An X-linked enzymatic deficiency that impairs the hexose monophosphate shunt, reducing NADPH and glutathione production, thereby rendering red blood cells susceptible to oxidative stress–induced hemolysis.

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked recessive (XLR) inherited enzymopathy of red blood cells (RBCs), characterised by reduced or absent activity of the G6PD enzyme. This enzyme is the rate-limiting first step of the pentose phosphate pathway (hexose monophosphate shunt), which is the sole source of NADPH in mature RBCs. Without adequate NADPH, RBCs cannot regenerate reduced glutathione (GSH) and are therefore vulnerable to oxidative stress, leading to episodic or chronic haemolytic anaemia.

Breaking down the name:

Glucose-6-phosphate = the substrate (glucose with a phosphate group at carbon-6)
Dehydrogenase = "de-" (removal) + "hydrogen" + "-ase" (enzyme) → an enzyme that removes hydrogen atoms, transferring them to NADP⁺ to form NADPH

In Cantonese, G6PD deficiency is colloquially known as 蠶豆症 (literally "fava bean disease"), reflecting the classic trigger of haemolysis by fava beans (broad beans) ^[1]^[2].

2. Epidemiology

G6PD deficiency is the most common RBC enzymopathy worldwide, affecting approximately 400 million people ^[1]^[2]
Prevalence is highest in regions historically endemic for Plasmodium falciparum malaria: sub-Saharan Africa, the Mediterranean basin, the Middle East, South/Southeast Asia, and Southern China
The geographic overlap with malaria is not coincidental — G6PD deficiency confers a selective survival advantage against malaria (heterozygote advantage, similar to sickle cell trait). The mechanism involves oxidative stress in the parasitised G6PD-deficient RBC leading to early phagocytosis before the parasite can complete its life cycle ^[3]^[4]

In Hong Kong, G6PD deficiency affects approximately 4.5% of males and 0.5% of females ^[1]^[2]

This makes it the most common inborn error of metabolism in the Hong Kong Chinese population
The most common variant in the Chinese population is G6PD Canton (c.1376G > T, Class II) ^[1]^[2]
Other common Chinese variants include: G6PD Kaiping, G6PD Chinese-5
Universal newborn screening for G6PD deficiency has been implemented in Hong Kong since 1984 (by cord blood quantitative G6PD assay) ^[5]

High Yield – Hong Kong Context

G6PD deficiency is part of the universal newborn screening programme in Hong Kong. Any neonate identified is given a card listing drugs and substances to avoid. This is extremely important because neonatal jaundice in G6PD-deficient neonates can progress to kernicterus if not recognised promptly.

Because G6PD is encoded on the X chromosome (Xq28), the condition predominantly affects males (hemizygous: only one X chromosome)
Females can be affected in several scenarios ^[2]:
- Homozygous females (both X chromosomes carry mutant allele) — rare, requires both parents to carry mutation
- Heterozygous females with skewed X-inactivation (lyonization) — if by chance a greater proportion of cells inactivate the normal X, the female effectively has a large population of G6PD-deficient RBCs and can be symptomatic
- Turner syndrome (45,X) with the single X carrying the mutation

3. Anatomy and Function: Red Cell Metabolism and the Pentose Phosphate Pathway

Understanding G6PD deficiency requires understanding why RBCs are uniquely vulnerable to oxidative stress.

Mature RBCs are essentially "bags of haemoglobin." They have:

No nucleus → cannot synthesise new proteins or enzymes
No mitochondria → cannot perform oxidative phosphorylation
No endoplasmic reticulum → cannot repair damaged proteins

This means that:

All ATP must come from anaerobic glycolysis (Embden-Meyerhof pathway)
All NADPH must come from the pentose phosphate pathway (hexose monophosphate shunt)
Enzymes present at the time of RBC maturation are all they will ever have — and these enzymes gradually degrade over the 120-day RBC lifespan

Red cell metabolism relies on two main pathways ^[1]:

Pathway	Primary Product	Primary Function	Key Enzyme Deficiency
Embden-Meyerhof (Glycolysis)	ATP	Maintain cell volume, membrane integrity, ion gradients (Na⁺/K⁺-ATPase)	Pyruvate kinase deficiency (2nd most common RBC enzymopathy)
Pentose Phosphate Pathway (HMP shunt)	NADPH	Regenerate reduced glutathione (GSH) → protect against oxidative stress	G6PD deficiency (most common RBC enzymopathy)

Glucose-6-phosphate
        │
        ▼  ← G6PD (rate-limiting step)
6-Phosphogluconolactone + NADPH
        │
        ▼  ← 6-Phosphogluconate dehydrogenase
Ribulose-5-phosphate + NADPH

G6PD catalyses the first and rate-limiting step: oxidation of glucose-6-phosphate to 6-phosphogluconolactone, simultaneously reducing NADP⁺ → NADPH
NADPH is then used by glutathione reductase to regenerate reduced glutathione (GSH) from oxidised glutathione (GSSG)

                NADPH (from G6PD)
                     │
                     ▼
GSSG (oxidised) ──────────→ 2 GSH (reduced)
     ↑        Glutathione reductase
     │
Reactive oxygen species (H₂O₂, superoxide)
     │
     ▼
H₂O (detoxified) ←──────── GSH
              Glutathione peroxidase

GSH is the principal antioxidant defence in RBCs
It neutralises reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂) and superoxide radicals
Without GSH, ROS accumulate → oxidise haemoglobin → denatured haemoglobin precipitates as Heinz bodies → RBC membrane damage → haemolysis

First Principles – Why Only RBCs?

G6PD is expressed in all cells, not just RBCs. However, most nucleated cells have alternative sources of NADPH (e.g., NADP⁺-dependent isocitrate dehydrogenase, malic enzyme). Mature RBCs lack mitochondria and a nucleus, so the pentose phosphate pathway is their ONLY source of NADPH. This is why G6PD deficiency predominantly manifests as a red cell disease.

4. Aetiology

Gene: G6PD gene on Xq28 (long arm of X chromosome)
Inheritance: X-linked recessive (XLR) ^[1]^[2]
- Hemizygous males (XᴳY) are fully affected
- Heterozygous females (XᴳXⁿ) are carriers — but may be symptomatic due to lyonization
- Homozygous females (XᴳXᴳ) are fully affected — uncommon
Over 200 mutations have been described — most are missense point mutations (single amino acid substitutions) rather than large deletions
- Why? Because complete absence of G6PD is likely embryonically lethal — the pentose phosphate pathway is essential for nucleotide synthesis (ribose-5-phosphate) during cell division. Only partial loss of function is compatible with life.

Variant	Mutation	WHO Class	Residual Activity	Geography	Clinical Severity
G6PD B	Wild-type	IV	100%	Universal	Normal
G6PD A⁻	202G→A	III	10–60%	Africa	Mild–moderate
G6PD Canton	1376G→T	II	* < 10%*	Southern China/HK	Moderate–severe
G6PD Mediterranean	563C→T	II	< 10%	Mediterranean	Severe
G6PD Kaiping	1388G→A	II	< 10%	Southern China	Moderate–severe

In Hong Kong and Southern China, the most common variant is G6PD Canton (1376G > T, Class II) ^[1]^[2]

The WHO classification of G6PD variants (Classes I–V) reflects residual enzyme activity and clinical phenotype ^[1]^[2]:

WHO Class	Residual G6PD Activity	Clinical Phenotype
Class I	* < 10%*	Chronic non-spherocytic haemolytic anaemia (CNSHA) — haemolysis even without triggers
Class II	* < 10%*	Intermittent haemolysis only — triggered by oxidant exposure
Class III	10–60%	Intermittent haemolysis only — usually milder
Class IV	60–100%	No clinical significance (normal)
Class V	* > 100%*	No clinical significance

Why do Class I and Class II both have < 10% activity but different phenotypes?

The difference lies in the stability of the enzyme variant. Class I variants produce an enzyme so unstable that it degrades very rapidly even in young RBCs and reticulocytes, leading to chronic haemolysis. Class II variants produce an enzyme with a short half-life but with enough activity in young RBCs to avoid chronic haemolysis — episodes only occur when an oxidant stress overwhelms the already limited reserves.

G6PD deficiency provides protection against Plasmodium falciparum malaria ^[3]^[4]
The parasitised G6PD-deficient RBC is more susceptible to oxidative damage → early phagocytosis of the infected cell before the parasite can mature
This explains the high prevalence in malaria-endemic regions
Other conditions that similarly protect against malaria include: thalassaemia, HbS (sickle cell), HbC, HbE

5. Pathophysiology

Oxidant exposure (infection, drugs, fava beans) generates reactive oxygen species (ROS) within the RBC
Normally, ROS would be neutralised by GSH (reduced glutathione)
In G6PD deficiency, NADPH cannot be regenerated → GSH cannot be regenerated from GSSG
ROS accumulate → oxidise haemoglobin → methaemoglobin → further oxidation produces haemichromes → these precipitate as insoluble aggregates called Heinz bodies
Heinz bodies attach to the inner surface of the RBC membrane → splenic macrophages "bite off" the portion of membrane containing Heinz bodies → produces characteristic bite cells (aka degmacytes, from Greek "degma" = bite) and blister cells
Loss of membrane → reduced surface-area-to-volume ratio → RBCs become rigid and spherocytic → trapped and destroyed in the reticuloendothelial system (RES) → extravascular haemolysis
If oxidative damage is severe enough, RBC membrane integrity is completely compromised → intravascular haemolysis with haemoglobinaemia and haemoglobinuria

Haemolysis triggers an appropriate reticulocyte response. Reticulocytes and young RBCs have HIGH G6PD activity (because they are newly produced and the enzyme has not yet degraded). These young cells are RESISTANT to oxidant stress. Therefore, haemolysis typically resolves within ~1 week even with continuous oxidant exposure. ^[2]

This is a crucial concept:

G6PD enzyme activity declines over the lifespan of the RBC (normal half-life ~62 days)
The oldest RBCs have the lowest G6PD activity → they are destroyed first
The compensatory reticulocytosis floods the circulation with young, G6PD-rich cells
This is why in Class II/III variants, haemolysis is self-limiting and does not persist indefinitely

Exam Pearl – Self-Limiting Haemolysis

A common exam trap: during an acute haemolytic episode, the G6PD assay may be FALSELY NORMAL because the old, deficient cells have been destroyed and replaced by young reticulocytes with higher enzyme activity. Always repeat the G6PD assay 2–3 months after the acute episode to confirm the diagnosis.

Neonatal jaundice is the most clinically significant manifestation in the neonatal period ^[5]^[6]
The mechanism is two-fold:
1. Increased bilirubin production: haemolysis → ↑ unconjugated bilirubin
2. Decreased bilirubin conjugation: G6PD deficiency is associated with impaired hepatic conjugation of bilirubin (the neonatal liver is already immature, and G6PD deficiency further impairs the NADPH-dependent processes in hepatocytes)
The neonatal jaundice in G6PD deficiency is typically unconjugated hyperbilirubinaemia
Can progress to kernicterus (bilirubin encephalopathy) if severe and untreated — this is a devastating, preventable complication

6. Classification

As detailed above ^[2]:

Class	Enzyme Activity	Clinical Significance
I	< 10%	Chronic non-spherocytic haemolytic anaemia
II	< 10%	Intermittent acute haemolysis
III	10–60%	Intermittent acute haemolysis (usually milder)
IV	60–100%	Normal, no clinical significance
V	> 100%	Normal, no clinical significance

Note: In 2022, the WHO updated the classification to a simplified system with 3 groups (A: severely deficient, B: moderately deficient, C: normal/near-normal). However, the traditional 5-class system remains widely used in HKUMed teaching and examinations. Know both.

Pattern	Description	Typical Variants
Acute haemolytic anaemia	Episodes triggered by drugs, infection, fava beans	Class II (Canton, Mediterranean), Class III (A⁻)
Neonatal jaundice	Severe unconjugated hyperbilirubinaemia in newborns	Any variant, especially Class II
Chronic non-spherocytic haemolytic anaemia (CNSHA)	Ongoing haemolysis without triggers	Class I (rare, severe variants)
Asymptomatic	Detected only on screening	Class III–V

7. Triggers of Haemolysis

This is one of the most clinically important and most examined aspects of G6PD deficiency.

All triggers share a common mechanism: they generate oxidative stress within the RBC, overwhelming the already-compromised antioxidant defences.

Oxidising drugs that precipitate haemolysis in G6PD deficiency — must be memorised ^[2]^[7]:

Category	Specific Drugs	Mechanism
Anti-malarials	Primaquine, chloroquine (high dose), dapsone	Generate ROS via redox cycling
Antibiotics	Co-trimoxazole (TMP-SMX), nitrofurantoin, nalidixic acid, sulfonamides	Sulfone/sulfonamide groups generate free radicals
Analgesics	Aspirin (high dose), phenacetin	Oxidant metabolites
Others	Rasburicase, methylene blue, naphthalene (mothballs)	Direct oxidant action

High Yield – Two Haematology Drugs Requiring G6PD Check

Two drugs used in haematology where you MUST check G6PD status before starting ^[7]:

Co-trimoxazole — used for PJP (Pneumocystis jirovecii pneumonia) prophylaxis in immunocompromised patients
Rasburicase — a recombinant urate oxidase used for tumour lysis syndrome prophylaxis/treatment; it generates H₂O₂ as a byproduct, which is directly toxic to G6PD-deficient RBCs

Both are absolutely contraindicated in G6PD deficiency.

Fava beans (broad beans, 蠶豆) — contain the oxidant compounds vicine and convicine, which are hydrolysed to divicine and isouramil, potent generators of ROS
- This is why the condition is called "favism" or 蠶豆症
- Not all G6PD-deficient individuals react to fava beans — the response depends on the variant, individual susceptibility, and amount consumed
- Even breast milk from a mother who has eaten fava beans can trigger haemolysis in a G6PD-deficient neonate

Infection is the most common trigger of haemolytic episodes globally ^[2]
Mechanism: activated neutrophils during infection undergo a respiratory burst (oxidative burst), generating large amounts of H₂O₂ and superoxide radicals → these diffuse into nearby RBCs → oxidative damage
Common infections: pneumonia, hepatitis, typhoid, urinary tract infections

Naphthalene (mothballs) — a classic exam trigger; babies placed in clothes stored with mothballs
Henna — topical application in some cultures
Diabetic ketoacidosis — the metabolic acidosis generates oxidative stress
Neonatal period itself — the transition from fetal to neonatal circulation involves increased oxygen exposure → oxidative stress

8. Clinical Features

Most individuals with G6PD deficiency are completely asymptomatic between episodes of haemolysis ^[2]. The clinical features appear when the patient is exposed to an oxidant trigger. The exception is Class I variants (chronic non-spherocytic haemolytic anaemia), which manifest with ongoing haemolysis.

Symptom	Pathophysiological Basis
Sudden onset of jaundice (yellowish skin and sclera)	Occurs 2–4 days after drug/oxidant exposure ^[2]. Haemolysis → ↑ unconjugated bilirubin → bilirubin deposits in skin and sclera. Onset is delayed because it takes time for the oxidant to accumulate, damage enough RBCs, and for bilirubin to rise.
Pallor	Anaemia from RBC destruction → ↓ haemoglobin → ↓ skin/mucosal perfusion
Dark urine (coca-cola/tea-coloured)	Haemoglobinuria ^[2] — intravascular haemolysis → free haemoglobin in plasma exceeds haptoglobin binding capacity → filtered by kidneys → dark urine. Also urobilinogen (from bilirubin metabolism) contributes.
Fatigue, dizziness, dyspnoea on exertion	Anaemia → ↓ oxygen delivery to tissues → compensatory ↑ heart rate, ↑ respiratory rate
Back pain / loin pain	Free haemoglobin is toxic to renal tubules → acute tubular injury. Also massive haemolysis causes "haemolysis-associated back pain" possibly related to RBC debris and splenic distension.
Abdominal pain	Splenic sequestration of damaged RBCs → splenic distension
Fever	Often the precipitating infection itself causes fever; also haemolysis per se can cause low-grade fever due to release of inflammatory mediators from destroyed RBCs
Neonatal jaundice (in newborns)	Typically presents day 2–3 of life; combination of ↑ bilirubin production (haemolysis) + ↓ bilirubin conjugation (immature liver + G6PD-related hepatocyte dysfunction)

Sign	Pathophysiological Basis
Jaundice (scleral icterus first)	Bilirubin has high affinity for elastin in the sclera → earliest visible sign of jaundice, detectable at bilirubin > ~35 µmol/L
Pallor (conjunctival, palmar)	↓ Haemoglobin → ↓ colour in well-perfused mucous membranes
Tachycardia	Compensatory response to anaemia → ↑ cardiac output to maintain oxygen delivery
Hypotension (in severe cases)	Massive haemolysis → haemodynamic instability
Splenomegaly (mild, in chronic variants)	Spleen is the site of extravascular haemolysis; in Class I (chronic haemolysis), chronic work hypertrophy of splenic macrophages → mild splenomegaly
Hepatomegaly (uncommon)	RES activation in liver (Kupffer cells phagocytosing damaged RBCs)
No splenomegaly in acute episodic variants	Haemolysis is self-limiting and transient → no chronic splenic workload

Neonatal jaundice is the most important clinical manifestation in the paediatric setting ^[5]^[6]:

Typically presents within the first week of life (often day 2–5)
May present without an obvious trigger — the neonatal transition itself (↑ O₂ exposure, immature antioxidant systems, immature liver conjugation) is sufficient
Jaundice in G6PD-deficient neonates is more severe and prolonged than physiological jaundice
Risk of kernicterus (bilirubin encephalopathy) if unconjugated bilirubin crosses the blood-brain barrier → deposits in basal ganglia, hippocampus, cranial nerve nuclei
- Early signs: lethargy, poor feeding, high-pitched cry, hypotonia
- Late signs: opisthotonus, seizures, hearing loss, cerebral palsy

High Yield – Neonatal Jaundice Pathogenesis in G6PD Deficiency

The hyperbilirubinaemia in G6PD-deficient neonates is NOT purely from haemolysis. Studies show that impaired hepatic bilirubin conjugation (via UGT1A1 enzyme, which requires NADPH-dependent processes) is a major contributor. This is why some G6PD-deficient neonates develop severe jaundice without overt evidence of haemolysis (normal reticulocyte count, normal haemoglobin). Co-inheritance of a UGT1A1 promoter polymorphism (Gilbert syndrome) further increases the risk ^[6].

Rare, severe G6PD variants with < 10% activity and extreme enzyme instability
Presentation:
- Chronic anaemia from birth
- Persistent reticulocytosis
- Neonatal jaundice (often severe)
- Splenomegaly
- Gallstones (pigment stones from chronic bilirubin turnover)
- Folate deficiency (due to ↑ erythropoietic demand consuming folate stores)
- Iron overload (from chronic haemolysis → ↑ iron absorption, ± transfusional iron)

These are part of the clinical picture and often asked in exams:

Finding	Pathophysiological Basis
Heinz bodies (seen with supravital staining, e.g., crystal violet)	Precipitated denatured haemoglobin aggregates within the RBC
Bite cells (degmacytes)	Splenic macrophages "bite" out portions of RBC membrane containing Heinz bodies, leaving a semicircular defect
Blister cells	Variant of bite cell where the Heinz body is removed leaving a clear "blister" area
Spherocytes	Loss of membrane surface area (from splenic pitting) → ↓ surface-area-to-volume ratio → spherical shape
Polychromasia	Reticulocytes stain bluish-grey due to residual RNA → indicates compensatory marrow response
Schistocytes (rare, if severe)	Fragmentation from severe intravascular damage

9. Important Associations and Context

When encountering elevated unconjugated bilirubin, always consider ^[8]:

Increased production — haemolysis (including G6PD deficiency), ineffective erythropoiesis
Decreased conjugation — Gilbert syndrome (UGT1A1 polymorphism), Crigler-Najjar syndrome
Combined — G6PD-deficient neonates (haemolysis + impaired conjugation)

Every prescriber in Hong Kong should check G6PD status before prescribing known oxidant drugs. The hospital system flags G6PD-deficient patients in their electronic medical records. This is a patient safety issue.

Methemoglobinaemia: G6PD deficiency impairs NADPH-dependent methemoglobin reductase → reduced capacity to convert methaemoglobin back to functional haemoglobin. Treatment of methaemoglobinaemia with methylene blue is contraindicated in G6PD deficiency (methylene blue requires NADPH from G6PD to act as an electron carrier) ^[9]
Gallstones: chronic haemolysis → ↑ bilirubin → pigment gallstones (especially in Class I variants and chronic haemolysis of any cause including thalassaemia)

Clinical Feature	Pathophysiology Chain
Jaundice	↓G6PD → ↓NADPH → ↓GSH → oxidative damage → haemolysis → ↑unconjugated bilirubin → jaundice
Dark urine	Intravascular haemolysis → free Hb in plasma → haemoglobinuria
Pallor/anaemia	RBC destruction → ↓Hb
Self-limiting haemolysis	Old RBCs (low G6PD) destroyed first → reticulocytosis → young RBCs have high G6PD → resistant → haemolysis stops ~1 week
Bite cells	Heinz bodies pitted by splenic macrophages → membrane loss
Neonatal jaundice	Haemolysis + impaired hepatic conjugation → severe unconjugated hyperbilirubinaemia
Kernicterus	Unconjugated bilirubin crosses immature BBB → basal ganglia deposition → neurological damage

High Yield Summary

G6PD deficiency is the most common RBC enzymopathy worldwide (~400M affected), inherited in an X-linked recessive pattern
In Hong Kong: 4.5% males, 0.5% females affected; most common variant is G6PD Canton (1376G > T, Class II)
G6PD is the rate-limiting enzyme of the pentose phosphate pathway, the sole source of NADPH in mature RBCs → required to regenerate reduced glutathione (GSH) → the principal RBC antioxidant
Without NADPH/GSH, oxidant exposure → Heinz bodies (precipitated denatured Hb) → bite cells → extravascular haemolysis (± intravascular if severe)
Haemolysis is self-limiting (~1 week) because reticulocytes have high G6PD activity
Triggers: infection (most common), drugs (primaquine, co-trimoxazole, rasburicase, dapsone, sulfonamides), fava beans, naphthalene (mothballs)
Always check G6PD before prescribing co-trimoxazole or rasburicase
Neonatal jaundice can be severe → risk of kernicterus; mechanism involves BOTH haemolysis AND impaired hepatic conjugation
G6PD assay may be falsely normal during acute haemolysis (young cells with high activity) → repeat 2–3 months later
G6PD deficiency confers protection against P. falciparum malaria (heterozygote advantage)

Active Recall - G6PD Deficiency (Definition, Epidemiology, Pathophysiology, Clinical Features)

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.376, Red Cell Enzymopathies / G6PD Deficiency) ^[2] Senior notes: Ryan Ho Haemtology.pdf (p.40, Section 2.5.2.2 – G6PD Deficiency) ^[3] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Malaria section – conditions protecting against malaria) ^[4] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (Malaria section – conditions protecting against malaria) ^[5] Lecture slides: GC 157. Paediatric Chemical Pathology.pdf ^[6] Lecture slides: Unconjugated hyperbilirubinemia in newborns ≥35 weeks of gestation_ Etiology and pathogenesis - UpToDate.pdf ^[7] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (p.16, Rasburicase and Co-trimoxazole in G6PD) ^[8] Senior notes: Block A - Introduction to GI_Hepatology investigations (LFT, Endoscopy).pdf (p.8, Unconjugated bilirubin causes) ^[9] Senior notes: Block A - Many members of the family have anaemia.pdf (p.8, Methemoglobinaemia and G6PD)

Differential Diagnosis of G6PD Deficiency

G6PD deficiency does not present with a single, unique clinical picture. Instead, it manifests through three main clinical syndromes — each of which has its own broad differential:

Acute haemolytic anaemia (the most common presentation in older children/adults)
Neonatal jaundice (the most clinically important presentation in neonates)
Chronic non-spherocytic haemolytic anaemia (CNSHA) (rare, Class I variants)

The differential diagnosis must therefore be structured around these presenting syndromes, because when a patient arrives, you do not yet know the cause — you are working backwards from clinical features to diagnosis.

3. Differential Diagnosis of Acute Haemolytic Anaemia

When a patient (typically a young male in Hong Kong) presents with sudden onset of jaundice, pallor, dark urine, and anaemia — especially 2–4 days after a drug exposure or infection — the working diagnosis is acute haemolysis. The differential includes:

These are defects inherent to the RBC itself. Except for paroxysmal nocturnal haemoglobinuria (PNH), they are all inherited.

Condition	Mechanism	Key Differentiating Features
G6PD deficiency	↓NADPH → ↓GSH → oxidative damage → Heinz bodies, bite cells	XLR inheritance; episodic haemolysis triggered by oxidant exposure; Heinz bodies and bite cells on blood film; self-limiting in ~1 week ^[2]
Pyruvate kinase (PK) deficiency	↓ATP → RBC energy failure → chronic haemolysis	AR inheritance; chronic Coombs-negative haemolytic anaemia from birth; better tolerated due to ↑2,3-BPG (right shift of O₂-Hb curve); no Heinz bodies; echinocytes on film ^[1]^[2]
Hereditary spherocytosis (HS)	Membrane protein defects (spectrin, ankyrin, band 3) → ↓surface area → spherocytes → splenic trapping	AD inheritance (most common); spherocytes on film; ↑MCHC; positive osmotic fragility test or eosin-5-maleimide (EMA) binding test; splenomegaly; Coombs-negative
Hereditary elliptocytosis (HE)	Spectrin/protein 4.1 defects → elliptical RBCs → variable haemolysis	AD; elliptocytes > 25% on film; usually mild haemolysis
Unstable haemoglobinopathies (e.g., Hb Köln)	Unstable Hb chains precipitate → Heinz bodies → extravascular haemolysis	AD inheritance; Heinz bodies present (like G6PD); isopropanol/heat instability test positive; triggered by infections ^[9]
Sickle cell disease	HbS polymerises under deoxygenation → sickle-shaped RBCs	Sickling on film; Hb electrophoresis shows HbS; vaso-occlusive crises; very rare in Hong Kong Chinese (mainly in African populations)
Paroxysmal nocturnal haemoglobinuria (PNH)	Acquired somatic mutation in PIGA → loss of GPI-anchored complement regulators (CD55, CD59) → complement-mediated intravascular haemolysis	Acquired, not inherited; intravascular haemolysis; haemoglobinuria (classically morning); flow cytometry shows loss of CD55/CD59; associated with aplastic anaemia and thrombosis ^[10]

High Yield – How to differentiate Heinz body causes

Both G6PD deficiency and unstable haemoglobinopathies (e.g., Hb Köln) produce Heinz bodies. However:

In G6PD deficiency: Heinz bodies appear only during acute episodes and disappear between episodes (as the enzyme-deficient cells are cleared)
In unstable haemoglobinopathies: Heinz bodies may be present chronically and are detected by the isopropanol stability test or heat instability test (which precipitates unstable Hb)
G6PD assay is definitive for G6PD deficiency; Hb electrophoresis + instability test for unstable Hb ^[9]

These are external forces acting on an otherwise normal RBC.

Condition	Mechanism	Key Differentiating Features
Autoimmune haemolytic anaemia (AIHA) — warm or cold type	Autoantibodies (IgG warm / IgM cold) coat RBCs → phagocytosis (warm) or complement-mediated lysis (cold)	Coombs test (DAT) positive — this is THE key differentiator; G6PD deficiency is Coombs-NEGATIVE; spherocytes on film; associated with SLE, lymphoma, CLL, drugs
Drug-induced immune haemolysis	Drug acts as hapten or triggers autoantibody production → immune-mediated RBC destruction	Positive DAT; temporal relationship to drug (e.g., penicillin, cephalosporins, methyldopa); can mimic G6PD if the drug also happens to be an oxidant
Transfusion reaction (acute haemolytic)	ABO-incompatible transfusion → complement activation → intravascular haemolysis	Temporal relationship to transfusion; positive DAT; fever, back pain, hypotension, haemoglobinuria
Microangiopathic haemolytic anaemia (MAHA)	Mechanical shearing of RBCs through damaged microvasculature (TTP, HUS, DIC, malignant HT, pre-eclampsia)	Schistocytes on film (fragmented RBCs); thrombocytopaenia; ↑LDH; Coombs-negative; NO Heinz bodies
Infection-related haemolysis	Direct parasitisation of RBCs (malaria), toxin-mediated lysis (Clostridium perfringens), or oxidative stress from infection	Thick/thin film for malaria; blood cultures; travel history; note that infection is also the most common TRIGGER of G6PD haemolysis, so these can coexist ^[2]
Wilson disease — fulminant hepatic failure	Massive copper release → Coombs-negative haemolytic anaemia + acute liver failure	Young patient with unexplained acute liver failure + Coombs-negative haemolysis; low caeruloplasmin; KF rings on slit lamp; very high 24h urine copper ^[11]
Toxins / chemicals	Arsine gas, lead, copper, spider/snake venom → direct RBC damage	Occupational/exposure history; basophilic stippling (lead); specific toxicology

Exam Pearl – Coombs Test is Key

The Direct Antiglobulin Test (DAT / Coombs test) is the single most important initial test to differentiate the cause of haemolysis:

DAT positive → immune-mediated haemolysis (AIHA, drug-induced, transfusion reaction)
DAT negative → intrinsic RBC defect (G6PD deficiency, PK deficiency, HS, haemoglobinopathy) OR non-immune extrinsic cause (MAHA, infection, toxin, PNH, Wilson disease)

G6PD deficiency is always Coombs-negative ^[1]^[2]. If you have a Coombs-positive haemolysis, look for AIHA, not G6PD.

4. Differential Diagnosis of Neonatal Jaundice (Unconjugated Hyperbilirubinaemia)

When a neonate presents with jaundice in the first week of life, the differential is broad. G6PD deficiency is an important cause of severe neonatal jaundice, particularly in Hong Kong given the 4.5% male prevalence ^[5]^[6].

Mechanism	Conditions	How to Distinguish from G6PD
Physiological jaundice	Immature hepatic conjugation + ↑RBC turnover in all neonates	Mild, peaks day 3–5, resolves by day 7–10 in term infants; bilirubin usually < 200 µmol/L; no anaemia; normal G6PD assay
Breast milk jaundice	β-glucuronidase in breast milk deconjugates bilirubin in gut → ↑enterohepatic circulation	Late onset (after day 4–7), prolonged but benign; no anaemia; normal G6PD
ABO incompatibility	Maternal anti-A or anti-B IgG crosses placenta → haemolysis	Mother O, baby A or B; DAT positive (usually weakly); spherocytes on film; early onset (day 1); G6PD normal
Rh isoimmunisation	Maternal anti-D IgG crosses placenta → severe haemolysis	Mother Rh-negative, baby Rh-positive; strongly DAT positive; severe anaemia, hydrops in worst cases; G6PD normal
G6PD deficiency	Haemolysis + impaired hepatic conjugation	Newborn screening positive; male predominance; may or may not have evidence of haemolysis (reticulocytosis, anaemia); often NO obvious trigger ^[5]^[6]
Gilbert syndrome	UGT1A1 promoter polymorphism → ↓bilirubin conjugation	Mild unconjugated hyperbilirubinaemia; no haemolysis; no anaemia; usually presents in adolescence, but co-inheritance with G6PD deficiency dramatically increases risk of severe neonatal jaundice ^[12]
Crigler-Najjar syndrome (Type I and II)	Absent (Type I) or severely reduced (Type II) UGT1A1 activity → very high unconjugated bilirubin	Type I: presents in first days, severe (bilirubin > 340 µmol/L), unresponsive to phenobarbital; Type II: responds to phenobarbital
Cephalhaematoma / birth trauma	Extravascular blood → reabsorption of haemoglobin → ↑bilirubin production	Visible swelling; limited by suture lines (cephalhaematoma) or not (subgaleal haemorrhage); no ongoing haemolysis
Polycythaemia	↑RBC mass → ↑bilirubin production from normal RBC turnover	Haematocrit > 65%; plethoric appearance; SGA or IDM neonate
Sepsis	Infection → haemolysis + impaired hepatic function + ↑bilirubin	Unwell neonate; fever/hypothermia; feeding difficulties; ↑CRP/WCC
Hypothyroidism (congenital)	↓Thyroid hormones → ↓hepatic conjugation + ↓GI motility (↑enterohepatic circulation)	Prolonged jaundice; detected on newborn screening (↑TSH)

High Yield – Neonatal Jaundice and G6PD Co-Inheritance with Gilbert Syndrome

Co-inheritance of G6PD deficiency AND Gilbert syndrome (UGT1A1 polymorphism) dramatically increases the risk of severe neonatal jaundice and kernicterus. This is because:

G6PD deficiency → ↑bilirubin production (haemolysis) + ↓hepatic NADPH-dependent conjugation
Gilbert syndrome → ↓UGT1A1 activity → further ↓conjugation
The "double hit" overwhelms the neonatal bilirubin clearance system ^[6]^[12]

Timing	Likely Causes
< 24 hours (pathological — always investigate)	Rh/ABO isoimmunisation, congenital infection (TORCH), G6PD (if triggered in utero)
Day 2–5	Physiological jaundice, G6PD deficiency, ABO incompatibility, sepsis, cephalhaematoma
> 2 weeks (prolonged jaundice)	Breast milk jaundice, hypothyroidism, Crigler-Najjar, biliary atresia (but this causes conjugated hyperbilirubinaemia — different entity), G6PD (if ongoing trigger)

This differential is relevant when considering the rare Class I G6PD variants that cause chronic non-spherocytic haemolytic anaemia (CNSHA).

Condition	Key Differentiating Features
G6PD deficiency (Class I)	Chronic Coombs-negative haemolysis; Heinz bodies intermittently; G6PD assay shows very low activity even between episodes ^[2]
Pyruvate kinase deficiency	AR inheritance; chronic Coombs-negative haemolysis; echinocytes on film; NO Heinz bodies; PK activity assay diagnostic; better tolerated due to ↑2,3-BPG ^[1]^[2]
Hereditary spherocytosis	AD; spherocytes + ↑MCHC; positive EMA binding test / osmotic fragility; splenomegaly
Thalassaemia (intermedia/major)	Hypochromic microcytic anaemia; target cells, basophilic stippling; Hb electrophoresis shows ↑HbA2/HbF (β-thal) or HbH inclusions (α-thal) ^[13]^[14]
Chronic AIHA	DAT positive; spherocytes; associated with SLE, lymphoma
Sickle cell disease	HbS on electrophoresis; sickle cells; vaso-occlusive phenotype
Unstable haemoglobinopathies (e.g., Hb Köln)	Heinz bodies chronically present; isopropanol instability test positive; AD inheritance ^[9]

The term "congenital non-spherocytic haemolytic anaemia" (CNSHA) refers to chronic haemolytic anaemia WITHOUT spherocytes — this narrows the differential to RBC enzyme defects (G6PD Class I, PK deficiency) and some unstable haemoglobinopathies [10_sr]

This table directly compares the key differentials for acute haemolysis — the most common exam scenario:

Feature	G6PD Deficiency	AIHA (Warm)	Hereditary Spherocytosis	PK Deficiency	MAHA
Inheritance	XLR	Acquired	AD	AR	Acquired
Onset	Episodic, triggered	Variable	Chronic with crises	Chronic from birth	Acute
DAT (Coombs)	Negative	Positive	Negative	Negative	Negative
Blood film	Bite cells, Heinz bodies, blister cells	Spherocytes, polychromasia	Spherocytes	Echinocytes	Schistocytes
MCHC	Normal	Normal/↑	↑ (characteristic)	Normal	Normal
Trigger	Oxidant stress, infection, fava beans	Often none; SLE, drugs, lymphoma	Infection can worsen	None specific	Underlying condition (TTP, HUS, DIC)
Heinz bodies	Present (during episode)	Absent	Absent	Absent	Absent
Splenomegaly	Usually absent (unless Class I)	Often present	Present	Variable	Variable
Reticulocyte	↑ (appropriate)	↑	↑	↑	↑
Self-limiting	Yes (~1 week)	No (unless trigger removed)	No	No	Depends on cause

Given the ethnic demographics and prevalence patterns in Hong Kong:

Scenario	Most Important Differentials	Why
Male neonate with severe jaundice	G6PD deficiency vs ABO incompatibility vs sepsis vs physiological	G6PD affects 4.5% of HK males; universal newborn screening exists ^[1]^[2]^[5]
Hypochromic microcytic anaemia	Thalassaemia trait vs iron deficiency anaemia	Both extremely common in HK; distinguish by RDW (normal in thal, ↑ in IDA), RBC count (↑ in thal, ↓ in IDA), ferritin, and Hb electrophoresis ^[13]^[14]
Young male with acute jaundice + dark urine after taking Chinese herbal medicine	G6PD deficiency (many Chinese herbal remedies contain oxidant compounds)	High index of suspicion; ask about herbal medicine use
Chronic haemolysis in a child	G6PD Class I, PK deficiency, HS, thalassaemia intermedia	Rare in HK but must consider; G6PD assay, PK assay, EMA binding, Hb electrophoresis

High Yield – Wilson Disease as a Mimic

Wilson disease presenting with fulminant hepatic failure can mimic G6PD deficiency because it causes Coombs-negative haemolytic anaemia. The key clues:

Young patient with no obvious cause for liver failure
Coombs-negative haemolysis + acute liver failure
Low caeruloplasmin, KF rings, high urine copper
The haemolysis is caused by massive release of free copper from necrotic hepatocytes, which is directly toxic to RBC membranes ^[11]

Exam Pearl – The Importance of Repeating the G6PD Assay

During an acute haemolytic episode, the G6PD assay may be FALSELY NORMAL because:

The most deficient (oldest) RBCs have been selectively destroyed
Compensatory reticulocytosis floods blood with young RBCs that have high G6PD activity
This artificially raises the measured enzyme level

Always repeat the G6PD assay 2–3 months after the acute episode to confirm the diagnosis ^[2]

When a patient develops haemolysis after drug exposure, differentiate:

Mechanism	Examples	Differentiating Feature
G6PD-related oxidant haemolysis	Primaquine, dapsone, co-trimoxazole, rasburicase, nitrofurantoin	Requires G6PD deficiency; Coombs-negative; Heinz bodies; dose-dependent; self-limiting ^[7]
Drug-induced immune haemolysis	Penicillin (hapten), cephalosporins, methyldopa (autoantibody), quinidine (immune complex)	Can occur in G6PD-normal patients; Coombs-positive; not self-limiting until drug withdrawn
Direct RBC toxicity	Ribavirin, dapsone (also causes methaemoglobinaemia irrespective of G6PD)	Dose-dependent; Coombs-negative; bite cells may be seen

For acute haemolysis in a Hong Kong patient:

G6PD deficiency — most common enzymopathy; 4.5% male prevalence in HK

AIHA — DAT positive; look for underlying cause (SLE, lymphoma)

Thalassaemia — common in HK but usually presents as chronic microcytic anaemia, not acute haemolysis

MAHA (TTP/HUS/DIC) — schistocytes; thrombocytopaenia; different clinical context

For neonatal jaundice in Hong Kong:

G6PD deficiency — universal newborn screening; check result

Physiological jaundice — mild, self-limiting

ABO incompatibility — mother O, baby A/B; weakly DAT positive

Gilbert syndrome co-inheritance — potentiates G6PD-related jaundice

High Yield Summary – Differential Diagnosis

The differential diagnosis of G6PD deficiency is structured around its three presentation patterns: acute haemolytic anaemia, neonatal jaundice, and chronic haemolytic anaemia (CNSHA)
The DAT (Coombs test) is the single most important test to separate immune (DAT+) from non-immune (DAT−) haemolysis — G6PD deficiency is always Coombs-negative
Heinz bodies and bite cells on blood film point to G6PD deficiency or unstable haemoglobinopathies (e.g., Hb Köln) — differentiate with G6PD assay vs Hb instability test
In Hong Kong, the top differentials for a male neonate with severe jaundice are G6PD deficiency, ABO incompatibility, and sepsis
Co-inheritance of G6PD deficiency + Gilbert syndrome (UGT1A1 polymorphism) dramatically increases risk of severe neonatal jaundice
Wilson disease (Coombs-negative haemolysis + liver failure in young patient) is an important mimic
Drug-induced haemolysis may be G6PD-related (oxidant, Coombs-negative) or immune-mediated (Coombs-positive) — always check both
G6PD assay may be falsely normal during acute haemolysis — repeat in 2–3 months

Active Recall - G6PD Deficiency Differential Diagnosis

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.376, Red Cell Enzymopathies / G6PD Deficiency) ^[2] Senior notes: Ryan Ho Haemtology.pdf (p.39–40, Section 2.5.2 Red Cell Enzymopathies, Section 2.5.2.2 G6PD Deficiency) ^[5] Lecture slides: GC 157. Paediatric Chemical Pathology.pdf ^[6] Lecture slides: Unconjugated hyperbilirubinemia in newborns ≥35 weeks of gestation_ Etiology and pathogenesis - UpToDate.pdf ^[7] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (p.16, Rasburicase and Co-trimoxazole in G6PD) ^[9] Senior notes: Block A - Many members of the family have anaemia.pdf (p.8, Hb Köln and methemoglobinaemia) ^[10] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1468, PNH and aplastic anaemia workup) ^[11] Senior notes: Block A - Patients with non-viral chronic liver diseases.pdf (p.2, Wilson disease and Coombs-negative haemolysis) ^[12] Senior notes: Ryan Ho GI.pdf (p.292, Gilbert syndrome and bilirubin metabolism) ^[13] Senior notes: Block A - Many members of the family have anaemia.pdf (p.6–13, Thalassaemia vs IDA differentiation) ^[14] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (p.5, Hypochromic microcytic anaemia in HK)

Diagnostic Criteria, Diagnostic Algorithm, and Investigation Modalities for G6PD Deficiency

Unlike many conditions, G6PD deficiency does not have a single "diagnostic criteria checklist" like the Jones criteria for rheumatic fever or the ACR/EULAR criteria for SLE. Instead, diagnosis rests on two pillars:

Confirming haemolysis — proving that RBCs are being destroyed at an accelerated rate
Confirming G6PD deficiency — demonstrating reduced enzyme activity as the underlying cause

In the neonatal context, diagnosis may precede any clinical event because of universal newborn screening in Hong Kong.

The diagnostic approach therefore has three layers:

Layer 1: Confirm haemolysis (is this actually haemolysis?)
Layer 2: Characterise the haemolysis (intravascular vs extravascular? Immune vs non-immune?)
Layer 3: Identify G6PD deficiency as the cause (enzyme assay ± molecular testing)

There is no formal international "diagnostic criteria set" for G6PD deficiency, but the practical diagnostic requirements are:

Criterion	Detail
1. Clinical suspicion	Episodic acute haemolytic anaemia triggered by oxidant exposure (drugs, infection, fava beans), OR severe neonatal jaundice, OR chronic haemolytic anaemia (Class I)
2. Laboratory evidence of haemolysis	↑ Reticulocyte count, ↑ unconjugated bilirubin, ↑ LDH, ↓ haptoglobin, ± haemoglobinuria
3. Characteristic blood film	Bite cells (degmacytes), blister cells, ghost/hemi-ghost cells, polychromasia; Heinz bodies on supravital staining ^[15]
4. Negative DAT (Coombs test)	Excludes immune-mediated haemolysis
5. Reduced G6PD enzyme activity	Quantitative G6PD assay showing reduced activity (measured as U/g Hb) — the definitive diagnostic test ^[1]^[2]
6. (Optional) Molecular/genetic confirmation	Identification of specific G6PD mutation (e.g., G6PD Canton 1376G > T) by DNA analysis — not required for clinical diagnosis but useful for family counselling and research

High Yield – When is Diagnosis Made WITHOUT an Acute Episode?

In Hong Kong, the most common scenario for diagnosis is universal newborn screening — the neonate is identified as G6PD-deficient from cord blood BEFORE any clinical event occurs. The parents receive a G6PD deficiency card listing substances to avoid. This is a screening diagnosis, not a clinical diagnosis triggered by symptoms ^[5].

3. Key Investigations — Organised by Diagnostic Layer

These investigations answer: "Are RBCs being destroyed faster than normal?"

Investigation	Expected Finding in G6PD Haemolysis	Why / Mechanism
Complete Blood Count (CBC)	↓ Haemoglobin (normocytic or slightly macrocytic during recovery due to reticulocytosis)	RBC destruction → anaemia. MCV may be normal acutely; in recovery, ↑ reticulocytes (which are larger than mature RBCs) can push MCV up
Reticulocyte count	↑↑ (appropriate reticulocytosis)	Bone marrow responds to anaemia by releasing immature RBCs (reticulocytes) early. This is an "appropriate" response — the marrow is healthy, unlike in aplastic anaemia
Unconjugated (indirect) bilirubin	↑↑	Haem from destroyed RBCs is metabolised to unconjugated bilirubin by the reticuloendothelial system. It is unconjugated because it has not yet been processed by the liver. An elevation of unconjugated bilirubin suggests increased production → haemolysis or G6PD deficiency ^[8]
Lactate dehydrogenase (LDH)	↑↑	LDH is an intracellular enzyme abundant in RBCs (LDH-1 and LDH-2 isoenzymes). When RBCs are destroyed, LDH is released into the plasma. It is a sensitive but non-specific marker of haemolysis
Serum haptoglobin	↓↓ (often undetectable)	Haptoglobin is a plasma protein that binds free haemoglobin. During haemolysis, free Hb is released → binds haptoglobin → Hb-haptoglobin complex is cleared by the liver → serum haptoglobin drops. Haptoglobin is the most sensitive marker of haemolysis — it can be unmeasurably low even with mild haemolysis
Plasma free haemoglobin	↑ (especially in intravascular haemolysis)	When intravascular haemolysis is severe enough that haptoglobin is completely consumed, free Hb circulates in plasma, giving it a pink/red tinge (haemoglobinaemia)
Urine haemoglobin / haemosiderin	Positive (in intravascular haemolysis)	Free Hb filtered by kidneys → haemoglobinuria (dark urine). Over time, renal tubular cells absorb and store iron as haemosiderin → shed into urine (positive urine haemosiderin = chronic/recurrent intravascular haemolysis)

The classic "haemolysis panel": ↑ reticulocyte count + ↑ unconjugated bilirubin + ↑ LDH + ↓ haptoglobin. All four together provide strong evidence of haemolysis.

High Yield – LFT in the Context of Haemolysis

When you see elevated total bilirubin on a liver function test, always ask for bilirubin fractionation (direct/indirect). If the unconjugated fraction is predominantly elevated with normal transaminases, the cause is NOT liver disease but either:

Increased production of unconjugated bilirubin → haemolysis, G6PD deficiency
Decreased conjugation → Gilbert syndrome, Crigler-Najjar ^[8]

This is a common exam trap — don't reflexively think "liver disease" when you see high bilirubin.

These investigations answer: "Is this immune or non-immune? Intravascular or extravascular?"

Investigation	Expected Finding	Interpretation
Direct Antiglobulin Test (DAT / Coombs test)	NEGATIVE	G6PD deficiency causes NON-IMMUNE haemolysis. If DAT is positive, think AIHA, not G6PD. This is the single most important branching point in the haemolysis workup ^[2]^[15]^[16]
Peripheral blood film (PBS)	Bite cells, blister cells, ghost cells, hemi-ghost cells, polychromasia; ± spherocytes ^[15]	Bite cells = splenic macrophages have "bitten out" Heinz bodies from the RBC membrane. Ghost/hemi-ghost cells = severely oxidised RBCs with pale cytoplasm where denatured Hb has been removed (like a "ghost" of the RBC). Polychromasia = reticulocytes staining bluish due to residual RNA → appropriate marrow response
Heinz body preparation (supravital stain)	Heinz bodies PRESENT	Heinz bodies are clumps of denatured, precipitated haemoglobin visible with supravital staining (crystal violet or methyl violet). They are NOT visible on standard Wright-Giemsa stain. They are the pathological hallmark of oxidative damage to Hb. Present during acute G6PD episodes and in unstable haemoglobinopathies ^[15]

Exam Pearl – Heinz Bodies vs Howell-Jolly Bodies

Students frequently confuse these:

Heinz bodies = precipitated denatured haemoglobin, seen ONLY with supravital staining → G6PD deficiency, unstable Hb
Howell-Jolly bodies = nuclear remnants (DNA fragments) visible on standard Wright-Giemsa stain → seen post-splenectomy, hyposplenism

They look superficially similar (both are dark inclusions) but represent completely different pathology and require different staining techniques.

Key peripheral blood film findings in G6PD deficiency ^[15]:

Finding	Appearance	Pathophysiology
Bite cell (degmacyte)	RBC with a semicircular "bite" removed from the edge	Splenic macrophages physically remove the portion of membrane containing Heinz bodies
Blister cell	RBC with a clear area of cytoplasm separated from the haemoglobinised area	Variant of bite cell; Heinz body removed leaving a "blister"
Ghost cell / hemi-ghost cell	Pale RBC with haemoglobin concentrated to one pole	Massive oxidative damage → most Hb denatured and removed; the RBC is essentially an empty shell
Polychromasia	Larger, bluish-grey RBCs on Wright-Giemsa	Reticulocytes with residual RNA; stain blue rather than pink
Heinz bodies	Dark inclusions on supravital stain (crystal violet)	Precipitated denatured haemoglobin

Investigation	Details	Key Points
Quantitative G6PD enzyme assay	Measures G6PD activity in RBC haemolysate, expressed as U/g Hb	THE definitive diagnostic test. Uses a spectrophotometric method: G6PD converts NADP⁺ → NADPH, which absorbs light at 340 nm. Rate of absorbance change = enzyme activity. Normal reference range varies by lab but typically > 7–10 U/g Hb
Qualitative / semi-quantitative screening tests	Fluorescent spot test (Beutler test): a drop of blood is incubated with G6PD substrates; NADPH production is detected by UV fluorescence	Used for newborn screening in Hong Kong (cord blood). Simple and cheap. A spot that does NOT fluoresce = G6PD deficient (no NADPH produced). False negatives can occur during acute haemolysis (reticulocytes have higher G6PD → may fluoresce normally)
Molecular / DNA analysis	PCR-based detection of specific G6PD mutations (e.g., G6PD Canton 1376G > T, G6PD Kaiping 1388G > A)	Not required for routine clinical diagnosis. Useful for: (1) confirming diagnosis in borderline cases, (2) identifying female heterozygotes (enzyme assay may be normal due to lyonization), (3) family counselling, (4) research

High Yield – The G6PD Assay Timing Trap

Do NOT rely on a G6PD assay taken during an acute haemolytic episode.

During acute haemolysis:

The oldest, most G6PD-deficient RBCs are selectively destroyed
Compensatory reticulocytosis produces young RBCs with HIGH G6PD activity
The measured G6PD level in the remaining cells may be falsely NORMAL

Always repeat the G6PD assay 2–3 months after the episode when the reticulocyte count has normalised and the RBC population reflects a normal age distribution ^[2]^[15]

Universal newborn screening for G6PD deficiency has been implemented in Hong Kong since 1984 ^[5].

Aspect	Detail
Sample	Cord blood (collected at delivery)
Method	Quantitative G6PD enzyme assay on cord blood haemolysate; some centres use the fluorescent spot test as initial screen
Cut-off	Activity below a defined threshold (typically < 4.6 U/g Hb for males; female heterozygotes are harder to detect — see below)
Follow-up	Positive screen → confirmatory quantitative assay → if confirmed, parents receive a G6PD deficiency card listing substances to avoid
Coverage	All live births in Hong Kong

High Yield – Limitations of Newborn Screening in Females

Female heterozygotes are difficult to detect by quantitative G6PD assay because their RBC population is a mosaic (due to random X-inactivation / lyonization):

Some RBCs express the normal G6PD allele (high activity)
Some RBCs express the mutant G6PD allele (low activity)
The measured AVERAGE enzyme activity may be within normal range, even though a substantial proportion of cells are deficient

This is why molecular / DNA testing is more reliable for identifying female carriers. However, routine newborn screening uses enzyme assay, so some female carriers will be missed ^[1]^[2].

6. Special Investigation Scenarios

Investigation	Purpose	Expected Finding
Total and direct bilirubin	Confirm unconjugated hyperbilirubinaemia	Predominantly unconjugated (indirect) bilirubin; if direct bilirubin is elevated, think biliary atresia or neonatal hepatitis — different entity
G6PD newborn screening result	Check if already identified	Should be available from cord blood screen in HK
CBC + reticulocyte count	Assess for anaemia and haemolysis	May be normal (jaundice can occur without significant anaemia — impaired conjugation component) ^[6]
DAT (Coombs)	Exclude immune haemolysis (ABO/Rh incompatibility)	Negative in G6PD; positive in ABO/Rh
Maternal and infant blood groups	Screen for ABO/Rh incompatibility	Mother O, baby A/B = at risk for ABO incompatibility
Peripheral blood film	Look for bite cells, spherocytes	Bite cells → G6PD; spherocytes → ABO/Rh or HS
TSH (from newborn screening)	Exclude congenital hypothyroidism	Prolonged jaundice + hypothyroid features
Infection screen (if indicated)	Exclude sepsis as trigger/cause	Blood culture, CRP, WCC

Investigation	Purpose
G6PD assay	Will show very low activity even between episodes (Class I: < 10% with severe instability)
Haemoglobin electrophoresis / HPLC	Exclude thalassaemia and haemoglobinopathies (↑HbA2/HbF in β-thal, HbH in α-thal) ^[13]
EMA binding test / osmotic fragility	Exclude hereditary spherocytosis (positive in HS, negative in G6PD)
PK activity assay	Exclude pyruvate kinase deficiency if G6PD is normal
Isopropanol / heat instability test	Exclude unstable haemoglobinopathies (e.g., Hb Köln)
Iron studies (ferritin, TSAT)	Assess for iron overload from chronic haemolysis or transfusions
Folate level	Chronic haemolysis → ↑ erythropoietic demand → folate depletion

7. Interpretation Guide — Putting It All Together

Parameter	Result	Interpretation
Hb	↓↓ (e.g., 6–8 g/dL)	Significant anaemia from RBC destruction
MCV	Normal or slightly ↑	Normal during acute phase; ↑ if reticulocytosis is brisk (reticulocytes are macrocytic)
WCC	Often ↑	Reactive leukocytosis (stress response, or underlying infection as trigger)
Platelets	Normal or ↑	Reactive thrombocytosis
Reticulocyte count	↑↑ (e.g., 10–20%)	Appropriate marrow response — the marrow is working hard to replace destroyed cells
Unconjugated bilirubin	↑↑	From haem catabolism of destroyed RBCs
LDH	↑↑	Released from lysed RBCs
Haptoglobin	↓↓ (often undetectable)	Consumed by binding free haemoglobin
DAT (Coombs)	Negative	Non-immune haemolysis
PBS	Bite cells, ghost cells, blister cells, polychromasia	Pathognomonic of oxidative haemolysis ^[15]
Heinz body prep	Positive	Denatured Hb precipitates
G6PD assay	May be falsely normal (repeat in 2–3 months)	Young RBCs with high G6PD activity dominate
Urinalysis	Haemoglobinuria (dark urine), ± haemosiderinuria	Intravascular haemolysis component

This is a commonly examined scenario — a case of haemolysis where the Coombs test result is the key pivot ^[16]:

Feature	DAT-Positive (Immune)	DAT-Negative (Non-immune)
Example	AIHA (warm or cold), drug-induced immune haemolysis, transfusion reaction	G6PD deficiency, PK deficiency, HS, MAHA, PNH
Mechanism	Antibody ± complement coating RBCs → phagocytosis or complement lysis	Intrinsic RBC defect (enzyme, membrane, Hb) or mechanical/toxic damage
PBS	Spherocytes ++	Bite cells (G6PD), schistocytes (MAHA), spherocytes (HS), echinocytes (PK def)
Treatment	Steroids, rituximab, splenectomy (for AIHA)	Avoid trigger, transfuse, supportive

High Yield GC Exam Point – Case 6 from Haematology Intro Lecture

From the GC Haematology Introduction to Haematological investigations lecture ^[16]:

A 35-year-old woman with:

Hb 7.5 g/dL, WCC 9.5, Plt 200
PBS: spherocytes ++
Reticulocyte count: 10%
Total bilirubin: 55 µmol/L
LDH: 950 IU/L
DAT: positive for IgG and complement

This is a classic case of warm AIHA (DAT positive for IgG + complement, spherocytes, haemolysis markers elevated).

How does this differ from G6PD?

G6PD: DAT negative, bite cells (not spherocytes), Heinz bodies present, triggered by oxidant exposure
AIHA: DAT positive, spherocytes (not bite cells), no Heinz bodies, often no obvious trigger

The DAT is the key branching investigation.

In the neonatal context, the critical question is not just "is bilirubin elevated?" but "is it dangerously high?" ^[5]^[6]

Aspect	Detail
Bilirubin thresholds	Use hour-specific bilirubin nomograms (e.g., Bhutani nomogram) — a bilirubin of 200 µmol/L at 24 hours is far more concerning than the same level at 72 hours
Risk stratification	G6PD deficiency is listed as a major risk factor for severe hyperbilirubinaemia in all neonatal jaundice guidelines → lowers the phototherapy threshold
Exchange transfusion threshold	If bilirubin rises despite intensive phototherapy and reaches exchange-transfusion threshold (varies by gestational age and risk factors), exchange transfusion is indicated to prevent kernicterus
Why G6PD neonates are high-risk	Dual mechanism: haemolysis + impaired conjugation → bilirubin can rise rapidly and unpredictably ^[6]

Investigation	Category	Key Finding in G6PD	Timing Caveat
CBC	Confirm haemolysis	↓ Hb, ± ↑ MCV	Hb may be normal initially
Reticulocyte count	Confirm haemolysis	↑↑	Peaks 5–7 days after onset
Unconjugated bilirubin	Confirm haemolysis	↑↑	—
LDH	Confirm haemolysis	↑↑	Non-specific (also ↑ in tissue damage)
Haptoglobin	Confirm haemolysis	↓↓	Most sensitive marker
DAT (Coombs)	Characterise	Negative	—
PBS	Characterise	Bite cells, ghost cells, polychromasia	—
Heinz body prep	Characterise	Positive	Only during acute episode
G6PD enzyme assay	Confirm cause	Low activity	May be falsely normal during acute episode — repeat in 2–3 months
Molecular testing	Confirm cause	Specific mutation identified	Not required routinely
Newborn screening	Screening	Low G6PD on cord blood	May miss female heterozygotes

High Yield Summary – Diagnosis of G6PD Deficiency

Diagnosis = clinical suspicion + haemolysis markers + negative DAT + characteristic blood film + G6PD enzyme assay
G6PD enzyme assay (quantitative, spectrophotometric) is the definitive diagnostic test — measures rate of NADPH production from G6PD activity in RBC haemolysate
CRITICAL CAVEAT: G6PD assay may be falsely normal during acute haemolysis due to reticulocytosis — always repeat 2–3 months later ^[2]^[15]
DAT (Coombs) is NEGATIVE in G6PD deficiency — if positive, think AIHA instead
Peripheral blood film: bite cells, ghost/hemi-ghost cells, blister cells, polychromasia; Heinz bodies on supravital staining (crystal violet)
Haemolysis panel: ↑ reticulocytes, ↑ unconjugated bilirubin, ↑ LDH, ↓ haptoglobin
In Hong Kong, universal newborn screening (cord blood G6PD assay) identifies most cases before clinical presentation ^[5]
Female heterozygotes may have normal enzyme assay due to lyonization — molecular testing is more reliable for carrier detection ^[1]^[2]
In neonates, always use hour-specific bilirubin nomograms and remember G6PD deficiency is a major risk factor for severe hyperbilirubinaemia ^[6]

Active Recall - G6PD Deficiency Diagnosis and Investigations

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.376, Red Cell Enzymopathies / G6PD Deficiency) ^[2] Senior notes: Ryan Ho Haemtology.pdf (p.39–40, Section 2.5.2, Section 2.5.2.2 G6PD Deficiency) ^[5] Lecture slides: GC 157. Paediatric Chemical Pathology.pdf ^[6] Lecture slides: Unconjugated hyperbilirubinemia in newborns ≥35 weeks of gestation_ Etiology and pathogenesis - UpToDate.pdf ^[8] Senior notes: Block A - Introduction to GI_Hepatology investigations (LFT, Endoscopy).pdf (p.8, Unconjugated bilirubin causes) ^[13] Senior notes: Block A - Many members of the family have anaemia.pdf (p.6–30, Thalassaemia workup and Hb electrophoresis) ^[15] Senior notes: Block A - Family history of anaemia_ inherited causes of anaemia; haemolytic anaemia; aplastic anaemia.pdf (p.21, G6PD peripheral blood film findings and assay timing) ^[16] Lecture slides: Haematology Introduction to Haematological investigations (CBP, Clotting).pdf (p.30, Case 6 – AIHA with DAT positive)

Management of G6PD Deficiency

G6PD deficiency is an inherited enzymopathy caused by mutations in the G6PD gene on the X chromosome. Because:

Mature RBCs lack a nucleus → they cannot upregulate gene expression or synthesise new enzyme
The defect is germline (present in every nucleated cell) → it cannot be corrected by any currently available pharmacotherapy

Therefore, the management of G6PD deficiency is fundamentally:

Prevention — avoiding oxidant triggers to prevent haemolytic episodes
Supportive — managing acute haemolytic crises when they occur
Monitoring and long-term care — especially for chronic (Class I) variants

There is no specific curative treatment for G6PD deficiency. Management is SUPPORTIVE — blood transfusion, folic acid, and stopping/avoiding offending agents. ^[15]

3. Acute Haemolytic Episode — Detailed Management

The single most important step is to STOP the offending agent immediately ^[15].

Action	Rationale
Stop the offending drug	The drug is generating oxidative stress → removing it halts further ROS production → allows the remaining (younger, more G6PD-rich) RBCs to survive
Treat the underlying infection	Infection is the most common trigger of G6PD haemolysis ^[2] → appropriate antibiotics/antivirals reduce the neutrophil oxidative burst that damages RBCs
Remove environmental exposure	Naphthalene (mothballs), henna, fava beans — remove the source of oxidant

Why Stopping the Drug Works — First Principles

Remember that haemolysis in G6PD deficiency is self-limiting ^[2]. Once the oxidant trigger is removed:

The oldest, most enzyme-depleted RBCs have already been destroyed
Compensatory reticulocytosis produces young RBCs with HIGH G6PD activity
These young cells are resistant to oxidant stress
Haemolysis typically resolves within ~1 week even without specific treatment

Removing the trigger simply prevents ADDITIONAL waves of oxidant damage while the body's natural compensatory mechanism takes over.

Intervention	Indication	Mechanism / Rationale
IV fluid hydration	All patients with significant haemolysis	Maintains renal perfusion; dilutes free haemoglobin in renal tubules; prevents haemoglobin cast formation → reduces risk of acute kidney injury (AKI) from haemoglobinuria
Alkalinisation of urine (sodium bicarbonate in IV fluids)	Considered in severe intravascular haemolysis with haemoglobinuria	Free haemoglobin is more soluble and less toxic to renal tubules at alkaline pH → reduces risk of tubular obstruction and AKI
Monitor urine output	All patients	Early detection of AKI; target urine output > 0.5 mL/kg/h
Monitor renal function (creatinine, urea)	All patients with dark urine / haemoglobinuria	Free haemoglobin is directly nephrotoxic → haemoglobinuric AKI. Rising creatinine warrants nephrology input
Serial Hb monitoring	All patients	Track nadir of anaemia (usually 3–5 days); guide transfusion decisions

Packed RBC transfusion is indicated for severe anaemia or haemodynamic compromise ^[15].

Aspect	Detail
Indication	Symptomatic severe anaemia (Hb < 7 g/dL as general threshold, but clinical judgement applies — transfuse earlier if haemodynamically unstable, ongoing haemolysis, or comorbidities); children and neonates have different thresholds
Product	Packed RBCs (leucodepleted, cross-matched)
Special consideration	The transfused (normal) RBCs have NORMAL G6PD activity → they are resistant to the oxidant stress. This is why transfusion is effective even if the trigger has not yet been fully removed — you are effectively replacing enzyme-deficient cells with enzyme-replete cells
Volume	Typically 10–15 mL/kg in paediatric patients; 1 unit at a time in adults with reassessment
Monitoring	Standard transfusion monitoring; check post-transfusion Hb at 1 hour

High Yield – Transfusion in G6PD Deficiency

There is no contraindication to transfusing standard packed RBCs to G6PD-deficient patients. The donor RBCs are G6PD-normal and will function properly. You do NOT need to request "G6PD-deficient" blood or any special product. Simply use standard leucodepleted, cross-matched packed RBCs.

Folic acid supplementation should be given during and after any haemolytic episode, as in any cause of haemolysis ^[15].

Aspect	Detail
Rationale	Haemolysis → compensatory ↑ erythropoiesis → ↑ folate consumption (folate is a cofactor for DNA synthesis in rapidly dividing erythroid precursors). If folate stores are depleted, the marrow cannot mount an adequate reticulocyte response → megaloblastic crisis
Dose	5 mg PO daily during acute episode; 1–5 mg PO daily for chronic haemolysis
Duration	Throughout the haemolytic episode and for a few weeks after recovery

4. Neonatal Jaundice Management

Neonatal jaundice is the most dangerous manifestation of G6PD deficiency because of the risk of kernicterus ^[5]^[6].

Aspect	Detail
Indication	Bilirubin level above phototherapy threshold on hour-specific bilirubin nomogram (e.g., Bhutani nomogram / NICE guidelines). G6PD deficiency is a recognised risk factor that LOWERS the phototherapy threshold ^[6]
Mechanism	Blue-green light (wavelength 460–490 nm) converts unconjugated bilirubin in the skin into water-soluble photo-isomers (lumirubin) that can be excreted in bile and urine WITHOUT requiring hepatic conjugation. This effectively bypasses the conjugation defect
Types	Conventional phototherapy (single light source); intensive phototherapy (multiple light sources, higher irradiance) for rapidly rising bilirubin
Monitoring	Serial total serum bilirubin every 4–6 hours under phototherapy; monitor Hb (for ongoing haemolysis); ensure adequate hydration (phototherapy increases insensible water loss); shield eyes (risk of retinal damage)

Aspect	Detail
Indication	Bilirubin continues to rise despite intensive phototherapy and reaches exchange-transfusion threshold on nomogram; OR signs of acute bilirubin encephalopathy (lethargy, hypotonia, high-pitched cry, opisthotonus)
Mechanism	Removes the neonate's bilirubin-laden, G6PD-deficient blood and replaces it with donor blood that has: (1) NORMAL G6PD activity, (2) LOW bilirubin, (3) functional albumin with bilirubin-binding capacity. This rapidly reduces circulating bilirubin AND removes the enzyme-deficient cells
Volume	Double-volume exchange (approximately 160 mL/kg in term neonates) — replaces ~85% of the neonate's blood
Risks	Electrolyte disturbances (hypocalcaemia from citrate in donor blood), infection, thrombocytopaenia, line complications, NEC, cardiac arrhythmias
Setting	Must be performed in a neonatal ICU with continuous monitoring

High Yield – Why G6PD Lowers the Phototherapy Threshold

Standard neonatal jaundice guidelines (AAP, NICE) list G6PD deficiency as a "neurotoxicity risk factor" that lowers the bilirubin threshold for phototherapy and exchange transfusion. This is because:

Bilirubin can rise rapidly and unpredictably due to the dual mechanism (haemolysis + impaired conjugation)
G6PD-deficient neonates are at higher risk of kernicterus at any given bilirubin level
The jaundice may recur even after phototherapy if the trigger persists or new triggers emerge ^[6]

For the rare Class I G6PD variants with chronic non-spherocytic haemolytic anaemia (CNSHA):

Intervention	Rationale	Details
Daily folic acid	Chronic ↑ erythropoiesis → chronic folate consumption	1–5 mg PO daily lifelong
Regular transfusions	If chronically symptomatic anaemia or Hb consistently low	Aim to maintain Hb above a target (individualised); monitor for iron overload
Iron chelation therapy	Transfusional iron overload (each unit of packed RBCs contains ~200–250 mg iron; the body has no physiological mechanism to excrete excess iron)	Three chelators: Desferrioxamine (DFO) SC/IV, Deferiprone (DFP) PO, Deferasirox PO ^[17]
Splenectomy	The spleen is the primary site of extravascular haemolysis; removing it can reduce the rate of RBC destruction	Consider if severely symptomatic or transfusion-dependent; defer until > 6 years old to reduce risk of overwhelming post-splenectomy infection (OPSI) ^[2]
Post-splenectomy measures	OPSI is life-threatening — encapsulated organisms (pneumococcus, meningococcus, Haemophilus)	Vaccinations (PCV13/PPSV23, MenACWY, Hib) at least 2 weeks before elective splenectomy; lifelong penicillin prophylaxis (especially in children); patient education about fever management
Monitoring for complications	Gallstones (pigment stones), iron overload, folate deficiency, aplastic crisis (parvovirus B19)	Regular USS abdomen, ferritin, folate levels, reticulocyte count

5.1 Iron Chelation — Why and How

Iron overload is a serious complication of chronic haemolysis and repeated transfusions ^[17].

Chelator	Route	Advantages	Disadvantages / Side Effects
Desferrioxamine (DFO)	SC infusion 3–5 nights/week, or IV	Gold standard; long track record	Compliance is a major issue (requires 8–12h subcutaneous infusion); ototoxicity, retinal changes, ARDS
Deferiprone (DFP)	PO three times daily	Oral; good cardiac iron chelation	Agranulocytosis (monitor WCC); GI upset; arthropathy
Deferasirox	PO once daily	Oral; convenient; once-daily dosing	GI upset, LFT derangement, renal toxicity (↑Cr), hypersensitivity

Iron Overload – First Principles

Why does iron overload occur in chronic haemolysis even WITHOUT transfusion?

Chronic haemolysis → ineffective erythropoiesis → bone marrow signals "need more iron" → suppression of hepcidin (the master iron-regulatory hormone produced by the liver)
Suppressed hepcidin → increased intestinal iron absorption via ferroportin on enterocytes
Over months to years, this leads to iron accumulation in the liver, heart, and endocrine organs
Iron is toxic because it catalyses Fenton/Haber-Weiss reactions: Fe²⁺ + H₂O₂ → Fe³⁺ + OH• + OH⁻ → hydroxyl radicals → tissue damage

Add transfusional iron on top of this, and the overload accelerates dramatically ^[17].

6. Prevention — The Cornerstone of G6PD Management

Prevention is more important than treatment in G6PD deficiency. The goal is to avoid oxidant triggers so that haemolytic episodes never occur in the first place.

Every G6PD-deficient individual (or their parents/guardians) should receive ^[15]:

A G6PD deficiency card — listing the condition and substances to avoid
Verbal and written education about:
- The nature of the condition (lifelong, inherited, X-linked)
- Specific drugs, foods, and chemicals to avoid (see table below)
- Recognition of haemolysis symptoms (jaundice, dark urine, pallor, fatigue)
- When to seek medical attention (dark urine, rapid-onset jaundice, severe pallor)
Medical alert bracelet (optional but recommended for severe variants)

Comprehensive list of oxidant triggers ^[2]^[7]^[15]:

Category	Specific Substances	Why They Are Dangerous
Anti-malarials	Primaquine, dapsone, chloroquine (high-dose)	Generate ROS via redox cycling
Antibiotics	Co-trimoxazole (TMP-SMX), nitrofurantoin, nalidixic acid, sulfonamides	Sulfone moiety generates free radicals
Anti-cancer/haematology	Rasburicase	Generates H₂O₂ as a direct byproduct of uric acid metabolism
Analgesics	Aspirin (high-dose), phenacetin	Oxidant metabolites; low-dose aspirin is generally safe
Chemical agents	Naphthalene (mothballs), methylene blue, henna	Direct oxidant action
Foods	Fava beans (broad beans, 蠶豆)	Contain vicine/convicine → divicine/isouramil → potent ROS generators
Chinese herbal medicines	Various (e.g., those containing Rhizoma Coptidis / 黄连)	Often contain uncharacterised oxidant compounds; always ask about herbal medicine use

High Yield – Two Drugs That MUST Have G6PD Checked Before Starting

Two drugs used in haematology/infectious disease where you must ALWAYS check G6PD status before starting ^[7]:

Co-trimoxazole (Septrin, TMP-SMX) — used for PJP (Pneumocystis jirovecii pneumonia) prophylaxis in immunocompromised patients. The sulphamethoxazole component is a sulphonamide → oxidant haemolysis in G6PD deficiency. Alternative for PJP prophylaxis in G6PD-deficient patients: inhaled pentamidine or atovaquone
Rasburicase — a recombinant urate oxidase used for tumour lysis syndrome (TLS) prophylaxis/treatment. It converts uric acid to allantoin, generating H₂O₂ as a byproduct → directly toxic to G6PD-deficient RBCs. Alternative for TLS in G6PD-deficient patients: allopurinol or febuxostat (xanthine oxidase inhibitors that do not generate H₂O₂)

Both are absolutely contraindicated in G6PD deficiency.

Principle	Detail
Always check G6PD status before prescribing known oxidant drugs	In Hong Kong, the electronic medical record system flags G6PD-deficient patients → prescribers receive an alert
Use safe alternatives when available	E.g., Inhaled pentamidine instead of co-trimoxazole for PJP prophylaxis; allopurinol instead of rasburicase for TLS
Low-dose aspirin is generally safe	The oxidant stress from low-dose aspirin (75–100 mg) is usually insufficient to cause clinically significant haemolysis; high-dose aspirin is avoided
Paracetamol is safe	Does not generate significant oxidative stress at therapeutic doses
Inform the anaesthetic team	Certain anaesthetic agents (e.g., methylene blue used as a surgical dye) are contraindicated

Aspect	Detail
Inheritance pattern	X-linked recessive: affected father (XᴳY) → all daughters are carriers (XᴳXⁿ), all sons are unaffected (XⁿY); carrier mother (XᴳXⁿ) → 50% sons affected, 50% daughters carriers
Family screening	Test siblings of affected individual; consider testing female relatives (may be carriers with skewed lyonization)
Prenatal counselling	Generally NOT considered severe enough to warrant prenatal diagnosis or termination — the condition is manageable with avoidance of triggers. However, families should be informed about the condition

7. Special Management Scenarios

Methylene blue is used as the first-line treatment for acquired methaemoglobinaemia — but it is CONTRAINDICATED in G6PD deficiency.

Aspect	Explanation
Normal mechanism	Methylene blue is reduced by NADPH (produced by G6PD) → leucomethylene blue → reduces metHb back to functional Hb
Problem in G6PD deficiency	Without adequate NADPH, methylene blue CANNOT be reduced → it remains in its oxidised form → paradoxically acts as an oxidant itself → worsens haemolysis AND fails to treat methaemoglobinaemia
Alternative for methaemoglobinaemia in G6PD	Ascorbic acid (vitamin C) as a direct reducing agent (slower, less effective); exchange transfusion in severe cases

Issue	Detail
Primaquine	Used for P. vivax/ovale radical cure (kills liver hypnozoites) — contraindicated in G6PD deficiency (severe haemolysis)
Tafenoquine	Newer 8-aminoquinoline — also contraindicated in G6PD deficiency
Alternative	Chloroquine alone (without primaquine) for suppressive therapy; accept risk of relapse. For P. falciparum: artemisinin-based combination therapy (ACT) is generally safe in G6PD deficiency
Irony	G6PD deficiency protects against severe malaria, yet the drugs used to treat/prevent malaria can cause haemolysis in these patients

Consideration	Detail
Inform the anaesthetic team of G6PD status	Avoid oxidant drugs (methylene blue as surgical dye, some anaesthetic agents)
Avoid hypothermia and acidosis	Both can exacerbate oxidative stress
Post-operative monitoring	Watch for delayed haemolysis (2–4 days post-exposure) if any suspect drug was inadvertently given

This is often more useful clinically than the "avoid" list:

Drug	Safety	Notes
Paracetamol	Safe	No significant oxidant activity
Low-dose aspirin (75–100 mg)	Generally safe	Therapeutic antiplatelet dose is insufficient to cause haemolysis in most variants
Penicillins, cephalosporins	Safe	Not oxidant
Macrolides (azithromycin, clarithromycin)	Safe	Not oxidant
Metformin	Safe	Not oxidant
ACE inhibitors, ARBs, beta-blockers	Safe	Not oxidant
Trimethoprim alone (without sulphamethoxazole)	Safe	It is the sulphamethoxazole component that is the problem, not trimethoprim

Modality	Indication	Contraindications / Caveats
Stop offending agent	ALL acute haemolytic episodes	—
IV fluid hydration	Significant haemolysis, haemoglobinuria	Fluid overload in neonates/cardiac patients
Packed RBC transfusion	Severe anaemia (Hb < 7 g/dL) or haemodynamic compromise	Standard transfusion precautions; no special products needed
Folic acid	All haemolytic episodes; daily for chronic haemolysis ^[15]	—
Phototherapy	Neonatal jaundice above threshold	Shield eyes; monitor hydration
Exchange transfusion	Neonatal jaundice approaching exchange threshold or signs of encephalopathy	NICU setting; risk of electrolyte disturbance
Iron chelation	Transfusion-dependent chronic variants with iron overload	DFO: compliance; DFP: agranulocytosis; Deferasirox: renal/hepatic toxicity ^[17]
Splenectomy	Chronic, severely symptomatic haemolysis (Class I)	Defer until > 6 years old; vaccinate and start penicillin prophylaxis ^[2]
Patient education + avoidance	ALL G6PD-deficient individuals — lifelong	—

High Yield Summary – Management of G6PD Deficiency

No specific curative treatment exists — management is supportive + preventive ^[15]
Acute episode management: Stop offending agent → IV fluids → transfuse if severe → folic acid → monitor renal function
Haemolysis is self-limiting (~1 week) because reticulocytes have high G6PD activity ^[2]
Neonatal jaundice: phototherapy (intensive if needed) → exchange transfusion if approaching threshold or signs of encephalopathy; G6PD deficiency lowers the phototherapy threshold ^[6]
Chronic (Class I): folic acid daily, regular transfusions ± iron chelation, consider splenectomy if severely symptomatic
PREVENTION is the cornerstone: patient education, G6PD card, avoid oxidant drugs/foods/chemicals
Two drugs requiring mandatory G6PD check: co-trimoxazole (PJP prophylaxis) and rasburicase (TLS prophylaxis) — both absolutely contraindicated in G6PD deficiency ^[7]
Methylene blue is CONTRAINDICATED — requires NADPH from G6PD to function; paradoxically worsens haemolysis
Safe drugs include: paracetamol, low-dose aspirin, penicillins, macrolides, trimethoprim alone
Iron chelation for transfusion-dependent patients: DFO (SC), DFP (PO), Deferasirox (PO) ^[17]

Active Recall - G6PD Deficiency Management

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.376, Red Cell Enzymopathies / G6PD Deficiency) ^[2] Senior notes: Ryan Ho Haemtology.pdf (p.39–40, Section 2.5.2, Section 2.5.2.2 G6PD Deficiency) ^[5] Lecture slides: GC 157. Paediatric Chemical Pathology.pdf ^[6] Lecture slides: Unconjugated hyperbilirubinemia in newborns ≥35 weeks of gestation_ Etiology and pathogenesis - UpToDate.pdf ^[7] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (p.16, Rasburicase and Co-trimoxazole in G6PD) ^[15] Senior notes: Block A - Family history of anaemia_ inherited causes of anaemia; haemolytic anaemia; aplastic anaemia.pdf (p.21, G6PD treatment: supportive, blood transfusion, folic acid, stop offending agents) ^[17] Senior notes: Maksim Medicine Notes.pdf (p.159, Iron chelation therapy — DFO, DFP, Deferasirox)

Complications of G6PD Deficiency

G6PD deficiency is often described as a "benign" condition because most patients (Class II/III variants) are asymptomatic between episodes. However, complications can be severe and even life-threatening, particularly:

During acute haemolytic crises — the complications arise from the sudden destruction of RBCs and the downstream effects of free haemoglobin and bilirubin
In the neonatal period — the immature brain is uniquely vulnerable to unconjugated bilirubin
In chronic haemolysis (Class I) — the complications mirror those of any chronic haemolytic anaemia (thalassaemia, hereditary spherocytosis, etc.)

The complications can be conceptualised by their temporal relationship:

Timing	Complications
Acute (during haemolytic episode)	Severe anaemia, haemodynamic shock, AKI, haemoglobinuria, DIC (rare)
Neonatal	Severe neonatal jaundice, kernicterus (bilirubin encephalopathy)
Chronic (Class I variants or recurrent episodes)	Gallstones, iron overload, folate deficiency, aplastic crisis, splenomegaly, growth retardation

2. Acute Complications

Aspect	Detail
Mechanism	Massive, rapid RBC destruction → precipitous drop in Hb (can fall from normal to < 5 g/dL within 24–48 hours in severe cases) → inadequate oxygen delivery to tissues
Why it can be dangerous	If the rate of Hb drop outpaces the body's compensatory mechanisms (↑ heart rate, ↑ cardiac output, ↑ 2,3-BPG), tissue hypoxia ensues → organ dysfunction. Children and the elderly are most vulnerable because of limited cardiac reserve
Clinical features	Tachycardia, hypotension, altered consciousness, lactic acidosis, end-organ damage (myocardial ischaemia, renal hypoperfusion)
Management	Urgent packed RBC transfusion; IV fluids; ICU admission if haemodynamically unstable

This is one of the most important acute complications and the one most commonly examined.

Aspect	Detail
Mechanism	Severe intravascular haemolysis → free haemoglobin exceeds haptoglobin binding capacity → free Hb filtered by glomeruli → precipitates in renal tubules (especially in acidic, concentrated urine) → tubular obstruction and direct oxidative injury to renal tubular epithelial cells → acute tubular necrosis (ATN)
Why free Hb is nephrotoxic	Free haemoglobin releases its haem group → free haem contains iron in the Fe²⁺/Fe³⁺ state → catalyses Fenton reaction (Fe²⁺ + H₂O₂ → Fe³⁺ + OH• + OH⁻) → hydroxyl radicals cause lipid peroxidation and oxidative injury to tubular cells. Free Hb also scavenges nitric oxide (NO) → renal vasoconstriction → further ischaemic injury
Risk factors for AKI	Dehydration (concentrated urine), acidic urine pH (promotes Hb precipitation), pre-existing renal disease, massive haemolysis
Prevention	Aggressive IV fluid hydration to maintain dilute, alkaline urine; some centres add sodium bicarbonate to IV fluids (urinary alkalinisation)
Clinical clue	Dark "coca-cola" coloured urine + rising creatinine + oliguria
Outcome	Usually reversible with adequate hydration; may require temporary renal replacement therapy (haemodialysis) if severe

First Principles – Why Is the Kidney Specifically Vulnerable?

The kidney concentrates urine → as glomerular filtrate passes through the tubules, free haemoglobin becomes progressively MORE concentrated. In acidic urine (pH < 5.5), haemoglobin denatures and precipitates as casts, physically blocking the tubular lumen. Additionally, the proximal tubular cells actively reabsorb filtered Hb via megalin/cubilin receptors → intracellular haem release → iron-mediated oxidative damage. This is why maintaining a dilute, alkaline urine is protective.

Aspect	Detail
Mechanism	Massive intravascular haemolysis → release of RBC membrane phospholipids (particularly phosphatidylserine, normally confined to the inner membrane leaflet) → these act as a procoagulant surface → activate the coagulation cascade → widespread microvascular thrombosis → consumption of clotting factors and platelets → paradoxical bleeding
When to suspect	Falling platelets + prolonged PT/APTT + ↑ D-dimers + bleeding from multiple sites in the context of massive haemolysis
Frequency	Uncommon — only in the most severe, fulminant episodes

3. Neonatal Complications

Neonatal jaundice is the most clinically significant complication in the paediatric setting ^[5]^[6]^[18].

Aspect	Detail
Mechanism	Dual mechanism: (1) ↑ bilirubin production from haemolysis, and (2) impaired hepatic conjugation — the neonatal liver is immature AND G6PD deficiency reduces NADPH availability for UGT1A1-dependent conjugation ^[6]
Severity	More severe and less predictable than physiological jaundice; bilirubin can rise rapidly, sometimes exceeding exchange-transfusion thresholds within hours
Why G6PD neonates are high-risk	G6PD deficiency is listed as a "neurotoxicity risk factor" in AAP/NICE guidelines → lowers phototherapy and exchange transfusion thresholds ^[6]
Monitoring	Hour-specific bilirubin nomograms; serial measurements every 4–6 hours if above threshold

Kernicterus is the most devastating complication of G6PD deficiency and is entirely PREVENTABLE with appropriate monitoring and treatment ^[5]^[6]^[18].

"Kernicterus" derives from German: kern = nucleus (of the brain) + icterus = jaundice — literally "yellow staining of the brain nuclei."

Aspect	Detail
Pathophysiology	Unconjugated bilirubin is lipid-soluble → in neonates with very high levels (typically > 340 µmol/L in term infants, lower in preterms), unconjugated bilirubin crosses the immature blood-brain barrier (BBB) → deposits in specific brain regions with high metabolic activity
Regions affected	Basal ganglia (globus pallidus, subthalamic nuclei), hippocampus, cranial nerve nuclei (especially auditory — cochlear nuclei and inferior colliculus), cerebellum
Why these regions?	These areas have the highest metabolic rate and oxygen consumption in the neonatal brain → they are most vulnerable to the mitochondrial toxicity caused by unconjugated bilirubin (bilirubin uncouples oxidative phosphorylation and inhibits mitochondrial enzymes)
Acute bilirubin encephalopathy (ABE) — early, reversible phase	Lethargy, poor feeding, hypotonia → progressing to high-pitched cry, irritability, hypertonia, retrocollis (neck arching), opisthotonus (trunk arching), fever, seizures
Chronic bilirubin encephalopathy (CBE) — late, irreversible phase	Tetrad of classic kernicterus: (1) Choreoathetoid cerebral palsy (basal ganglia damage), (2) Sensorineural hearing loss (auditory nerve/cochlear nuclei damage — often the earliest and most consistent finding), (3) Upward gaze palsy (midbrain damage), (4) Dental enamel dysplasia (bilirubin toxicity to developing tooth enamel)
Prevention	Universal newborn screening for G6PD (identifies at-risk neonates); close bilirubin monitoring; early phototherapy; exchange transfusion when indicated

High Yield – Kernicterus in G6PD Deficiency

Kernicterus from G6PD deficiency remains a cause of preventable neonatal brain damage worldwide. In Hong Kong, universal newborn screening and high awareness have made this rare, but it still occurs when:

G6PD-deficient neonates are exposed to triggers (naphthalene, herbal medicines, fava bean consumption by breastfeeding mother)
Post-discharge follow-up is inadequate (bilirubin can rise rapidly after hospital discharge)
Co-inheritance of Gilbert syndrome (UGT1A1 polymorphism) further impairs conjugation ^[6]^[18]

Sensorineural hearing loss is often the FIRST and most common permanent sequela — this is why newborn hearing screening is important in G6PD-deficient neonates with a history of significant jaundice.

4. Chronic Complications (Primarily Class I Variants, but Also Recurrent Episodic Haemolysis)

These complications are shared with other causes of chronic haemolytic anaemia and arise from the ongoing consequences of accelerated RBC destruction and compensatory erythropoiesis.

Aspect	Detail
Mechanism	Chronic haemolysis → chronic ↑ unconjugated bilirubin → liver conjugates it and excretes it in bile as conjugated bilirubin → bile becomes supersaturated with calcium bilirubinate → precipitation → black pigment gallstones
Why "pigment" stones?	Unlike the cholesterol stones of the "4 Fs" (fat, female, forty, fertile), these are composed of calcium bilirubinate and appear black/dark brown. They are directly caused by excess bilirubin in bile. In Hong Kong, when you see pigment gallstones in a young patient, always think of chronic haemolysis — thalassaemia or G6PD deficiency ^[1]
Clinical features	May be asymptomatic; biliary colic (RUQ pain radiating to right shoulder, worse after fatty meals); complications include cholecystitis, choledocholithiasis, cholangitis, gallstone pancreatitis
Relevance to G6PD	Mainly Class I variants (chronic haemolysis); also possible with frequently recurring acute episodes
Management	Cholecystectomy (laparoscopic) if symptomatic; USS abdomen for surveillance

Aspect	Detail
Mechanism	Two pathways: (1) Transfusional iron overload — each unit of packed RBCs contains ~200–250 mg iron; the body has no physiological mechanism to excrete excess iron ^[17]. (2) Increased intestinal absorption — chronic haemolysis/ineffective erythropoiesis → hepcidin suppression → ↑ ferroportin-mediated iron absorption from the gut
Organs affected	Liver (hepatic fibrosis → cirrhosis), Heart (dilated cardiomyopathy → heart failure — the most common cause of death in transfusion-dependent patients), Endocrine organs (pituitary → hypogonadism; pancreas → diabetes; thyroid → hypothyroidism)
Why iron is toxic	Iron catalyses the Fenton reaction: Fe²⁺ + H₂O₂ → Fe³⁺ + OH• + OH⁻ → hydroxyl radicals → lipid peroxidation → cell membrane damage and DNA damage → organ fibrosis
Monitoring	Serum ferritin (> 1000 µg/L = significant overload); liver and cardiac MRI T2* (quantifies tissue iron)
Management	Iron chelation: Desferrioxamine (DFO), Deferiprone (DFP), Deferasirox ^[17]; venesection is NOT appropriate in anaemic patients (unlike hereditary haemochromatosis)
Relevance to G6PD	Mainly Class I variants requiring regular transfusions; rarely significant in Class II/III with occasional haemolysis

Aspect	Detail
Mechanism	Chronic ↑ erythropoiesis → ↑ demand for folate (vitamin B9, essential cofactor for thymidylate synthase in DNA synthesis) → if dietary intake does not match the increased demand, folate stores become depleted → megaloblastic crisis (bone marrow fails to produce adequate reticulocytes)
Clinical significance	A patient with chronic haemolysis who develops folate deficiency will have a paradoxical drop in reticulocyte count and worsening anaemia — the marrow can no longer compensate
Prevention	Daily folic acid supplementation (1–5 mg PO) in all patients with chronic haemolysis ^[15]

Aspect	Detail
Mechanism	Parvovirus B19 has specific tropism for erythroid precursors (binds to P antigen / globoside on erythroid progenitors) → infects and destroys erythroid precursors in the bone marrow → temporary cessation of erythropoiesis (lasts ~7–10 days — the duration of viraemia)
Why it matters in G6PD	In a normal person, a 7–10 day pause in erythropoiesis causes only a minimal Hb drop (RBC lifespan is 120 days). But in chronic haemolysis, RBC lifespan is already shortened → the patient depends on a high rate of compensatory erythropoiesis to maintain Hb → if erythropoiesis suddenly stops, Hb drops precipitously → aplastic crisis
Clinical features	Sudden severe pallor; worsening fatigue; reticulocyte count drops to near-zero (reticulocytopaenia); absence of jaundice (haemolysis stops too because new RBCs are not being produced); may have preceding viral prodrome (fever, rash — "slapped cheek" appearance, arthralgia)
Outcome	Self-limiting (immunity develops → clears virus → erythropoiesis resumes); may require transfusion support during the nadir
Key distinction	Aplastic crisis ≠ aplastic anaemia. Aplastic crisis is a temporary arrest of erythropoiesis due to parvovirus B19 infection, not a permanent bone marrow failure

Exam Pearl – Aplastic Crisis vs Haemolytic Crisis

Students often confuse these:

Haemolytic crisis: triggered by oxidant stress → ↑↑ reticulocytes (marrow is working overtime), ↑ bilirubin, ↑ LDH → jaundice and dark urine
Aplastic crisis: triggered by parvovirus B19 → ↓↓ reticulocytes (marrow arrested), NO jaundice (no new RBCs being destroyed), Hb drops with NO compensatory response

The reticulocyte count is the key differentiator: HIGH in haemolytic crisis, NEAR-ZERO in aplastic crisis.

Aspect	Detail
Mechanism	The spleen is the primary site of extravascular haemolysis. In Class I variants with chronic haemolysis, the splenic macrophages are chronically activated → work hypertrophy of the red pulp → mild to moderate splenomegaly
Clinical significance	Usually mild; rarely causes hypersplenism (cytopaenias from sequestration). Splenomegaly is NOT a feature of Class II/III variants (episodic haemolysis only)
Management	Splenectomy considered only if severely symptomatic or transfusion-dependent ^[2]

Aspect	Detail
Mechanism	Chronic anaemia → chronic tissue hypoxia → impaired somatic growth. Additionally, iron overload damaging the pituitary (hypogonadotropic hypogonadism) and thyroid can impair growth and pubertal development
Relevance	Only in severe Class I variants with chronically low Hb and/or transfusional iron overload affecting the endocrine system

These are complications that arise from the management of G6PD deficiency itself.

Complication	Source	Mechanism
Transfusion reactions	Packed RBC transfusion	Febrile non-haemolytic, allergic, delayed haemolytic (especially with repeated transfusions → alloimmunisation to minor blood group antigens)
Transfusional iron overload	Repeated transfusions	As above (Section 4.2) — each unit = ~250 mg iron ^[17]
Chelation drug toxicity	Iron chelation therapy	DFO: ototoxicity, retinal toxicity, ARDS; DFP: agranulocytosis; Deferasirox: GI upset, renal toxicity, hepatotoxicity ^[17]
Post-splenectomy complications	Splenectomy (for Class I)	Overwhelming post-splenectomy infection (OPSI) — encapsulated organisms (Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae); lifelong infection risk; requires pre-splenectomy vaccination + lifelong penicillin prophylaxis ^[2]
Thrombosis post-splenectomy	Splenectomy	↑ platelet count post-splenectomy (loss of splenic sequestration) → reactive thrombocytosis → increased thrombotic risk, especially portal vein thrombosis

Drug Trigger	Specific Complication Risk	Why
Co-trimoxazole	Severe haemolysis in immunocompromised patient already vulnerable	Patient may already be anaemic from underlying disease (HIV, malignancy); haemolysis on top of this can be fatal ^[7]
Rasburicase	Haemolysis + methaemoglobinaemia	Rasburicase generates H₂O₂ (oxidant) AND can cause methaemoglobinaemia — double jeopardy in G6PD-deficient patients where methylene blue (the standard metHb treatment) is also contraindicated ^[7]
Primaquine	Severe intravascular haemolysis + AKI	One of the strongest oxidant triggers; can cause life-threatening haemolysis especially in Class II (Mediterranean, Canton) variants

Complication	Class I (CNSHA)	Class II/III (Episodic)	Neonatal
Severe acute anaemia	During crises	During triggered episodes	Possible
AKI (haemoglobinuric)	During crises	During severe episodes	Rare
Neonatal jaundice	Yes	Yes	Primary concern
Kernicterus	Risk if unrecognised	Risk if unrecognised	Most important complication
Pigment gallstones	Common (chronic haemolysis)	Uncommon	No
Iron overload	If transfusion-dependent	Rare	No
Folate deficiency	If not supplemented	Rare	No
Aplastic crisis (B19)	Risk	Minimal	No
Splenomegaly	Mild–moderate	No	No

High Yield Summary – Complications of G6PD Deficiency

Kernicterus (bilirubin encephalopathy) is the MOST devastating and PREVENTABLE complication — caused by unconjugated bilirubin crossing the immature BBB and depositing in basal ganglia, auditory nuclei, and cerebellum ^[5]^[6]^[18]
Kernicterus tetrad: choreoathetoid CP, sensorineural hearing loss (earliest/most common), upward gaze palsy, dental enamel dysplasia
Acute kidney injury (haemoglobinuric nephropathy) — free Hb precipitates in renal tubules + iron-mediated oxidative damage; prevented by aggressive hydration and urinary alkalinisation
Pigment gallstones — from chronic bilirubin turnover; think chronic haemolysis when a young patient in HK has gallstones
Iron overload — from repeated transfusions (each unit = ~250 mg Fe) and increased intestinal absorption; monitor with ferritin + MRI T2*; treat with chelation ^[17]
Aplastic crisis — parvovirus B19 infects erythroid precursors → temporary erythropoietic arrest; distinguish from haemolytic crisis by near-zero reticulocyte count
Folate deficiency — chronic erythropoietic demand depletes folate stores; prevented by daily supplementation ^[15]
Rasburicase is doubly dangerous — causes haemolysis (H₂O₂ generation) AND methaemoglobinaemia, and methylene blue (the metHb antidote) is also contraindicated in G6PD ^[7]
Most complications in Class II/III variants are acute and self-limiting; chronic complications (gallstones, iron overload, splenomegaly) are mainly in Class I (CNSHA)

Active Recall - G6PD Deficiency Complications

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.376, Red Cell Enzymopathies / G6PD Deficiency — haemolysis complications: NNJ, gallstones, splenomegaly, folate deficiency) ^[2] Senior notes: Ryan Ho Haemtology.pdf (p.39–40, Section 2.5.2, Section 2.5.2.2 G6PD Deficiency — haemolysis complications, splenectomy timing, chronic haemolysis complications) ^[5] Lecture slides: GC 157. Paediatric Chemical Pathology.pdf ^[6] Lecture slides: Unconjugated hyperbilirubinemia in newborns ≥35 weeks of gestation_ Etiology and pathogenesis - UpToDate.pdf ^[7] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (p.16, Rasburicase and Co-trimoxazole in G6PD) ^[15] Senior notes: Block A - Family history of anaemia_ inherited causes of anaemia; haemolytic anaemia; aplastic anaemia.pdf (p.21, G6PD treatment and complications) ^[17] Senior notes: Maksim Medicine Notes.pdf (p.159, Iron chelation therapy — DFO, DFP, Deferasirox; iron overload complications) ^[18] Lecture slides: Paediatrics in Review - Jaundice - Newborn to Age 2 Months.pdf (p.5, G6PD deficiency and neonatal jaundice/kernicterus risk)

High Yield Summary

G6PD deficiency is the most common RBC enzymopathy worldwide (~400M affected), inherited in an X-linked recessive pattern
In Hong Kong: 4.5% males, 0.5% females affected; most common variant is G6PD Canton (1376G > T, Class II)
G6PD is the rate-limiting enzyme of the pentose phosphate pathway, the sole source of NADPH in mature RBCs → required to regenerate reduced glutathione (GSH) → the principal RBC antioxidant
Without NADPH/GSH, oxidant exposure → Heinz bodies (precipitated denatured Hb) → bite cells → extravascular haemolysis (± intravascular if severe)
Haemolysis is self-limiting (~1 week) because reticulocytes have high G6PD activity
Triggers: infection (most common), drugs (primaquine, co-trimoxazole, rasburicase, dapsone, sulfonamides), fava beans, naphthalene (mothballs)
Always check G6PD before prescribing co-trimoxazole or rasburicase
Neonatal jaundice can be severe → risk of kernicterus; mechanism involves BOTH haemolysis AND impaired hepatic conjugation
G6PD assay may be falsely normal during acute haemolysis (young cells with high activity) → repeat 2–3 months later
G6PD deficiency confers protection against P. falciparum malaria (heterozygote advantage)

High Yield Summary – Differential Diagnosis

The differential diagnosis of G6PD deficiency is structured around its three presentation patterns: acute haemolytic anaemia, neonatal jaundice, and chronic haemolytic anaemia (CNSHA)
The DAT (Coombs test) is the single most important test to separate immune (DAT+) from non-immune (DAT−) haemolysis — G6PD deficiency is always Coombs-negative
Heinz bodies and bite cells on blood film point to G6PD deficiency or unstable haemoglobinopathies (e.g., Hb Köln) — differentiate with G6PD assay vs Hb instability test
In Hong Kong, the top differentials for a male neonate with severe jaundice are G6PD deficiency, ABO incompatibility, and sepsis
Co-inheritance of G6PD deficiency + Gilbert syndrome (UGT1A1 polymorphism) dramatically increases risk of severe neonatal jaundice
Wilson disease (Coombs-negative haemolysis + liver failure in young patient) is an important mimic
Drug-induced haemolysis may be G6PD-related (oxidant, Coombs-negative) or immune-mediated (Coombs-positive) — always check both
G6PD assay may be falsely normal during acute haemolysis — repeat in 2–3 months

High Yield Summary – Diagnosis of G6PD Deficiency

Diagnosis = clinical suspicion + haemolysis markers + negative DAT + characteristic blood film + G6PD enzyme assay
G6PD enzyme assay (quantitative, spectrophotometric) is the definitive diagnostic test — measures rate of NADPH production from G6PD activity in RBC haemolysate
CRITICAL CAVEAT: G6PD assay may be falsely normal during acute haemolysis due to reticulocytosis — always repeat 2–3 months later ^[2]^[15]
DAT (Coombs) is NEGATIVE in G6PD deficiency — if positive, think AIHA instead
Peripheral blood film: bite cells, ghost/hemi-ghost cells, blister cells, polychromasia; Heinz bodies on supravital staining (crystal violet)
Haemolysis panel: ↑ reticulocytes, ↑ unconjugated bilirubin, ↑ LDH, ↓ haptoglobin
In Hong Kong, universal newborn screening (cord blood G6PD assay) identifies most cases before clinical presentation ^[5]
Female heterozygotes may have normal enzyme assay due to lyonization — molecular testing is more reliable for carrier detection ^[1]^[2]
In neonates, always use hour-specific bilirubin nomograms and remember G6PD deficiency is a major risk factor for severe hyperbilirubinaemia ^[6]

High Yield Summary – Management of G6PD Deficiency

No specific curative treatment exists — management is supportive + preventive ^[15]
Acute episode management: Stop offending agent → IV fluids → transfuse if severe → folic acid → monitor renal function
Haemolysis is self-limiting (~1 week) because reticulocytes have high G6PD activity ^[2]
Neonatal jaundice: phototherapy (intensive if needed) → exchange transfusion if approaching threshold or signs of encephalopathy; G6PD deficiency lowers the phototherapy threshold ^[6]
Chronic (Class I): folic acid daily, regular transfusions ± iron chelation, consider splenectomy if severely symptomatic
PREVENTION is the cornerstone: patient education, G6PD card, avoid oxidant drugs/foods/chemicals
Two drugs requiring mandatory G6PD check: co-trimoxazole (PJP prophylaxis) and rasburicase (TLS prophylaxis) — both absolutely contraindicated in G6PD deficiency ^[7]
Methylene blue is CONTRAINDICATED — requires NADPH from G6PD to function; paradoxically worsens haemolysis
Safe drugs include: paracetamol, low-dose aspirin, penicillins, macrolides, trimethoprim alone
Iron chelation for transfusion-dependent patients: DFO (SC), DFP (PO), Deferasirox (PO) ^[17]

High Yield Summary – Complications of G6PD Deficiency

Kernicterus (bilirubin encephalopathy) is the MOST devastating and PREVENTABLE complication — caused by unconjugated bilirubin crossing the immature BBB and depositing in basal ganglia, auditory nuclei, and cerebellum ^[5]^[6]^[18]
Kernicterus tetrad: choreoathetoid CP, sensorineural hearing loss (earliest/most common), upward gaze palsy, dental enamel dysplasia
Acute kidney injury (haemoglobinuric nephropathy) — free Hb precipitates in renal tubules + iron-mediated oxidative damage; prevented by aggressive hydration and urinary alkalinisation
Pigment gallstones — from chronic bilirubin turnover; think chronic haemolysis when a young patient in HK has gallstones
Iron overload — from repeated transfusions (each unit = ~250 mg Fe) and increased intestinal absorption; monitor with ferritin + MRI T2*; treat with chelation ^[17]
Aplastic crisis — parvovirus B19 infects erythroid precursors → temporary erythropoietic arrest; distinguish from haemolytic crisis by near-zero reticulocyte count
Folate deficiency — chronic erythropoietic demand depletes folate stores; prevented by daily supplementation ^[15]
Rasburicase is doubly dangerous — causes haemolysis (H₂O₂ generation) AND methaemoglobinaemia, and methylene blue (the metHb antidote) is also contraindicated in G6PD ^[7]
Most complications in Class II/III variants are acute and self-limiting; chronic complications (gallstones, iron overload, splenomegaly) are mainly in Class I (CNSHA)

Glucose-6-phosphate Dehydrogenase Deficiency

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

2. Epidemiology

3. Anatomy and Function: Red Cell Metabolism and the Pentose Phosphate Pathway

4. Aetiology

5. Pathophysiology

6. Classification

7. Triggers of Haemolysis

8. Clinical Features

9. Important Associations and Context

Active Recall - G6PD Deficiency (Definition, Epidemiology, Pathophysiology, Clinical Features)

Differential Diagnosis of G6PD Deficiency

3. Differential Diagnosis of Acute Haemolytic Anaemia

4. Differential Diagnosis of Neonatal Jaundice (Unconjugated Hyperbilirubinaemia)

Active Recall - G6PD Deficiency Differential Diagnosis

References

Diagnostic Criteria, Diagnostic Algorithm, and Investigation Modalities for G6PD Deficiency

3. Key Investigations — Organised by Diagnostic Layer

6. Special Investigation Scenarios

7. Interpretation Guide — Putting It All Together

Active Recall - G6PD Deficiency Diagnosis and Investigations

References

Management of G6PD Deficiency

3. Acute Haemolytic Episode — Detailed Management

4. Neonatal Jaundice Management

5.1 Iron Chelation — Why and How

6. Prevention — The Cornerstone of G6PD Management

7. Special Management Scenarios

Active Recall - G6PD Deficiency Management

References

Complications of G6PD Deficiency

2. Acute Complications

3. Neonatal Complications

4. Chronic Complications (Primarily Class I Variants, but Also Recurrent Episodic Haemolysis)

Active Recall - G6PD Deficiency Complications

References

On this page

Glucose-6-phosphate Dehydrogenase Deficiency

1. Definition

2. Epidemiology

2.1 Global Burden

2.2 Hong Kong Prevalence

2.3 Sex Distribution

3. Anatomy and Function: Red Cell Metabolism and the Pentose Phosphate Pathway

3.1 Unique Features of Mature RBCs

3.2 Two Key Metabolic Pathways

3.3 The Pentose Phosphate Pathway in Detail

3.4 Why Reduced Glutathione (GSH) Matters

4. Aetiology

4.1 Genetics

4.2 Common Variants by Geography

4.3 Why Variant Class Matters

4.4 Evolutionary Advantage

5. Pathophysiology

5.1 The Central Problem

5.2 Step-by-Step Mechanism

5.3 Why Haemolysis is Self-Limiting

5.4 Pathophysiology of Neonatal Jaundice in G6PD Deficiency

6. Classification

6.1 WHO Classification of G6PD Variants (by enzyme activity)

6.2 Clinical Classification (by presentation pattern)

7. Triggers of Haemolysis

7.1 Mechanism of Triggers

7.2 Drug Triggers

7.3 Food Triggers

7.4 Infection

7.5 Other Triggers

8. Clinical Features

8.1 General Principle

8.2 Symptoms (with pathophysiological basis)

8.3 Signs (with pathophysiological basis)

8.4 Neonatal Presentation

8.5 Chronic Non-Spherocytic Haemolytic Anaemia (CNSHA) — Class I Variants

8.6 Peripheral Blood Film Findings (Clinical Signs on Investigation)

9. Important Associations and Context

9.1 Unconjugated Hyperbilirubinaemia Differential

9.2 G6PD and Drug Prescribing

9.3 G6PD and Other Conditions

10. Summary of Key Pathophysiological Connections

Active Recall - G6PD Deficiency (Definition, Epidemiology, Pathophysiology, Clinical Features)

References

1. Why Differential Diagnosis Matters for G6PD Deficiency

2. Conceptual Framework: The Three Clinical Scenarios

3. Differential Diagnosis of Acute Haemolytic Anaemia

3.1 Intracorpuscular (Intrinsic RBC) Causes

3.2 Extracorpuscular (Extrinsic to the RBC) Causes

4. Differential Diagnosis of Neonatal Jaundice (Unconjugated Hyperbilirubinaemia)

4.1 Structured Approach by Mechanism

4.2 Timing of Onset as a Differential Clue

5. Differential Diagnosis of Chronic Haemolytic Anaemia

6. Discriminating Features: A Comparative Table

7. Special Differential Considerations in Hong Kong

8. Diagnostic Approach to Differentiation — Stepwise Algorithm

9. Drugs That Cause Haemolysis: G6PD vs Non-G6PD Mechanisms

10. Summary: Top Differentials to Always Consider

Active Recall - G6PD Deficiency Differential Diagnosis

1. Overview: What Are We Trying to Achieve Diagnostically?

2. Diagnostic Criteria

3. Key Investigations — Organised by Diagnostic Layer

3.1 Layer 1: Confirm Haemolysis

3.2 Layer 2: Characterise the Haemolysis

3.3 Layer 3: Confirm G6PD Deficiency as the Cause

4. Diagnostic Algorithm

5. Newborn Screening Programme — Hong Kong

6. Special Investigation Scenarios

6.1 Neonatal Jaundice Workup When G6PD Deficiency is Suspected

6.2 Chronic Haemolytic Anaemia Workup (Suspected Class I G6PD Deficiency)

7. Interpretation Guide — Putting It All Together

7.1 Classic Acute G6PD Haemolysis Lab Profile

7.2 Comparison: DAT-Positive vs DAT-Negative Haemolysis

8. Neonatal Bilirubin Interpretation — Special Considerations

9. Summary Table: All Investigations at a Glance

Active Recall - G6PD Deficiency Diagnosis and Investigations

1. Principles of Management — Why There Is No "Cure"

2. Management Algorithm

3. Acute Haemolytic Episode — Detailed Management

3.1 Step 1: Remove the Trigger