation
- This contributes to iron overload even without transfusion (due to suppressed hepcidin from ineffective erythropoiesis → increased intestinal iron absorption) [1][2]
- The mechanism: ineffective erythropoiesis → high erythroferrone → suppresses hepcidin → increased ferroportin activity → increased iron absorption from the gut
Iron overload occurs through two mechanisms [1][2]:
| Mechanism | Pathophysiology |
|---|---|
| Transfusional iron overload | Each unit of packed RBCs contains ~200-250 mg iron; with no physiological excretion mechanism, repeated transfusions cause progressive iron accumulation |
| Ineffective erythropoiesis | Even non-transfused patients accumulate iron because ineffective erythropoiesis → ↑erythroferrone → ↓hepcidin → ↑intestinal iron absorption |
"Haemolysis due to PK deficiency — unknown mechanism" is noted in the lecture material [1][2]. While the precise molecular cascade leading to red cell destruction is not fully elucidated, the prevailing understanding centres on ATP depletion → cation leak → membrane rigidity → splenic trapping as described above.
6. Classification
PK deficiency has a wide clinical spectrum depending on the nature and combination of PKLR mutations:
| Severity | Hb Range | Transfusion Need | Typical Genotype |
|---|---|---|---|
| Mild | 9-12 g/dL | Rarely/never needed | Often compound heterozygote with one mild variant |
| Moderate | 7-9 g/dL | Intermittent (infections, aplastic crises) | Variable |
| Severe | < 7 g/dL | Regular transfusions required | Homozygous severe/null mutations |
| Very severe / Hydrops fetalis | Incompatible with life (rare) | In utero transfusions | Homozygous null mutations |
- Missense mutations: most common; produce an enzyme with reduced activity or stability — variable severity
- Null/nonsense mutations: no functional enzyme produced — generally more severe
- Splice-site mutations: variable effect on protein production
- Compound heterozygotes: two different mutations — phenotype depends on the "less severe" of the two alleles (the allele producing some residual activity determines the clinical floor)
If inherited, think in terms of the components of the RBC, and how they can go wrong → either the membrane, the metabolism, or the haemoglobin [3]:
| Component | Defect | Example |
|---|---|---|
| Membrane | Structural protein deficiency | Hereditary spherocytosis, hereditary elliptocytosis |
| Metabolism (Enzymopathy) | Glycolytic enzyme defect | Pyruvate kinase deficiency |
| Pentose phosphate pathway defect | G6PD deficiency | |
| Haemoglobin | Quantitative (reduced synthesis) | Thalassaemia |
| Qualitative (abnormal structure) | Sickle cell disease, Hb Köln |
7. Clinical Features
| Symptom | Pathophysiological Basis |
|---|---|
| Fatigue / exercise intolerance / dyspnoea on exertion | Anaemia → reduced oxygen-carrying capacity; however, often milder than expected due to ↑2,3-BPG shifting the curve right |
| Jaundice (intermittent or chronic) | Extravascular haemolysis → increased unconjugated bilirubin production exceeding hepatic conjugation capacity → clinical icterus (typically mild/fluctuating) |
| Dark urine | Increased urobilinogen excretion (from bilirubin metabolism in the gut, reabsorbed and excreted by kidneys); NOT haemoglobinuria (which occurs in intravascular haemolysis) |
| Pallor | Anaemia (low haemoglobin) |
| Right upper quadrant / biliary pain | Chronic bilirubin overproduction → pigmented gallstones (calcium bilirubinate) → can present with biliary colic or cholecystitis |
| Neonatal jaundice (NNJ) [1][2] | Neonatal presentation is common; haemolysis in the newborn produces unconjugated hyperbilirubinaemia that may require phototherapy or exchange transfusion; the neonatal liver has immature conjugation capacity, worsening the picture |
| Symptoms of aplastic crisis | Parvovirus B19 infection → infects erythroid precursors → temporary cessation of erythropoiesis → rapid drop in Hb in a patient already dependent on high erythroid turnover; presents with sudden worsening pallor, fatigue, tachycardia |
| Symptoms of iron overload (late) | Fatigue, joint pains, erectile dysfunction, glucose intolerance — from iron deposition in liver, heart, pancreas, joints, pituitary |
| Growth retardation / delayed puberty (in severe paediatric cases) | Chronic anaemia + iron overload affecting the pituitary/hypothalamic axis |
| Sign | Pathophysiological Basis |
|---|---|
| Pallor (conjunctival, palmar) | Low haemoglobin |
| Jaundice / Icterus (scleral, skin) | Elevated unconjugated bilirubin from haemolysis |
| Variable splenomegaly [1][2] | The spleen is the primary site of extravascular haemolysis → work hypertrophy of splenic macrophages + trapping of abnormal RBCs → progressive splenic enlargement |
| Hepatomegaly | Extramedullary haematopoiesis (the liver attempts to compensate for anaemia by producing RBCs outside the marrow); also contributes to iron deposition |
| Gallstones (pigmented) [1][2] | Chronic unconjugated hyperbilirubinaemia → excess bilirubin excreted into bile → precipitation as calcium bilirubinate stones; may be detected as early as childhood |
| Leg ulcers (rare, in severe cases) | Chronic haemolysis + tissue hypoxia + microvascular disease → impaired healing in dependent areas |
| Skeletal changes / frontal bossing (severe cases, rare in PK deficiency) | Marrow expansion from chronic erythroid hyperplasia — more characteristic of severe thalassaemia but can occur in severe PK deficiency |
| Signs of iron overload: bronze skin, hepatomegaly, cardiomegaly | Iron deposition in skin (melanin + haemosiderin), liver (haemosiderosis/cirrhosis), heart (dilated cardiomyopathy) |
| Post-splenectomy findings: Howell-Jolly bodies, target cells, elevated platelet count | Absence of splenic filtration function → Howell-Jolly bodies (nuclear remnants) persist; loss of splenic platelet pooling → thrombocytosis |
| Finding | Explanation |
|---|---|
| Echinocytes (burr cells / crenated cells) | ATP depletion → loss of normal membrane lipid asymmetry and cytoskeletal integrity → spiculated projections; this is the characteristic morphology (NOT spherocytes, hence "non-spherocytic" haemolytic anaemia) |
| Acanthocytes | Irregular spiculated cells, also from membrane damage |
| Reticulocytosis | Compensatory marrow response to haemolysis; reticulocyte count typically 5-15% (higher post-splenectomy) |
| Polychromasia | Reticulocytes stain bluish with Wright stain due to residual RNA |
| Nucleated red blood cells (NRBCs) | Seen post-splenectomy (spleen normally removes NRBCs from circulation) |
| Absence of spherocytes | Distinguishes from hereditary spherocytosis and autoimmune haemolytic anaemia |
Chronic, Coombs'-negative haemolytic anaemia: usually from birth [1][2] Haemolysis Cx: NNJ, gallstones, jaundice, variable splenomegaly, folate deficiency [1][2] Iron overload due to ineffective erythropoiesis or transfusional iron overload [1][2]
Classic Exam Presentation
A neonate or infant presents with persistent jaundice and anaemia. Direct Coombs test is negative. Blood film shows echinocytes (not spherocytes). Family history may reveal consanguinity or affected siblings. Reticulocyte count is elevated. Think PK deficiency!
Why Coombs-Negative?
The direct Coombs (direct antiglobulin) test detects antibodies or complement bound to the RBC surface. PK deficiency is an intrinsic, non-immune red cell defect — there are no antibodies on the RBC surface. Therefore, the DAT is negative. A positive Coombs test would point you towards autoimmune haemolytic anaemia instead.
8. Special Clinical Considerations
- PK deficiency commonly presents with neonatal jaundice (NNJ) [1][2] that may be severe enough to require exchange transfusion
- The combination of haemolysis + immature hepatic conjugation in neonates creates a "perfect storm" for severe unconjugated hyperbilirubinaemia → risk of kernicterus if untreated
- Must be differentiated from other causes of neonatal haemolytic jaundice: ABO/Rh incompatibility, G6PD deficiency, hereditary spherocytosis
- Parvovirus B19 has tropism for erythroid precursors (binds P antigen/globoside on erythroid progenitors)
- In healthy individuals, the transient reticulocytopenia is clinically insignificant
- In PK deficiency (or any chronic haemolytic anaemia), the already shortened RBC lifespan means the patient is dependent on high marrow output → even a brief halt in erythropoiesis causes a precipitous drop in Hb (aplastic crisis)
- Presents with sudden worsening of anaemia + absent reticulocytes (reticulocyte count drops to near zero)
- Increased haemolytic stress during pregnancy → may require transfusion support
- Folic acid supplementation is essential (increased demand from both haemolysis and pregnancy)
- Folate deficiency develops because chronic haemolysis drives compensatory erythroid hyperplasia → increased folate consumption for DNA synthesis in rapidly dividing erythroid precursors [1][2]
- If folate is depleted, the marrow cannot mount an adequate compensatory response → worsening anaemia (megaloblastic arrest superimposed on haemolytic anaemia)
| Parameter | Expected Finding | Explanation |
|---|---|---|
| Hb | ↓ (variable, 4-12 g/dL) | Chronic haemolytic anaemia |
| MCV | Normal or slightly ↑ | Reticulocytosis raises MCV (reticulocytes are larger) |
| Reticulocyte count | ↑ (typically 5-15%; up to 40-70% post-splenectomy) | Compensatory marrow response |
| Unconjugated bilirubin | ↑ | Haemolysis → haem breakdown → unconjugated bilirubin |
| LDH | ↑ | Released from lysed RBCs |
| Haptoglobin | ↓ | Consumed by binding free haemoglobin released during haemolysis |
| Direct Coombs test | Negative | Non-immune haemolysis |
| Peripheral blood film | Echinocytes, polychromasia, no spherocytes | Characteristic morphology |
| Serum ferritin / Transferrin saturation | ↑ (progressive) | Iron overload from ineffective erythropoiesis ± transfusions |
| Folate | May be ↓ | Consumed by compensatory erythroid hyperplasia |
High Yield Summary
Pyruvate Kinase Deficiency — Key Points for Exams:
- 2nd most common RBC enzymopathy (after G6PD deficiency); most common glycolytic enzyme defect causing hereditary non-spherocytic haemolytic anaemia
- Autosomal recessive inheritance (PKLR gene, chromosome 1q22) — affects males and females equally
- Pathophysiology: ↓PK → ↓ATP → RBC membrane dysfunction → rigid, echinocytic cells → extravascular haemolysis in spleen
- Key compensatory mechanism: Metabolic block → ↑upstream intermediates → ↑2,3-BPG → right-shifted O₂-dissociation curve → anaemia is better tolerated than expected for the Hb level
- Clinical features: Chronic Coombs-negative haemolytic anaemia (from birth), NNJ, jaundice, splenomegaly, pigmented gallstones, folate deficiency, iron overload
- Blood film: Echinocytes (NOT spherocytes) — hence "non-spherocytic" haemolytic anaemia
- Iron overload occurs via TWO mechanisms: transfusional + ineffective erythropoiesis (↓hepcidin)
- Diagnosis: ↓PK enzyme activity in RBC haemolysate and/or PKLR genetic testing
- Management: Folate supplementation, transfusion ± iron chelation, splenectomy (partial response), and newer agent mitapivat (first-in-class PK activator, FDA-approved 2022)
- Aplastic crisis: Parvovirus B19 → sudden worsening of anaemia + reticulocytopenia
Active Recall - Pyruvate Kinase Deficiency (Definition to Clinical Features)
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p376 — Red Cell Enzymopathies) [2] Senior notes: Ryan Ho Haemtology.pdf (p39 — Red Cell Enzymopathies, Pyruvate Kinase Deficiency) [3] Senior notes: Block A - Family history of anaemia_ inherited causes of anaemia; haemolytic anaemia; aplastic anaemia.pdf (Hemolytic Anemia section) [4] Lecture slides: GC 097. Many members of the family have anaemia (MED).pdf; GC 097. Many members of the family have anaemia (File 1).pdf
Differential Diagnosis of Pyruvate Kinase Deficiency
When you encounter a patient — typically a child or young adult — with chronic, Coombs-negative haemolytic anaemia, you need a systematic framework to work through the differentials. The key discriminating features that point towards PK deficiency are:
- Chronic (lifelong, not episodic)
- Non-immune (DAT/Coombs negative)
- Non-spherocytic (echinocytes, not spherocytes on film)
- Anaemia better tolerated than expected (due to ↑2,3-BPG)
- Autosomal recessive family history pattern
- No oxidant trigger relationship (distinguishing from G6PD deficiency)
The differential diagnosis of PK deficiency is essentially the differential of chronic Coombs-negative haemolytic anaemia, which can be organized by the component of the RBC that is defective.
If inherited, think in terms of the components of the RBC, and how they can go wrong → either the membrane, the metabolism, or the haemoglobin [3]
This is the single most important framework for inherited haemolytic anaemias, directly from GC lecture slides:
Disorders of red cell metabolism: Glucose-6-phosphate dehydrogenase deficiency, Pyruvate kinase deficiency, Deficiency of other glycolytic pathway enzymes, Congenital non-spherocytic haemolytic anaemia [4][5]
Detailed Differential Diagnosis Table
| Condition | Key Distinguishing Features from PK Deficiency | Why It's Different |
|---|---|---|
| Hereditary Spherocytosis (HS) | Spherocytes on blood film (not echinocytes); positive osmotic fragility test; AD inheritance (most commonly); splenomegaly common; MCHC often ↑ | HS is caused by defects in membrane cytoskeletal proteins (ankyrin, spectrin, band 3, protein 4.2) → progressive membrane loss → surface area decreases relative to volume → spherocyte formation. PK deficiency produces echinocytes, not spherocytes — the membrane is intact but ATP-depleted, causing spiculation rather than sphering. HS is "non-spherocytic" by definition does NOT apply. |
| Hereditary Elliptocytosis (HE) | Elliptocytes/ovalocytes on film; usually mild haemolysis; AD inheritance | Spectrin or protein 4.1 defects → RBCs become permanently elliptical after passing through microcirculation. Most patients have well-compensated haemolysis. Blood film morphology clearly different from PK deficiency. |
| Hereditary Stomatocytosis | Stomatocytes (mouth-like central pallor) on film; associated with Na⁺/K⁺ leak; splenectomy contraindicated (risk of thrombosis) | Abnormal membrane cation permeability → overhydrated or dehydrated RBCs. Rare. Distinguished by film morphology and the critical point that splenectomy is contraindicated (unlike PK deficiency where splenectomy is a therapeutic option). |
| Condition | Key Distinguishing Features from PK Deficiency | Why It's Different |
|---|---|---|
| G6PD Deficiency [1][3][5] | X-linked recessive (predominantly males affected; females variably affected by lyonization); episodic haemolysis triggered by oxidant stress (fava beans, drugs, infections) rather than chronic; Heinz bodies and bite cells on film during crisis; between crises, patient is entirely well with normal Hb; most common RBC enzymopathy, affects 400M worldwide, 4.5% M and 0.5% F in HK [1] | G6PD is in the pentose phosphate pathway → NADPH production → glutathione regeneration → protects against oxidative damage. Deficiency means RBCs cannot handle oxidant stress → acute, episodic intravascular and extravascular haemolysis. In contrast, PK deficiency causes chronic, baseline haemolysis because ATP production is permanently impaired. The inheritance pattern (XLR vs AR) and the clinical pattern (episodic vs chronic) are the biggest distinguishers. |
| Other glycolytic enzyme defects | Glucose phosphate isomerase deficiency, hexokinase deficiency, phosphofructokinase deficiency — all extremely rare; similar chronic non-spherocytic haemolysis picture | These are diagnosed by specific enzyme assays on RBC haemolysate. Clinically indistinguishable from PK deficiency without enzyme testing. Glucose phosphate isomerase deficiency is the 3rd most common glycolytic enzymopathy. |
| Pyrimidine 5'-nucleotidase deficiency | Prominent basophilic stippling on blood film (because the enzyme normally degrades pyrimidine nucleotides from ribosomal RNA in reticulocytes; without it, RNA remnants accumulate) | Rare. The striking basophilic stippling is the clue — not typically seen in PK deficiency. |
G6PD vs PK Deficiency — The Exam Favourite
| Feature | G6PD Deficiency | PK Deficiency |
|---|---|---|
| Inheritance | X-linked recessive | Autosomal recessive |
| Pathway affected | Pentose phosphate shunt | Glycolysis (Embden-Meyerhof) |
| Haemolysis pattern | Episodic (triggered by oxidants) | Chronic, baseline |
| Between crises | Normal Hb, normal film | Persistent anaemia |
| Film during haemolysis | Bite cells, Heinz bodies, blister cells | Echinocytes (burr cells) |
| DAT (Coombs) | Negative | Negative |
| Prevalence in HK | 4.5% males, 0.5% females | Rare |
| Neonatal presentation | NNJ (common) | NNJ (common) |
| Splenectomy role | Not typically indicated | Partial benefit (reduces haemolysis) |
| Condition | Key Distinguishing Features from PK Deficiency | Why It's Different |
|---|---|---|
| β-Thalassaemia major / intermedia | Microcytic, hypochromic anaemia (low MCV, low MCH); target cells; basophilic stippling; Hb electrophoresis shows ↑HbF and/or ↑HbA₂; typically presents at 6-12 months as HbF declines | Thalassaemia is a quantitative haemoglobin defect (reduced globin chain synthesis). PK deficiency has a normocytic or slightly macrocytic anaemia (from reticulocytosis). Hb electrophoresis is normal in PK deficiency. |
| HbH Disease (α-thalassaemia intermedia) | HbH inclusions on brilliant cresyl blue stain; microcytic anaemia; Hb electrophoresis shows HbH (β₄) band | HbH is a 3-gene deletion α-thalassaemia. The haemoglobin analysis is diagnostic. |
| Sickle Cell Disease (HbSS) | Sickle cells on film; painful vaso-occlusive crises; dactylitis in infants; positive sickle solubility test; Hb electrophoresis shows HbS | Qualitative Hb defect. Very rare in Hong Kong Chinese population — primarily affects those of African, Mediterranean, or Middle Eastern descent. Clinical presentation is dramatically different from PK deficiency. |
| Unstable Haemoglobin (e.g. Hb Köln) [6] | Heinz bodies on supravital stain; episodic haemolysis triggered by infections; heat instability test or isopropanol stability test positive; AD inheritance | Unstable haemoglobin chains unfold and precipitate → Heinz bodies → extravascular haemolysis [6]. Distinguished from PK deficiency by the presence of Heinz bodies, positive instability tests, and AD inheritance. PK deficiency does NOT produce Heinz bodies. |
These must be excluded when evaluating a patient with apparent chronic Coombs-negative haemolysis, even if the presentation seems inherited:
| Condition | Key Distinguishing Features | Why It's in the Differential |
|---|---|---|
| Paroxysmal Nocturnal Haemoglobinuria (PNH) | Acquired clonal disorder; intravascular haemolysis (haemoglobinuria, haemosiderinuria); flow cytometry shows loss of GPI-anchored proteins (CD55, CD59); associated with aplastic anaemia and thrombosis | PNH is the one acquired intrinsic red cell defect [3]. Coombs-negative. Can present insidiously. Distinguished by flow cytometry and the intravascular haemolysis pattern (dark morning urine). |
| Wilson Disease [7] | Coombs-negative haemolytic anaemia in the context of acute liver failure in a young patient; low ceruloplasmin; Kayser-Fleischer rings; elevated 24h urinary copper | "Young patients with no reason for fulminant liver failure, exclusion of all other common causes → only finding is low haemoglobin, Coombs negative haemolytic anemia" [7]. Copper-induced oxidative damage to RBCs. Important to consider in a young patient with Coombs-negative haemolysis + liver dysfunction. |
| Microangiopathic Haemolytic Anaemia (MAHA) — TTP, HUS, DIC | Schistocytes (fragmented RBCs) on film; thrombocytopenia; features of the underlying cause (renal failure in HUS, neurological features in TTP, consumptive coagulopathy in DIC) | MAHA causes Coombs-negative haemolysis but is intravascular (mechanical shearing) and produces schistocytes, not echinocytes. The blood film is diagnostic. |
| Drug-induced (non-immune) haemolysis | History of oxidant drugs (dapsone, rasburicase, phenazopyridine); can occur even in G6PD-normal individuals at high doses | Always take a drug history. Non-immune drug-induced haemolysis is Coombs-negative. |
| Clinical Scenario | DDx to Consider | How to Distinguish |
|---|---|---|
| Neonatal jaundice | ABO/Rh haemolytic disease of the newborn; G6PD deficiency; hereditary spherocytosis; Crigler-Najjar syndrome; Gilbert syndrome; breast milk jaundice; biliary atresia (conjugated) | DAT positive in immune-mediated HDN; G6PD assay; blood film morphology; bilirubin fractionation (unconjugated vs conjugated) |
| Unconjugated hyperbilirubinaemia in an adolescent/adult | Gilbert syndrome [8] — isolated mild intermittent jaundice, triggered by fasting/stress, bilirubin typically < 100 μmol/L, all other LFTs and haemolysis markers normal [8] | Gilbert syndrome has no anaemia, no reticulocytosis, normal haptoglobin, normal LDH — it is purely a conjugation defect (UGT1A1 deficiency), not a haemolytic condition. PK deficiency will have reticulocytosis + ↓haptoglobin + ↑LDH. |
| Chronic anaemia with iron overload | Thalassaemia intermedia; sideroblastic anaemia; myelodysplastic syndrome; congenital dyserythropoietic anaemia (CDA) | CDA in particular can mimic PK deficiency (chronic Coombs-negative haemolysis with ineffective erythropoiesis and iron overload). Distinguished by bone marrow morphology (binucleated/multinucleated erythroblasts in CDA) and specific serological/genetic tests. |
When faced with chronic Coombs-negative haemolytic anaemia, use this systematic approach:
Step 1: Confirm haemolysis
- ↑Reticulocytes, ↑unconjugated bilirubin, ↑LDH, ↓haptoglobin, negative DAT
Step 2: Exclude acquired causes
- Drug history, flow cytometry for PNH, peripheral film for schistocytes (MAHA), ceruloplasmin/copper studies if liver involvement
Step 3: Examine the blood film carefully
- Spherocytes → hereditary spherocytosis (confirm with osmotic fragility / EMA binding test)
- Echinocytes → PK deficiency (confirm with PK enzyme assay)
- Sickle cells → sickle cell disease (confirm with Hb electrophoresis)
- Target cells + microcytosis → thalassaemia (confirm with Hb electrophoresis, iron studies)
- Heinz bodies → G6PD deficiency or unstable Hb (confirm with G6PD assay or heat instability test)
- Bite cells → G6PD deficiency during acute crisis
- Schistocytes → MAHA (TTP/HUS/DIC)
- Basophilic stippling → thalassaemia or pyrimidine 5'-nucleotidase deficiency
Step 4: Targeted confirmatory testing
- PK enzyme activity in RBC haemolysate
- PKLR gene sequencing
- Hb electrophoresis/HPLC
- Specific enzyme assays (G6PD, glucose phosphate isomerase, etc.)
Exam Pitfall: Don't Forget PNH in the Differential
PNH is an acquired intrinsic RBC defect that is Coombs-negative. It is commonly associated with aplastic anaemia and can present insidiously. Always consider flow cytometry for GPI-anchored proteins (CD55/CD59) when evaluating unexplained Coombs-negative haemolysis — especially if there is concurrent cytopaenia, thrombosis, or dark urine (haemoglobinuria).
| Feature | PK Deficiency | Main Mimics |
|---|---|---|
| Haemolysis pattern | Chronic, baseline | G6PD = episodic; HS = chronic |
| DAT | Negative | AIHA = positive |
| Film morphology | Echinocytes | HS = spherocytes; G6PD crisis = bite cells; Thal = target cells |
| MCV | Normal/slightly ↑ | Thal = ↓↓; B12 def = ↑↑ |
| Inheritance | AR | G6PD = XLR; HS = AD (most) |
| Response to splenectomy | Partial improvement (Hb rises 1-3 g/dL; reticulocytosis paradoxically increases) | HS = good response; G6PD = not indicated |
| Hb electrophoresis | Normal | Thal = abnormal; SCD = HbS |
| Unique metabolic feature | ↑2,3-BPG → tolerates anaemia well | Not seen in other haemolytic anaemias to this degree |
High Yield Summary — Differential Diagnosis of PK Deficiency
- Framework: Chronic Coombs-negative haemolytic anaemia → think RBC component: Membrane (HS, HE) vs Metabolism (G6PD, PK) vs Haemoglobin (Thal, SCD, unstable Hb)
- G6PD vs PK: Episodic/oxidant-triggered vs chronic/baseline; XLR vs AR; pentose phosphate pathway vs glycolysis
- HS vs PK: Spherocytes vs echinocytes; AD vs AR; positive osmotic fragility vs PK enzyme assay
- Thalassaemia vs PK: Microcytic vs normocytic; abnormal Hb electrophoresis vs normal
- PNH: Acquired intrinsic; flow cytometry for CD55/CD59; intravascular haemolysis
- Wilson disease: Consider in young patient with Coombs-negative haemolysis + liver failure
- Gilbert syndrome: Isolated unconjugated hyperbilirubinaemia WITHOUT haemolysis markers — not a true DDx for haemolysis but included in DDx for jaundice
- Blood film is king — morphology narrows the differential more than any single blood test
Active Recall - Differential Diagnosis of Pyruvate Kinase Deficiency
References
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p376 — Red Cell Enzymopathies, G6PD Deficiency) [2] Senior notes: Ryan Ho Haemtology.pdf (p39 — Red Cell Enzymopathies, Pyruvate Kinase Deficiency) [3] Senior notes: Block A - Family history of anaemia_ inherited causes of anaemia; haemolytic anaemia; aplastic anaemia.pdf (Hemolytic Anaemia differential diagnosis) [4] Lecture slides: GC 047. Family history of anaemia.pdf (p30 — Disorders of Red Cell Metabolism) [5] Senior notes: Block A - Family history of anaemia_ inherited causes of anaemia; haemolytic anaemia; aplastic anaemia.pdf (p19 — Disorders of red cell metabolism list) [6] Senior notes: Block A - Many members of the family have anaemia.pdf (p8 — Haemoglobinopathies, Hb Köln) [7] Senior notes: Block A - Patients with non-viral chronic liver diseases.pdf (p2 — Wilson disease, Coombs-negative haemolytic anaemia) [8] Senior notes: Ryan Ho GI.pdf (p292 — Gilbert Syndrome)
Diagnostic Criteria, Algorithm & Investigations for Pyruvate Kinase Deficiency
Unlike conditions such as SLE (ACR/EULAR criteria) or rheumatic fever (Jones criteria), PK deficiency does not have a formalised set of diagnostic criteria from a professional society. Instead, the diagnosis rests on a combination of:
- Clinical picture consistent with chronic, non-spherocytic, Coombs-negative haemolytic anaemia
- Dx: by testing PK activity in RBC haemolysate or genetic testing [1][2]
The diagnostic process is therefore best understood as a stepwise algorithm — confirming haemolysis, excluding commoner/acquired causes, then arriving at the specific enzymopathy through targeted testing.
Based on the International PK Deficiency Working Group (PKDWG) and current haematological practice, the diagnosis of PK deficiency requires both:
| Criterion | Detail |
|---|---|
| A. Compatible clinical and laboratory picture | Chronic haemolytic anaemia with evidence of extravascular haemolysis (↑reticulocytes, ↑unconjugated bilirubin, ↑LDH, ↓haptoglobin), negative DAT, and absence of spherocytes on film |
| B. Confirmatory test (one or both) | 1. Reduced PK enzyme activity in RBC haemolysate (below the lower limit of the laboratory reference range, typically < 50% of normal), OR 2. Identification of biallelic (two) pathogenic variants in the PKLR gene by molecular genetic testing |
Why We Need BOTH Clinical + Confirmatory Evidence
You cannot diagnose PK deficiency on enzyme activity alone — because recent transfusion introduces donor RBCs with normal PK activity, falsely normalising the assay. Conversely, genetic testing alone may identify variants of uncertain significance. The clinical picture contextualises the laboratory result.
The approach follows a logical cascade: first confirm haemolysis, then exclude common/acquired causes, then perform targeted enzymopathy testing.
Step-by-Step Investigation Workup with Key Findings and Interpretation
The first question with any anaemia is whether the red cells are being destroyed faster than they are made. We use a panel of markers that, together, build the haemolysis picture.
| Investigation | Expected Finding in PK Deficiency | Interpretation / Why |
|---|---|---|
| Complete Blood Count (CBC) | ↓Hb (variable, 4-12 g/dL); MCV normal or slightly ↑; normal WCC and platelets (unless post-splenectomy → thrombocytosis) | Anaemia is the cardinal finding. MCV may be slightly elevated because reticulocytes are larger than mature RBCs (they still contain residual RNA and organelles). WCC and platelets are unaffected because PK-R is specific to the erythroid lineage. |
| Reticulocyte count | ↑ (typically 5-15%; up to 40-70% post-splenectomy) | The bone marrow is intact and compensating for peripheral red cell destruction. A high reticulocyte count in the context of anaemia = the marrow is responding appropriately, i.e. the problem is destruction (haemolysis) or loss (bleeding), not production failure. Post-splenectomy, the count rises further because reticulocytes are no longer being trapped/destroyed in the spleen. |
| Unconjugated (indirect) bilirubin | ↑ | Bilirubin is formed from the lysis of red blood cells (haem component) within the reticuloendothelial system, initially in the form of unconjugated bilirubin [9]. In extravascular haemolysis, macrophages break down trapped RBCs → haem → unconjugated bilirubin → exceeds hepatic conjugation capacity → serum level rises. |
| Serum LDH | ↑ | LDH is an intracellular enzyme abundant in RBCs. When RBCs are destroyed (whether intra- or extravascularly), LDH leaks into the plasma. It is a sensitive but non-specific marker of haemolysis (also elevated in tissue damage from any cause). |
| Serum haptoglobin | ↓ | Haptoglobin is a plasma protein that binds free haemoglobin released from lysed RBCs. The haptoglobin-haemoglobin complex is rapidly cleared by the liver. In ongoing haemolysis, haptoglobin consumption exceeds hepatic production → serum level falls. Note: haptoglobin deficiency is common in Chinese (1:1000) [10] — so a low haptoglobin alone is not sufficient evidence of haemolysis in this population; interpret in context. |
The Haemolysis Panel — A Package Deal
No single marker confirms haemolysis. You need the combination: ↑reticulocytes + ↑unconjugated bilirubin + ↑LDH + ↓haptoglobin. Any one marker in isolation can have alternative explanations (e.g. LDH rises in MI; bilirubin rises in Gilbert syndrome; haptoglobin is an acute phase reactant that can be falsely normal in inflammation despite haemolysis).
| Investigation | Expected Finding | Interpretation / Why |
|---|---|---|
| Direct Antiglobulin Test (DAT) | Negative [1][2] | The DAT detects IgG antibodies or complement (C3d) bound to the surface of the patient's RBCs. In PK deficiency, the haemolysis is intrinsic (the defect is inside the RBC) and non-immune (no antibodies are attacking the RBC surface) → DAT is negative. A positive DAT would redirect you towards autoimmune haemolytic anaemia (warm or cold type), which requires entirely different management (immunosuppression). |
Chronic, Coombs'-negative haemolytic anaemia [1][2] — this is a defining feature.
The blood film is one of the most powerful diagnostic tools in haematology. In PK deficiency, the morphology is subtly but importantly different from other haemolytic anaemias.
| Finding | Present in PK Deficiency? | Interpretation / Why |
|---|---|---|
| Echinocytes (burr cells / crenated cells) | ✓ Characteristic | ATP depletion → loss of membrane lipid asymmetry + impaired cytoskeletal maintenance → uniform, evenly spaced, short spicules projecting from the RBC surface. This is the hallmark morphology. |
| Polychromasia | ✓ | Reticulocytes stain basophilic (bluish-grey) with Romanowsky stains due to residual ribosomal RNA. Polychromasia on film = reticulocytosis = compensatory marrow response. |
| Spherocytes | ✗ Absent / rare | Absence of spherocytes is a key negative finding — hence "non-spherocytic haemolytic anaemia." This distinguishes PK deficiency from hereditary spherocytosis and warm AIHA. |
| Bite cells / Heinz bodies | ✗ Absent | These indicate oxidative haemolysis (G6PD deficiency). Not present in PK deficiency because the defect is in the glycolytic pathway, not the oxidant-defence pathway. |
| Target cells | May be present in small numbers | Non-specific; more prominent in thalassaemia and liver disease. |
| Nucleated RBCs (NRBCs) | May appear, especially post-splenectomy | Normally, the spleen removes NRBCs from circulation. Post-splenectomy, they persist. Also indicates markedly stressed erythropoiesis. |
| Howell-Jolly bodies | Post-splenectomy only | Nuclear remnants normally removed by the spleen. Their presence post-splenectomy confirms functional asplenia. |
Because G6PD deficiency is the most common RBC enzymopathy, affecting 400M worldwide, 4.5% males and 0.5% females in HK [1][3], it must be excluded before pursuing rarer diagnoses.
| Investigation | Detail | Interpretation |
|---|---|---|
| G6PD enzyme assay (quantitative spectrophotometric) | Measures G6PD activity in RBC haemolysate | Normal G6PD activity → G6PD deficiency excluded → proceed to PK testing. Reduced activity → G6PD deficiency confirmed. Caveat: During acute haemolysis, reticulocytes (which have higher G6PD activity) may cause a falsely normal result. Repeat testing 2-3 months after an acute episode in males; in females, genetic testing may be needed due to lyonization. |
Step 5: PK-Specific Confirmatory Testing
This is the definitive diagnostic step. There are two complementary approaches:
| Detail | Explanation |
|---|---|
| Specimen | RBC haemolysate (prepared from washed, leukocyte-depleted packed red cells) |
| Method | Spectrophotometric coupled-enzyme assay: PK converts PEP → pyruvate; pyruvate is then converted to lactate by lactate dehydrogenase (LDH) with consumption of NADH → measured as decrease in absorbance at 340 nm |
| Normal range | Varies by laboratory; typically 6-12 IU/gHb |
| Finding in PK deficiency | Reduced activity, usually < 50% of normal (homozygotes/compound heterozygotes); heterozygous carriers typically have ~50% activity and are clinically well |
| Interpretive pitfalls | See below |
Critical Pitfalls of the PK Enzyme Assay:
| Pitfall | Explanation | How to Avoid |
|---|---|---|
| Recent blood transfusion | Donor RBCs have normal PK activity → dilutes out the patient's deficient RBCs → falsely normal result | Wait ≥3 months after last transfusion before testing, OR use molecular genetic testing instead |
| High reticulocyte count | Reticulocytes have ~5-10x higher PK activity than mature RBCs (reticulocytes have residual mitochondria and higher metabolic activity) → may falsely elevate the measured PK activity into the low-normal range, masking a genuine deficiency | Always interpret PK activity relative to the reticulocyte count; some labs report a "PK/reticulocyte ratio" or "PK/hexokinase ratio" to adjust for this |
| Leukocyte contamination | WBCs contain PK-M2 isoform (different from PK-R) with high activity → contaminates the haemolysate → falsely normal result | Ensure thorough leukocyte depletion of the sample before assay |
| Certain PK mutants | Some missense mutations produce enzyme with normal Vmax but abnormal kinetics (e.g. altered substrate affinity or allosteric regulation) → normal activity under standard assay conditions but dysfunctional in vivo | If clinical suspicion is high despite normal PK activity, proceed to genetic testing |
Exam Pearl: When the PK Assay Can Be Misleading
Three situations where PK enzyme activity may be falsely normal:
- Post-transfusion (donor RBCs dilute the signal)
- High reticulocyte count (reticulocytes have higher PK activity)
- Leukocyte contamination of the haemolysate
In all three scenarios, if clinical suspicion remains, proceed to PKLR genetic testing which is not affected by these confounders.
| Detail | Explanation |
|---|---|
| Gene | PKLR (chromosome 1q22) |
| Method | Sanger sequencing or next-generation sequencing (NGS) panel targeting the PKLR gene; some laboratories use whole-exome sequencing (WES) for broader screening |
| Diagnostic finding | Identification of biallelic pathogenic variants (two variants — one from each parent): homozygous or compound heterozygous |
| Advantages over enzyme assay | Not affected by transfusion status, reticulocyte count, or sample contamination; provides definitive genotype for family counselling and prognostication; identifies the specific mutation which may correlate with disease severity |
| Limitations | Variants of uncertain significance (VUS) may be identified → require functional correlation; does not assess enzyme function directly; some deep intronic or regulatory variants may be missed by standard sequencing |
| When to use | (1) When enzyme assay is equivocal or confounded; (2) For carrier detection in family members; (3) For prenatal diagnosis in at-risk couples; (4) Increasingly used as first-line in centres with accessible molecular diagnostics |
Dx: by testing PK activity in RBC haemolysate or genetic testing [1][2]
Once PK deficiency is confirmed, a comprehensive baseline workup is needed to assess the consequences of chronic haemolysis and guide management.
| Investigation | Purpose | Key Findings & Interpretation |
|---|---|---|
| Iron studies (serum ferritin, transferrin saturation) | Assess for iron overload | Iron overload due to ineffective erythropoiesis or transfusional iron overload [1][2]. Ferritin > 500 μg/L warrants further evaluation; TSAT > 45% suggests iron overload. Ferritin is a surrogate marker for iron [11] — but remember it is also an acute phase reactant (can be falsely elevated in infection/inflammation). |
| Liver and cardiac MRI T2* | Quantify organ iron deposition | MRI T2* is the gold standard for non-invasive iron assessment. Liver T2* < 6.3 ms (corresponding to liver iron concentration > 7 mg/g dry weight) indicates significant overload. Cardiac T2* < 20 ms indicates cardiac iron loading; < 10 ms indicates severe loading with high risk of cardiomyopathy. |
| Serum folate | Assess for folate depletion | Folate deficiency can develop from increased consumption by compensatory erythroid hyperplasia [1][2]. Low folate → worsening anaemia due to megaloblastic arrest superimposed on the haemolytic anaemia. |
| Abdominal ultrasound | Assess splenomegaly and gallstones | Variable splenomegaly [1][2] from work hypertrophy of splenic macrophages trapping abnormal RBCs. Gallstones (pigmented, calcium bilirubinate) [1][2] from chronic unconjugated hyperbilirubinaemia. |
| Liver function tests (LFT) | Baseline hepatic assessment | May show elevated unconjugated bilirubin (from haemolysis). If transaminases or ALP are elevated, consider iron overload–related liver injury or gallstone-related biliary disease. |
| Haemoglobin electrophoresis / HPLC | Exclude concomitant haemoglobinopathy | Should be normal in isolated PK deficiency. Useful to exclude thalassaemia or structural Hb variants that may coexist (co-inheritance is described, though rare). |
| Echocardiography | Baseline cardiac function; assess for iron cardiomyopathy in chronically transfused patients | Chronic iron loading can cause dilated cardiomyopathy. Also, chronic anaemia itself → compensatory high-output state → eccentric cardiac remodelling over time. |
| Endocrine panel (in chronically transfused/iron-loaded patients) | Screen for iron-related endocrinopathies | Glucose (DM from pancreatic iron), TFTs (hypothyroidism), gonadotrophins and sex steroids (hypogonadism from pituitary iron), growth parameters in children. |
| Parvovirus B19 serology | If presenting with aplastic crisis (sudden ↓Hb + absent reticulocytes) | IgM positive = acute infection; IgG positive = past infection with immunity. Parvovirus B19 attacks erythroid precursors → transient red cell aplasia → crisis in patients dependent on high marrow output. |
Family Screening and Genetic Counselling
| Investigation | Who | Purpose |
|---|---|---|
| PK enzyme assay or PKLR genotyping in parents and siblings | Parents (obligate carriers), siblings (25% chance of being affected, 50% carriers) | Family screening is needed when one member is diagnosed [11]. Carriers have ~50% PK activity and are clinically well but need to be identified for reproductive counselling. |
| Prenatal diagnosis | At-risk couples (both parents are known carriers) | For at-risk couples, prenatal diagnosis is needed when pregnancy occurs [11]. Can be done by chorionic villus sampling (CVS) at 10-12 weeks or amniocentesis at 15-18 weeks, with targeted PKLR mutation analysis. |
| Stage | Investigation | What It Tells You |
|---|---|---|
| Haemolysis screen | CBC, reticulocytes, unconjugated bilirubin, LDH, haptoglobin | Confirms haemolysis is occurring |
| Immune vs non-immune | DAT (direct Coombs) | Negative = non-immune (PK def, HS, G6PD, etc.) |
| Morphology | Peripheral blood film | Echinocytes (not spherocytes) → points to enzymopathy |
| Exclude G6PD | G6PD enzyme assay | Normal → proceed to PK testing |
| Confirm PK deficiency | PK enzyme activity in RBC haemolysate | Low activity = diagnostic (interpret with reticulocyte count) |
| Confirm PK deficiency | PKLR gene sequencing | Biallelic pathogenic variants = definitive |
| Complications | Iron studies, MRI T2*, folate, USS abdomen, echo, Hb electrophoresis | Baseline and ongoing monitoring |
| Family | PK assay or genotyping in relatives | Carrier identification, prenatal counselling |
HbA1c Reliability in PK Deficiency
If a patient with PK deficiency also has diabetes, be aware that HbA1c will be falsely low because of the shortened RBC lifespan — glycosylated haemoglobin has less time to accumulate before the RBC is destroyed [12]. Use fructosamine or continuous glucose monitoring (CGM) instead for glycaemic assessment in these patients.
- PK deficiency is not currently included in most newborn screening programmes (unlike G6PD deficiency which is screened for in HK)
- However, a neonate presenting with severe unconjugated hyperbilirubinaemia and negative DAT should trigger consideration of PK deficiency, especially if:
- G6PD assay is normal
- Blood film shows no spherocytes
- Family history is suggestive (consanguinity, affected siblings)
- In this setting, PK enzyme assay and/or PKLR genotyping should be requested
High Yield Summary — Diagnosis of PK Deficiency
- No formal diagnostic criteria — diagnosis is biochemical + genetic
- Confirmed by testing PK activity in RBC haemolysate or genetic testing [1][2]
- Diagnostic algorithm: Confirm haemolysis → Negative DAT → Film shows echinocytes (non-spherocytic) → Exclude G6PD → PK enzyme assay ± PKLR sequencing
- PK enzyme assay pitfalls: Falsely normal after recent transfusion, with high reticulocyte count, or with leukocyte contamination. When in doubt, use genetic testing.
- Genetic testing identifies biallelic PKLR variants; not affected by transfusion or reticulocyte count
- Complication workup: Iron studies + MRI T2* (iron), folate level, USS abdomen (gallstones, spleen), cardiac assessment, endocrine screen
- Family screening is essential: carrier detection in parents/siblings; prenatal diagnosis for at-risk couples
- HbA1c is unreliable in PK deficiency due to shortened RBC lifespan → use alternative glycaemic markers
Active Recall - Diagnosis and Investigations of Pyruvate Kinase Deficiency
References
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p376 — Red Cell Enzymopathies, PK Deficiency) [2] Senior notes: Ryan Ho Haemtology.pdf (p39 — Red Cell Enzymopathies, Pyruvate Kinase Deficiency) [3] Senior notes: Block A - Family history of anaemia_ inherited causes of anaemia; haemolytic anaemia; aplastic anaemia.pdf (p19 — G6PD deficiency, Disorders of Red Cell Metabolism) [4] Lecture slides: GC 047. Family history of anaemia.pdf (p30 — Disorders of Red Cell Metabolism) [9] Senior notes: Block A - Introduction to GI_Hepatology investigations (LFT, Endoscopy).pdf (p8 — Bilirubin metabolism) [10] Senior notes: Block A - Fever after a blood transfusion_ transfusion and related problems.pdf (p19 — Haptoglobin deficiency in Chinese) [11] Senior notes: Block A - Many members of the family have anaemia.pdf (p6 — Family screening, ferritin, thalassaemia) [12] Senior notes: Block A - Polyuria and polydipsia_ glucose metabolism; diabetes mellitus; diabetic ketoacidosis.pdf (p4 — HbA1c limitations in haemolysis)
Management of Pyruvate Kinase Deficiency
PK deficiency management has historically been supportive — there is no traditional therapy that fixes the underlying enzyme defect. The goals are:
- Maintain adequate oxygen delivery (transfusions when needed)
- Prevent and treat complications of chronic haemolysis (folate depletion, gallstones, iron overload)
- Reduce haemolysis where possible (splenectomy)
- Disease-modifying therapy — a new era began with the approval of mitapivat (Pyrukynd®), the first-in-class PK activator (FDA-approved February 2022, EMA-approved 2023)
Mx: transfusion ± chelation, folate supplementation, splenectomy [1][2]
Treatment Modalities in Detail
| Aspect | Detail |
|---|---|
| Rationale | Chronic haemolysis drives compensatory erythroid hyperplasia in the bone marrow → rapidly dividing erythroid precursors consume large amounts of folate (vitamin B9) for DNA synthesis. If folate becomes depleted, the marrow cannot sustain adequate compensatory erythropoiesis → worsening anaemia with a megaloblastic component superimposed on the haemolytic anaemia. |
| Dose | Folic acid 1-2 mg/day for moderate/severe haemolysis; 5 mg/day if pregnant [13] |
| Duration | Lifelong in patients with ongoing haemolysis |
| Indication | All patients with PK deficiency who have evidence of ongoing haemolysis |
| Contraindications | Essentially none (folic acid is water-soluble and has an excellent safety profile); however, always exclude concomitant vitamin B12 deficiency before supplementing folate alone (folate can mask the haematological features of B12 deficiency while allowing neurological damage to progress) |
Folate supplementation is listed as a core component of management [1][2].
| Aspect | Detail |
|---|---|
| Rationale | When the bone marrow cannot compensate sufficiently for the rate of haemolysis, Hb drops to symptomatic or dangerous levels → transfusion provides exogenous, PK-normal RBCs that survive normally in the circulation |
| Indications | Transfusion if severe anaemia [1][2][13]; more specifically: symptomatic anaemia (fatigue, failure to thrive, exercise limitation), Hb < 7 g/dL (or individualised threshold based on symptoms and 2,3-BPG compensation), aplastic crisis (parvovirus B19), pregnancy, or peri-operative period |
| Transfusion programme | Severe cases may require regular transfusions every 2-6 weeks to maintain a target Hb (typically > 9-10 g/dL in children to support growth). Many patients with moderate disease only need intermittent transfusions during crises or intercurrent illness |
| Iron overload consequence | Iron in 500 mL of blood = 250 mg. The body does not have a good mechanism of iron excretion → prone to iron overload. Iron overload → toxic to the heart [14]. Each unit of packed RBCs delivers ~200-250 mg of iron that cannot be physiologically excreted → progressive accumulation in liver, heart, endocrine organs |
| Special considerations | Use leukocyte-depleted (leukoreduced) blood to reduce febrile non-haemolytic transfusion reactions and CMV transmission; extended phenotype matching is advisable to reduce alloimmunisation risk in chronically transfused patients |
Transfusion Threshold — Not Just a Number
In PK deficiency, patients tolerate anaemia better than expected due to elevated 2,3-BPG shifting the O₂-dissociation curve rightward. A patient with Hb 7 g/dL may be clinically well. Therefore, transfusion decisions should be guided by symptoms and functional status, not Hb level alone. Over-transfusing accelerates iron overload without proportional clinical benefit.
| Aspect | Detail |
|---|---|
| Rationale | Iron overload due to ineffective erythropoiesis or transfusional iron overload [1][2]. Iron accumulates through two pathways: (1) repeated transfusions (~250 mg Fe per unit); (2) increased intestinal absorption driven by ineffective erythropoiesis (↑erythroferrone → ↓hepcidin → ↑ferroportin activity). Iron deposits in parenchymal organs → Haber-Weiss reaction (iron catalyses H₂O₂ → reactive oxygen species) → organ damage [15] |
| Indications | Serum ferritin persistently > 1000 μg/L, or positive findings on MRI T2* for liver or heart iron loading; some guidelines recommend starting earlier (ferritin > 500-800 μg/L) especially in regularly transfused patients |
Chelation → Desferrioxamine (DFO), DFP and Deferasirox — bind to iron, excrete from urine and faeces [14]
| Agent | Route | Mechanism | Key Side Effects | Notes |
|---|---|---|---|---|
| Desferrioxamine (DFO) ("des-ferri-ox-amine" = removes iron using an oxamine chelator) | SC infusion 8-12h/day, 5-7 days/week; or IV | Hexadentate chelator that binds Fe³⁺ with very high affinity → chelate excreted in urine (gives reddish-orange colour) and faeces | Ototoxicity (sensorineural hearing loss), retinal toxicity (visual changes), injection site reactions, growth retardation in children if over-chelated, ARDS (rare) | Gold standard efficacy; major limitation is compliance due to prolonged subcutaneous infusion requirement [14][15] |
| Deferasirox (DFX) ("de-fer-asirox" = removes iron) | Oral (once daily, dispersible tablet or film-coated tablet) | Tridentate chelator; 2 molecules bind 1 Fe³⁺ → excreted primarily in faeces | GI upset (nausea, diarrhoea, abdominal pain), renal toxicity (↑creatinine, Fanconi syndrome — rare), hepatotoxicity (LFT derangement), rash | Most commonly used oral chelator; excellent compliance profile; requires regular monitoring of renal and hepatic function [15] |
| Deferiprone (DFP) ("de-feri-prone" = iron-prone removal) | Oral (three times daily) | Bidentate chelator; 3 molecules bind 1 Fe³⁺; particularly effective at removing cardiac iron | Agranulocytosis (1-2% — most serious; requires weekly FBC monitoring), arthropathy, GI upset, zinc deficiency | Preferred when cardiac iron loading is the primary concern; often used in combination with DFO for severe cardiac siderosis [15] |
| Chelator | Compliance | Cardiac Iron | Liver Iron | Monitoring Required |
|---|---|---|---|---|
| DFO | Poor (SC infusion) | Good | Good | Annual audiometry + ophthalmology |
| DFX | Good (oral OD) | Good | Good | RFT + LFT every 2 weeks initially, then monthly |
| DFP | Moderate (oral TDS) | Excellent | Moderate | Weekly FBC (agranulocytosis risk) |
Why Chelation Matters Even Without Transfusion
Many clinicians assume iron overload only occurs in transfusion-dependent patients. In PK deficiency, iron overload can develop even without transfusion due to ineffective erythropoiesis suppressing hepcidin → increased gut iron absorption. Monitor ferritin and consider chelation in non-transfused patients with rising iron stores.
4. Splenectomy
The spleen is the principal site of extravascular haemolysis in PK deficiency — rigid, ATP-depleted RBCs are trapped in the splenic cords and destroyed by macrophages. Removing the spleen reduces (but does not eliminate) haemolysis.
| Aspect | Detail |
|---|---|
| Indication | Transfusion-dependence or severely symptomatic anaemia [2][13]; specifically: patients requiring regular transfusions, or those with moderate-severe anaemia causing significant symptoms (growth failure, exercise limitation, poor quality of life) despite conservative measures |
| Timing | Usually deferred till > 5-6 years old to reduce risk of sepsis [2][13]. In young children, the spleen plays a critical role in immune defence against encapsulated bacteria. Removing it before the immune system has matured increases the risk of overwhelming post-splenectomy infection (OPSI). Some centres prefer age > 6y [2], others > 7y [13]. |
| Expected benefit | Hb typically rises by 1-3 g/dL; some patients become transfusion-independent. However, haemolysis is not completely eliminated — some extravascular destruction continues in the liver (hepatic macrophages / Kupffer cells). |
| Reticulocyte paradox | Post-splenectomy, the reticulocyte count often rises dramatically (sometimes to 40-70%) even though Hb improves. Why? Because the spleen was previously destroying many reticulocytes (which, despite having mitochondria, are still somewhat rigid and get trapped). Without splenic filtration, these reticulocytes survive and are counted in the peripheral blood. |
| Types | Total splenectomy (laparoscopic preferred) is standard. Partial splenectomy is sometimes considered in young children — preserves some splenic immune function while reducing haemolysis, but regrowth of splenic tissue may reduce long-term efficacy. |
| Contraindications | Hereditary stomatocytosis (splenectomy paradoxically increases thrombotic risk in this condition — but this is NOT PK deficiency, just worth knowing for the differential) |
Therapeutic splenectomy indications: decrease transfusion requirement in thalassaemia major / refractory ITP or AIHA / symptomatic relief in massive splenomegaly [16]. The same principle applies to PK deficiency — reducing the primary site of red cell destruction.
| Measure | Rationale |
|---|---|
| Vaccinations (at least 2 weeks before surgery): Pneumococcal (PCV13 + PPSV23), Meningococcal (MenACWY + MenB), Haemophilus influenzae type b (Hib), Influenza | The spleen is the primary filter for encapsulated bacteria. Post-splenectomy, patients are at lifelong risk of overwhelming post-splenectomy infection (OPSI) — fulminant sepsis with encapsulated organisms (S. pneumoniae > N. meningitidis > H. influenzae). Vaccination primes the adaptive immune system to partially compensate. |
| Patient and family education | Awareness of OPSI risk, importance of seeking urgent medical attention for any febrile illness |
| Measure | Detail |
|---|---|
| Lifelong prophylactic penicillin V | Typically phenoxymethylpenicillin 250-500 mg BD; reduces risk of pneumococcal sepsis. Some guidelines recommend lifelong in all asplenic patients; at minimum, for at least 2 years post-splenectomy and throughout childhood. |
| Booster vaccinations | Repeat PPSV23 every 5 years; annual influenza vaccination |
| MedicAlert bracelet | Identifies the patient as asplenic in an emergency → ensures prompt empirical antibiotics for febrile illness |
| Awareness of travel risks | Malaria is more severe in asplenic patients (the spleen normally removes parasitised RBCs); babesiosis is a life-threatening risk |
| Thrombotic risk monitoring | Post-splenectomy thrombocytosis is common (loss of splenic platelet pooling); some haemolytic anaemias carry an increased post-splenectomy venous thromboembolism (VTE) risk. Consider thromboprophylaxis if platelet count is markedly elevated. |
Post-Splenectomy Blood Film — Don't Be Alarmed
After splenectomy you will see: Howell-Jolly bodies (nuclear remnants), target cells, nucleated RBCs, thrombocytosis, and a markedly elevated reticulocyte count (40-70%). These are expected findings, not pathology. Spurious leukocytosis and increased platelet count are normal post-splenectomy consequences [16].
This is the most significant therapeutic advance in PK deficiency. Let's break down the name:
- Mita- = from "activate"
- -pivat = from "pyruvate kinase"
- So "mitapivat" = a drug that activates pyruvate kinase
| Aspect | Detail |
|---|---|
| Mechanism | Mitapivat is a small-molecule allosteric activator of PK-R. It binds to the PK-R tetramer at the allosteric activator site (the same site where the natural activator fructose-1,6-bisphosphate binds), stabilising the active R-state conformation of the enzyme → increases PK enzymatic activity → restores glycolytic flux → increases ATP production → improves RBC survival and reduces haemolysis |
| Regulatory status | FDA-approved (February 2022) for adults with PK deficiency; EMA-approved (November 2023); expanded approval to paediatric patients (≥1 year) in 2024 |
| Route / Dosing | Oral, twice daily (typical doses: 5 mg, 20 mg, or 50 mg BD; titrated to response) |
| Efficacy (ACTIVATE and ACTIVATE-T trials) | In non-transfusion-dependent patients: ~40% achieved Hb rise ≥ 1.5 g/dL sustained over 24 weeks; improvements in haemolysis markers (↓bilirubin, ↓reticulocytes, ↓LDH). In transfusion-dependent patients: ~40% achieved ≥ 33% reduction in transfusion burden |
| Key benefits | Disease-modifying (addresses the root cause); oral administration; improves Hb, reduces haemolysis markers, may reduce transfusion burden, may reduce iron overload progression |
| Side effects | Generally well tolerated; reported: insomnia, back pain, headache, dizziness, arthralgia. Important safety warning: must NOT be abruptly discontinued — requires gradual tapering over 2 weeks (abrupt cessation can cause acute haemolytic episodes due to sudden loss of PK activation in cells that have adapted to the drug) |
| Indications | Adults (and now children ≥ 1 year in some jurisdictions) with PK deficiency who have ongoing haemolytic anaemia (transfusion-dependent or non-transfusion-dependent) |
| Contraindications | Patients with certain PKLR genotypes producing no protein at all (null/null homozygotes) — the activator needs at least some residual PK protein to bind and activate; patients without any PK-R protein will not respond |
| Monitoring | Hb, reticulocyte count, unconjugated bilirubin, LDH — to assess treatment response; LFTs (rare hepatic effects reported) |
Mitapivat — Why It Works on the Molecular Level
PK-R normally exists in equilibrium between a less-active T-state and a more-active R-state. The natural allosteric activator fructose-1,6-bisphosphate (FBP) shifts the equilibrium towards the R-state. Many PK-deficiency mutations produce an enzyme that is structurally unstable or has reduced affinity for FBP → enzyme stays in the less-active T-state. Mitapivat mimics FBP, binding to the same allosteric site and locking the mutant enzyme in the active R-state → increased catalytic activity → more ATP production → less haemolysis. This only works if there is at least some PK-R protein present — hence null/null patients do not respond.
| Aspect | Detail |
|---|---|
| Rationale | HSCT replaces the patient's haematopoietic stem cells with those from a healthy donor → the new erythroid precursors produce normal PK-R → cure of haemolysis |
| Indication | Reserved for severe, transfusion-dependent PK deficiency with a matched sibling donor; considered when the burden of chronic transfusion and iron overload outweighs transplant risks |
| Efficacy | Curative — case series demonstrate normalisation of Hb and haemolysis markers post-engraftment |
| Risks | Graft-versus-host disease (GVHD), graft failure, transplant-related mortality (5-10%), infections, infertility from conditioning regimen |
| Current status | Allogeneic HSCT: NOT routinely used due to unfavourable risk-benefit ratio [2][13] — PK deficiency, while chronic, is generally manageable with supportive care ± mitapivat. HSCT risk is only justified in the most severe cases. |
| Therapy | Status | Mechanism |
|---|---|---|
| Gene therapy | Pre-clinical / early clinical trials | Lentiviral vector-mediated transfer of functional PKLR gene into autologous haematopoietic stem cells → corrected cells produce normal PK-R. Avoids donor-related risks of allogeneic HSCT. |
| Other PK activators | Research pipeline | Second-generation allosteric activators with potentially improved potency or pharmacokinetics |
8. Management of Specific Complications
| Complication | Management |
|---|---|
| Neonatal jaundice (NNJ) | Phototherapy (converts unconjugated bilirubin to water-soluble isomers for renal excretion); exchange transfusion if bilirubin approaches neurotoxic levels (threshold varies by gestational age and risk factors); IV immunoglobulin is NOT indicated (this is non-immune haemolysis) |
| Neonatal anaemia | Top-up transfusion if Hb drops to symptomatic/dangerous levels |
| Aspect | Management |
|---|---|
| Acute | Supportive transfusion to maintain Hb during the aplastic phase (typically self-limiting over 7-10 days as the immune system clears the virus and erythropoiesis resumes) |
| Prevention | No specific vaccine available; advise avoiding contact with known fifth disease (erythema infectiosum) outbreaks; IVIg has been used in immunocompromised patients with persistent parvovirus infection |
| Aspect | Management |
|---|---|
| Pigmented gallstones | Cholecystectomy if symptomatic (biliary colic, cholecystitis, cholangitis). Some centres perform concurrent cholecystectomy at the time of splenectomy if gallstones are present. Ursodeoxycholic acid (UDCA) is NOT effective for pigmented stones (it only dissolves cholesterol stones). |
| Organ | Monitoring | Target |
|---|---|---|
| Liver | Serum ferritin every 3-6 months; MRI T2* / FerriScan annually | Liver iron concentration < 7 mg/g dry weight |
| Heart | Cardiac MRI T2* annually in iron-loaded patients; echocardiography | Cardiac T2* > 20 ms (< 10 ms = severe, high risk of cardiomyopathy) |
| Endocrine | Annual screening: fasting glucose, TFTs, gonadotrophins, IGF-1 (children) | Early detection and treatment of iron-related endocrinopathies |
| Treatment | Indication | Key Points |
|---|---|---|
| Folate supplementation | All patients with ongoing haemolysis | 1-2 mg/d; 5 mg/d in pregnancy [1][2][13] |
| Transfusion | Severe/symptomatic anaemia, aplastic crisis | Leukodepleted; watch for iron overload [1][2] |
| Iron chelation | Ferritin > 1000 μg/L or positive MRI T2* | DFO, DFX, or DFP; choose based on organ iron and compliance [14][15] |
| Splenectomy | Transfusion-dependent or severely symptomatic | Defer to > 5-6y; pre-op vaccines; lifelong penicillin post-op [2][13][16] |
| Mitapivat | Haemolytic anaemia (TD or non-TD), ≥ 1y (some jurisdictions) | Oral PK activator; do NOT stop abruptly; requires some residual PK protein |
| Allogeneic HSCT | Severe TD disease with matched donor | Not routinely used — unfavourable risk-benefit ratio [2][13] |
| Gene therapy | Investigational | Pre-clinical/early clinical |
High Yield Summary — Management of PK Deficiency
- Supportive care is the backbone: folate supplementation, transfusion ± chelation, splenectomy [1][2]
- Folate: 1-2 mg/d (5 mg/d in pregnancy) — prevents megaloblastic arrest from folate depletion
- Transfusion: Guided by symptoms not just Hb level (patients tolerate anaemia well due to ↑2,3-BPG); each unit adds ~250 mg iron
- Iron chelation: DFO (SC — gold standard but compliance issues), DFX (oral — most used), DFP (oral — best for cardiac iron but risk of agranulocytosis); indicated even without transfusion due to ineffective erythropoiesis
- Splenectomy: Partial response (Hb rises 1-3 g/dL); defer to > 5-6y; pre-op vaccines; lifelong penicillin prophylaxis; post-splenectomy reticulocyte count rises paradoxically
- Mitapivat: First-in-class oral PK-R allosteric activator (FDA 2022); disease-modifying; must taper gradually (never stop abruptly); requires residual PK protein
- HSCT: Curative but reserved for severe cases — unfavourable risk-benefit ratio in most patients
- Post-splenectomy risks: OPSI (encapsulated bacteria), thrombocytosis/VTE risk
Active Recall - Management of Pyruvate Kinase Deficiency
References
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p376 — Red Cell Enzymopathies, PK Deficiency) [2] Senior notes: Ryan Ho Haemtology.pdf (p39 — Red Cell Enzymopathies, Pyruvate Kinase Deficiency) [13] Senior notes: Adrian Lui Pediatrics Notes.pdf (p375 — Management of hereditary spherocytosis; principles applied to PK deficiency) [14] Senior notes: Block A - Hematology Interactive Tutorial.pdf (p5 — Blood transfusion, iron overload, chelation agents) [15] Senior notes: Maksim Medicine Notes.pdf (p159 — Haemochromatosis / Iron overload, chelation therapy) [16] Senior notes: Block A - Splenomegaly_ common causes of splenomegaly; myeloproliferative diseases.pdf (p19 — Splenectomy indications, post-splenectomy complications)
Complications of Pyruvate Kinase Deficiency
PK deficiency is a lifelong condition, and its complications arise from three interrelated processes:
- Chronic haemolysis → bilirubin overproduction, folate depletion, splenomegaly
- Iron overload → organ damage (liver, heart, endocrine)
- Consequences of treatment → post-splenectomy risks, transfusion-related complications
Let's work through each complication systematically, explaining why it occurs from first principles.
1. Complications of Chronic Haemolysis
| Aspect | Detail |
|---|---|
| Pathophysiology | Haemolysis in the newborn generates large amounts of unconjugated bilirubin from haem catabolism. The neonatal liver has immature UDP-glucuronosyltransferase (UGT1A1) activity → limited ability to conjugate bilirubin → unconjugated bilirubin accumulates rapidly. PK deficiency provides the "supply-side" overload (excess bilirubin production) while neonatal hepatic immaturity provides the "clearance-side" bottleneck. |
| Clinical significance | NNJ is often the first presentation of PK deficiency [1][2]. Can range from mild phototherapy-responsive jaundice to severe hyperbilirubinaemia requiring exchange transfusion. |
| Dreaded complication | Kernicterus (bilirubin encephalopathy) — unconjugated bilirubin is lipid-soluble and crosses the blood-brain barrier → deposits in the basal ganglia, hippocampus, and brainstem nuclei → causes permanent neurological damage (choreoathetoid cerebral palsy, sensorineural hearing loss, oculomotor palsies, dental enamel hypoplasia). This is preventable with timely phototherapy or exchange transfusion. |
| Monitoring | Serial transcutaneous or serum bilirubin measurements; plotted on nomograms (e.g. NICE, AAP) to guide intervention thresholds. |
| Aspect | Detail |
|---|---|
| Pathophysiology | Chronic extravascular haemolysis → sustained overproduction of unconjugated bilirubin → hepatic conjugation and biliary excretion of bilirubin is increased → supersaturation of bile with calcium bilirubinate → precipitation and nucleation → formation of pigmented (black) gallstones. These are distinct from cholesterol gallstones — they are composed of calcium bilirubinate polymers and are NOT amenable to dissolution therapy (ursodeoxycholic acid only works on cholesterol stones). |
| Epidemiology | Can develop as early as childhood/adolescence; lifetime risk is very high in patients with chronic haemolytic anaemias. |
| Clinical consequences | Biliary colic (RUQ pain after fatty meals), acute cholecystitis (stone impaction in cystic duct → inflammation ± infection), choledocholithiasis (stone in common bile duct → obstructive jaundice with conjugated hyperbilirubinaemia, confusingly layered on top of the baseline unconjugated hyperbilirubinaemia from haemolysis), ascending cholangitis (Charcot triad: fever + jaundice + RUQ pain), gallstone pancreatitis (stone impaction at the ampulla of Vater → pancreatic duct obstruction). |
| Management | Cholecystectomy when symptomatic. Often performed concurrently with splenectomy to avoid a second surgery. |
| Aspect | Detail |
|---|---|
| Pathophysiology | The spleen is the principal site of extravascular haemolysis in PK deficiency. ATP-depleted, rigid RBCs are trapped in the splenic red pulp cords → phagocytosed by splenic macrophages → work hypertrophy of the reticuloendothelial system → progressive splenic enlargement. Additionally, in severe cases, the spleen may become a site of extramedullary haematopoiesis (the spleen "helps out" the overwhelmed bone marrow by producing RBCs itself), further contributing to enlargement. |
| Clinical consequences | Early satiety (stomach compression), LUQ discomfort/dragging sensation, cytopaenias from hypersplenism (the enlarged spleen excessively sequesters platelets and WBCs → thrombocytopaenia and leucopaenia in addition to the anaemia), and risk of splenic rupture (rare but life-threatening, especially after trauma — counsel patients to avoid contact sports). |
| Aspect | Detail |
|---|---|
| Pathophysiology | Chronic haemolysis drives a sustained compensatory erythroid hyperplasia in the bone marrow — the marrow is working overtime to replace destroyed RBCs. Rapidly dividing erythroid precursors require large amounts of folate (vitamin B9) as a one-carbon donor for thymidylate synthesis (the rate-limiting step in DNA replication). This increased demand, if not matched by dietary intake, leads to folate depletion. |
| Clinical consequence | Folate deficiency → impaired DNA synthesis → megaloblastic arrest of erythropoiesis → worsening anaemia with a macrocytic component superimposed on the haemolytic picture. This is a "double hit" — the marrow is already struggling to keep up, and now it cannot even divide properly. The reticulocyte count may paradoxically fall (the marrow can no longer compensate). |
| Prevention | Folate supplementation is a core management principle [1][2] — lifelong in all patients with ongoing haemolysis. |
| Aspect | Detail |
|---|---|
| Pathophysiology | Parvovirus B19 (the agent causing "fifth disease" / erythema infectiosum in healthy children) has a specific tropism for erythroid precursors — it binds to the P antigen (globoside) on the surface of erythroid progenitor cells → enters and destroys them → transient pure red cell aplasia lasting 7-10 days. In healthy individuals, this brief halt in erythropoiesis is clinically insignificant because normal RBC lifespan is ~120 days — losing 10 days of production barely dents the Hb. In PK deficiency, the RBC lifespan is already severely shortened → the patient is dependent on high marrow output to maintain Hb → even a brief cessation causes a precipitous drop in Hb (aplastic crisis). |
| Clinical features | Sudden worsening of pallor, fatigue, tachycardia, dyspnoea; reticulocyte count drops to near zero (the marrow has stopped producing RBCs); Hb may fall to life-threatening levels (< 4-5 g/dL). The jaundice may paradoxically improve (less haemolysis because fewer RBCs are being produced and destroyed). |
| Diagnosis | Parvovirus B19 IgM (acute infection); reticulocytopaenia in the context of worsening anaemia in a patient with known chronic haemolysis. |
| Management | Supportive transfusion; self-limiting once the immune system clears the virus and erythropoiesis resumes. |
| Contagion note | The patient is infectious before the rash appears (in healthy children); by the time the rash develops, viraemia has cleared. In aplastic crisis, there is no rash (the rash is immune-complex mediated and requires an intact immune response), so these patients remain viraemic and infectious for longer → infection control precautions are needed. |
Why Aplastic Crisis Is So Dangerous in Haemolytic Anaemias
Think of it like a factory running at 110% capacity to keep up with demand. If the factory suddenly shuts down for even a few days, the supply chain collapses catastrophically. In PK deficiency, the bone marrow is producing RBCs at a massively accelerated rate to compensate for haemolysis. Parvovirus B19 shuts the factory. Unlike someone with a normal 120-day RBC lifespan who can "coast" through the shutdown, the PK-deficient patient's already short-lived RBCs are destroyed faster than new ones can be made → acute, severe anaemia.
| Aspect | Detail |
|---|---|
| Pathophysiology | Chronic anaemia → tissue hypoxia in dependent areas with relatively poor perfusion (lower limbs, especially malleolar regions). Additionally, chronic haemolysis releases free haem and iron → local oxidative stress → endothelial damage → impaired wound healing. The combination of hypoxia, oxidative damage, and microvascular dysfunction creates an environment where even minor trauma fails to heal, forming chronic ulcers. |
| Prevalence | Uncommon in PK deficiency (more classically associated with sickle cell disease and hereditary spherocytosis). Seen mainly in severe, non-splenectomised patients with persistently low Hb. |
| Management | Optimise Hb (transfusion), compression dressings, wound care, infection prevention. |
2. Iron Overload and Its Organ-Specific Complications
Iron overload due to ineffective erythropoiesis or transfusional iron overload [1][2]
Iron overload is arguably the most important long-term complication of PK deficiency because it is progressive, initially silent, and causes irreversible organ damage if not detected and treated early.
| Mechanism | Pathophysiology |
|---|---|
| Transfusional | Each unit of packed RBCs contains ~200-250 mg iron. The body has no physiological mechanism for active iron excretion (only ~1 mg/day lost through desquamation). With chronic transfusions, iron accumulates inexorably. |
| Ineffective erythropoiesis | Expanded but ineffective erythropoiesis (destruction of erythroid precursors within the marrow) → increased secretion of erythroferrone (ERFE) by erythroblasts → ERFE suppresses hepatic hepcidin production → hepcidin is the master regulator of iron homeostasis that normally degrades ferroportin (the iron exporter on enterocytes and macrophages) → low hepcidin → ferroportin remains active → increased intestinal iron absorption well beyond physiological needs. This mechanism operates even in non-transfused patients. |
The pathophysiology of iron toxicity follows the same principle across all organs: non-transferrin-bound iron (NTBI) enters parenchymal cells → catalyses the Haber-Weiss / Fenton reaction (Fe²⁺ + H₂O₂ → Fe³⁺ + OH· + OH⁻) → generates reactive oxygen species (ROS) → oxidative damage to lipid membranes, proteins, and DNA → cell death and fibrosis.
| Organ | Complication | Clinical Features | Monitoring | Why This Organ? |
|---|---|---|---|---|
| Liver | Haemosiderosis → fibrosis → cirrhosis → risk of hepatocellular carcinoma | Hepatomegaly, deranged LFTs, eventually portal hypertension, ascites, variceal bleeding | Serum ferritin; MRI T2* (FerriScan); liver biopsy (rarely needed now) | The liver is the first organ to encounter absorbed iron via the portal circulation; hepatocytes actively take up NTBI via DMT1 and ZIP14 transporters |
| Heart | Iron-related dilated cardiomyopathy → heart failure; arrhythmias (especially supraventricular) | Dyspnoea, peripheral oedema, palpitations; can present as sudden cardiac death | Cardiac MRI T2* (< 20 ms = concerning; < 10 ms = severe); echocardiography (LVEF) | Cardiomyocytes take up NTBI avidly; cardiac iron is the leading cause of death in transfusion-dependent patients with inadequate chelation |
| Endocrine: Pituitary | Hypogonadotrophic hypogonadism | Delayed puberty (children), amenorrhoea (females), erectile dysfunction, decreased libido (males), infertility | Gonadotrophins (LH, FSH), sex steroids (oestradiol/testosterone) | The anterior pituitary gonadotrophs are exquisitely sensitive to iron; hypogonadism is often the earliest endocrinopathy |
| Endocrine: Pancreas | Diabetes mellitus (Type 3c / "bronze diabetes") | Polyuria, polydipsia, hyperglycaemia | Fasting glucose, HbA1c (but unreliable in haemolysis — use fructosamine instead) | Iron deposits in pancreatic beta cells → oxidative damage → impaired insulin secretion; compounded by insulin resistance from hepatic iron |
| Endocrine: Thyroid | Hypothyroidism | Fatigue, cold intolerance, weight gain, constipation | TSH, free T4 | Iron deposition in thyroid follicular cells |
| Endocrine: Parathyroid | Hypoparathyroidism | Hypocalcaemia, tetany, perioral paraesthesia | Calcium, PTH | Rare; iron deposits in parathyroid glands |
| Skin | Hyperpigmentation ("bronze" appearance) | Generalised darkening, especially sun-exposed areas | Clinical examination | Combination of iron-stimulated melanin production in the epidermis and haemosiderin deposition in the dermis |
| Joints | Arthropathy | Joint pain, stiffness, particularly 2nd and 3rd MCP joints | X-ray (squared-off bone ends, hook-like osteophytes — classic for haemochromatosis) | Iron deposition in synovium and articular cartilage → chondrocalcinosis → secondary degenerative arthritis |
| Bone | Osteoporosis / Osteopaenia | Fragility fractures, loss of height | DEXA scan | Iron toxicity to osteoblasts + hypogonadism (reduced sex steroids needed for bone maintenance) + chronic anaemia |
Cardiac Iron — The Leading Cause of Death in Transfusion-Dependent Haemolytic Anaemias
Cardiac complications from iron overload are the number one killer in inadequately chelated transfusion-dependent patients. Cardiac MRI T2* is the gold standard for detecting cardiac iron before clinical heart failure develops. A T2* < 10 ms indicates severe loading with imminent risk of fatal arrhythmia or heart failure. This is why regular chelation monitoring and compliance are critical.
3. Complications Related to Treatment
Splenectomy is a key therapeutic intervention but introduces its own set of lifelong risks.
| Complication | Pathophysiology | Prevention/Management |
|---|---|---|
| Overwhelming Post-Splenectomy Infection (OPSI) | The spleen is the primary filter for encapsulated bacteria (Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae type b). It contains specialised marginal-zone B cells that mount rapid T-independent antibody responses to polysaccharide capsular antigens. Without the spleen, these organisms can proliferate unchecked → fulminant sepsis with DIC, multi-organ failure, and death within hours of onset. Mortality of OPSI is 50-70% even with treatment. | Pre-splenectomy vaccination (PCV13, PPSV23, MenACWY, MenB, Hib); lifelong prophylactic penicillin V; MedicAlert bracelet; patient education to seek immediate medical attention for any febrile illness |
| Thrombocytosis and venous thromboembolism (VTE) | Loss of splenic platelet pooling (the spleen normally stores ~30% of circulating platelets) → rebound thrombocytosis (platelet count often > 600-1000 × 10⁹/L post-splenectomy). In haemolytic anaemias specifically, there is an additional prothrombotic state: free haemoglobin scavenges nitric oxide (a vasodilator and platelet inhibitor) → endothelial dysfunction + platelet hyperactivation → increased risk of portal vein thrombosis, DVT, PE. | Monitor platelet count; consider low-dose aspirin or anticoagulation if thrombocytosis is marked; maintain hydration; perioperative thromboprophylaxis |
| Post-splenectomy sepsis from other organisms | Asplenic patients are also at increased risk from intraerythrocytic parasites — malaria (the spleen normally removes parasitised RBCs) and Babesia (tick-borne, can be fulminant in asplenic individuals) | Malaria prophylaxis if travelling to endemic areas; awareness of babesiosis risk |
| Paradoxical reticulocytosis | Reticulocytes previously trapped/destroyed by the spleen now survive → reticulocyte count rises to 40-70% even as Hb improves. MCV may appear elevated (reticulocytes are larger). This is not pathological — it is an expected post-splenectomy finding. | Awareness; do not misinterpret as worsening haemolysis or a new pathology |
| Complication | Mechanism | Relevance to PK Deficiency |
|---|---|---|
| Iron overload (detailed above) | Each unit adds ~250 mg non-excretable iron | The most important transfusion complication in PK deficiency |
| Alloimmunisation | Exposure to non-self RBC antigens on donor blood → patient develops antibodies against minor blood group antigens (e.g. Rh, Kell, Kidd, Duffy) → future crossmatch becomes increasingly difficult; risk of delayed haemolytic transfusion reactions | Minimise by extended phenotype matching for chronically transfused patients |
| Febrile non-haemolytic transfusion reaction | Recipient antibodies react against donor leukocyte antigens → cytokine release → fever, rigors | Reduce risk with leukodepleted blood products |
| Transfusion-transmitted infections | Blood-borne pathogens (HBV, HCV, HIV — now extremely rare with screening; bacterial contamination of platelet products) | Standard blood bank screening protocols |
| Agent | Key Complications |
|---|---|
| Desferrioxamine | Ototoxicity (sensorineural hearing loss), retinal toxicity (reduced visual acuity, night blindness), growth retardation in children (over-chelation), injection-site reactions, ARDS (rare) |
| Deferasirox | Renal toxicity (↑creatinine, Fanconi syndrome), hepatotoxicity, GI upset, skin rash |
| Deferiprone | Agranulocytosis (1-2% — potentially fatal; requires weekly FBC monitoring), arthropathy, GI upset, zinc deficiency |
| Complication | Pathophysiology |
|---|---|
| Growth retardation and delayed puberty (paediatric) | Chronic anaemia → chronic tissue hypoxia → impaired growth; compounded by iron-overload–induced hypogonadotrophic hypogonadism |
| High-output cardiac state → cardiac remodelling | Chronic anaemia → compensatory ↑cardiac output (↑heart rate, ↑stroke volume) → eccentric left ventricular hypertrophy over years → may eventually progress to heart failure, even without iron overload |
| Exercise intolerance and reduced quality of life | Despite the compensatory ↑2,3-BPG effect, severe anaemia still limits exercise capacity and causes chronic fatigue |
| Extramedullary haematopoiesis | In severe cases, the bone marrow cannot meet erythropoietic demand even at maximal output → haematopoietic tissue expands to extra-medullary sites (liver, spleen, paravertebral regions) → hepatosplenomegaly, and rarely paravertebral masses that can cause spinal cord compression |
| Skeletal changes (rare in PK deficiency) | Chronic marrow expansion → cortical thinning, trabecular coarsening, frontal bossing, maxillary hyperplasia ("chipmunk facies") — these are far more characteristic of thalassaemia major but can occur in severe PK deficiency |
| Complication | Explanation |
|---|---|
| Worsening anaemia | Pregnancy increases plasma volume (haemodilution) and metabolic demand; haemolysis continues unabated → Hb may drop to levels requiring transfusion support |
| Increased folate demand | Both pregnancy and haemolysis increase folate requirements → high-dose folate supplementation (5 mg/day) is essential |
| Fetal implications | If both parents are carriers, there is a 25% chance the fetus is affected. Severe fetal PK deficiency can present with hydrops fetalis (massive fetal oedema from severe anaemia) — a rare but life-threatening presentation requiring in-utero transfusions and emergent delivery planning |
| Category | Complications |
|---|---|
| Haemolysis consequences [1][2] | NNJ, gallstones, jaundice, variable splenomegaly, folate deficiency |
| Iron overload [1][2] | Liver (cirrhosis, HCC risk), Heart (DCM, arrhythmias), Endocrine (hypogonadism, DM, hypothyroidism), Skin (bronze), Joints (arthropathy), Bone (osteoporosis) |
| Post-splenectomy | OPSI, thrombocytosis/VTE, parasitic infections, paradoxical reticulocytosis |
| Transfusion-related | Iron overload, alloimmunisation, FNHTR, TTI |
| Chelation-related | DFO: ototoxicity, retinal toxicity; DFX: renal/hepatotoxicity; DFP: agranulocytosis |
| Chronic anaemia | Growth retardation, high-output cardiac state, extramedullary haematopoiesis, exercise intolerance |
| Aplastic crisis | Parvovirus B19 → precipitous Hb drop, reticulocytopaenia |
| Pregnancy | Worsening anaemia, increased folate demand, risk of hydrops fetalis in affected fetus |
High Yield Summary — Complications of PK Deficiency
- Haemolysis complications: NNJ, gallstones, variable splenomegaly, folate deficiency [1][2] — know the pathophysiology of each
- Iron overload occurs through two mechanisms (transfusional + ineffective erythropoiesis) — even non-transfused patients can be iron-loaded
- Cardiac iron is the leading cause of death in inadequately chelated transfusion-dependent patients — monitor with cardiac MRI T2*
- Aplastic crisis (Parvovirus B19) — sudden severe anaemia + reticulocytopaenia; self-limiting but may need urgent transfusion
- OPSI post-splenectomy — fulminant sepsis with encapsulated bacteria; 50-70% mortality; prevented by vaccination + lifelong penicillin
- Gallstones are pigmented (calcium bilirubinate) — NOT amenable to UDCA dissolution
- HbA1c is falsely low in PK deficiency due to shortened RBC lifespan — use fructosamine for diabetic monitoring
- Iron overload endocrinopathies: hypogonadism is often the earliest; DM, hypothyroidism, hypoparathyroidism follow
- Pregnancy: worsening anaemia, increased folate demand, risk of hydrops fetalis if fetus is homozygous affected
Active Recall - Complications of Pyruvate Kinase Deficiency
References
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p376 — Red Cell Enzymopathies, PK Deficiency) [2] Senior notes: Ryan Ho Haemtology.pdf (p39 — Red Cell Enzymopathies, Pyruvate Kinase Deficiency) [4] Lecture slides: GC 047. Family history of anaemia.pdf (p3, p30 — Classification of haemolytic anaemia; Disorders of Red Cell Metabolism)
High Yield Summary
Pyruvate Kinase Deficiency — Key Points for Exams:
- 2nd most common RBC enzymopathy (after G6PD deficiency); most common glycolytic enzyme defect causing hereditary non-spherocytic haemolytic anaemia
- Autosomal recessive inheritance (PKLR gene, chromosome 1q22) — affects males and females equally
- Pathophysiology: ↓PK → ↓ATP → RBC membrane dysfunction → rigid, echinocytic cells → extravascular haemolysis in spleen
- Key compensatory mechanism: Metabolic block → ↑upstream intermediates → ↑2,3-BPG → right-shifted O₂-dissociation curve → anaemia is better tolerated than expected for the Hb level
- Clinical features: Chronic Coombs-negative haemolytic anaemia (from birth), NNJ, jaundice, splenomegaly, pigmented gallstones, folate deficiency, iron overload
- Blood film: Echinocytes (NOT spherocytes) — hence "non-spherocytic" haemolytic anaemia
- Iron overload occurs via TWO mechanisms: transfusional + ineffective erythropoiesis (↓hepcidin)
- Diagnosis: ↓PK enzyme activity in RBC haemolysate and/or PKLR genetic testing
- Management: Folate supplementation, transfusion ± iron chelation, splenectomy (partial response), and newer agent mitapivat (first-in-class PK activator, FDA-approved 2022)
- Aplastic crisis: Parvovirus B19 → sudden worsening of anaemia + reticulocytopenia
High Yield Summary — Differential Diagnosis of PK Deficiency
- Framework: Chronic Coombs-negative haemolytic anaemia → think RBC component: Membrane (HS, HE) vs Metabolism (G6PD, PK) vs Haemoglobin (Thal, SCD, unstable Hb)
- G6PD vs PK: Episodic/oxidant-triggered vs chronic/baseline; XLR vs AR; pentose phosphate pathway vs glycolysis
- HS vs PK: Spherocytes vs echinocytes; AD vs AR; positive osmotic fragility vs PK enzyme assay
- Thalassaemia vs PK: Microcytic vs normocytic; abnormal Hb electrophoresis vs normal
- PNH: Acquired intrinsic; flow cytometry for CD55/CD59; intravascular haemolysis
- Wilson disease: Consider in young patient with Coombs-negative haemolysis + liver failure
- Gilbert syndrome: Isolated unconjugated hyperbilirubinaemia WITHOUT haemolysis markers — not a true DDx for haemolysis but included in DDx for jaundice
- Blood film is king — morphology narrows the differential more than any single blood test
High Yield Summary — Diagnosis of PK Deficiency
- No formal diagnostic criteria — diagnosis is biochemical + genetic
- Confirmed by testing PK activity in RBC haemolysate or genetic testing [1][2]
- Diagnostic algorithm: Confirm haemolysis → Negative DAT → Film shows echinocytes (non-spherocytic) → Exclude G6PD → PK enzyme assay ± PKLR sequencing
- PK enzyme assay pitfalls: Falsely normal after recent transfusion, with high reticulocyte count, or with leukocyte contamination. When in doubt, use genetic testing.
- Genetic testing identifies biallelic PKLR variants; not affected by transfusion or reticulocyte count
- Complication workup: Iron studies + MRI T2* (iron), folate level, USS abdomen (gallstones, spleen), cardiac assessment, endocrine screen
- Family screening is essential: carrier detection in parents/siblings; prenatal diagnosis for at-risk couples
- HbA1c is unreliable in PK deficiency due to shortened RBC lifespan → use alternative glycaemic markers
High Yield Summary — Management of PK Deficiency
- Supportive care is the backbone: folate supplementation, transfusion ± chelation, splenectomy [1][2]
- Folate: 1-2 mg/d (5 mg/d in pregnancy) — prevents megaloblastic arrest from folate depletion
- Transfusion: Guided by symptoms not just Hb level (patients tolerate anaemia well due to ↑2,3-BPG); each unit adds ~250 mg iron
- Iron chelation: DFO (SC — gold standard but compliance issues), DFX (oral — most used), DFP (oral — best for cardiac iron but risk of agranulocytosis); indicated even without transfusion due to ineffective erythropoiesis
- Splenectomy: Partial response (Hb rises 1-3 g/dL); defer to > 5-6y; pre-op vaccines; lifelong penicillin prophylaxis; post-splenectomy reticulocyte count rises paradoxically
- Mitapivat: First-in-class oral PK-R allosteric activator (FDA 2022); disease-modifying; must taper gradually (never stop abruptly); requires residual PK protein
- HSCT: Curative but reserved for severe cases — unfavourable risk-benefit ratio in most patients
- Post-splenectomy risks: OPSI (encapsulated bacteria), thrombocytosis/VTE risk
High Yield Summary — Complications of PK Deficiency
- Haemolysis complications: NNJ, gallstones, variable splenomegaly, folate deficiency [1][2] — know the pathophysiology of each
- Iron overload occurs through two mechanisms (transfusional + ineffective erythropoiesis) — even non-transfused patients can be iron-loaded
- Cardiac iron is the leading cause of death in inadequately chelated transfusion-dependent patients — monitor with cardiac MRI T2*
- Aplastic crisis (Parvovirus B19) — sudden severe anaemia + reticulocytopaenia; self-limiting but may need urgent transfusion
- OPSI post-splenectomy — fulminant sepsis with encapsulated bacteria; 50-70% mortality; prevented by vaccination + lifelong penicillin
- Gallstones are pigmented (calcium bilirubinate) — NOT amenable to UDCA dissolution
- HbA1c is falsely low in PK deficiency due to shortened RBC lifespan — use fructosamine for diabetic monitoring
- Iron overload endocrinopathies: hypogonadism is often the earliest; DM, hypothyroidism, hypoparathyroidism follow
- Pregnancy: worsening anaemia, increased folate demand, risk of hydrops fetalis if fetus is homozygous affected
Hereditary Elliptocytosis
Hereditary elliptocytosis is a group of inherited red blood cell membrane disorders, primarily due to defects in spectrin or other cytoskeletal proteins, resulting in elliptical-shaped erythrocytes and variable degrees of hemolytic anemia.
Autoimmune Haemolytic Anaemia
Autoimmune haemolytic anaemia is a condition in which autoantibodies directed against self red blood cell antigens cause premature erythrocyte destruction, leading to anaemia.