G6PD DEFICIENCY

G6PD Deficiency — the most common human enzymopathy affecting 500 million people, causing acute hemolytic crises from common medications including antimalarials, antibiotics, and chemotherapy agents.

Whole genome sequencing identifies the complete G6PD genotype — including the variant-specific activity class needed to distinguish Class I (severe, chronic hemolysis) from Class II-III (trigger-dependent hemolysis) from Class IV (normal activity) — for safe prescribing of oxidant drugs.

G6PD

About this condition

G6PD Deficiency

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common human enzymopathy, affecting an estimated 400-500 million people worldwide. G6PD is encoded on the X chromosome (Xq28) and is the rate-limiting enzyme of the pentose phosphate pathway, which produces NADPH — required for regenerating reduced glutathione and protecting red blood cells from oxidative damage. G6PD-deficient red blood cells are highly susceptible to oxidative stress, leading to acute hemolytic episodes when triggered by oxidant drugs, infection, or fava bean consumption (favism). Neonatal hyperbilirubinemia is the most serious consequence in the newborn period.

Over 200 G6PD variants have been characterized, classified by WHO into functional activity classes: Class I (severely deficient, <10% activity, associated with chronic non-spherocytic hemolytic anemia); Class II (severely deficient, <10% activity, acute hemolysis on oxidant exposure); Class III (moderately deficient, 10-60% activity, hemolysis only with strong oxidant triggers); Class IV (normal activity, 60-150%); Class V (increased activity). The most common Class II variants are G6PD Mediterranean (c.563C>T, most common in Mediterranean, Middle Eastern, and Indian populations), G6PD A- (c.202G>A + c.376A>G, most common in African ancestry), and G6PD Canton (c.1376G>T, common in Southeast Asian populations). G6PD A- causes Class III disease; G6PD Mediterranean causes Class II disease with more severe hemolytic episodes.

Clinically significant drug interactions include: primaquine and tafenoquine (malaria prophylaxis — require Class B (normal or mildly deficient) G6PD status by FDA), rasburicase (contraindicated in G6PD deficiency — causes acute life-threatening hemolysis), dapsone, nitrofurantoin, methylene blue (important as an antidote for methemoglobinemia — cannot be used in G6PD deficiency), and several chemotherapy agents (adriamycin, doxorubicin at high doses). CPIC Level A guidelines exist for rasburicase and G6PD. The FDA requires G6PD testing before prescribing tafenoquine (KrintafelL) for malaria treatment.

G6PD is X-linked. Hemizygous males express the full G6PD phenotype of their single allele. Heterozygous females have two X chromosomes — one normal, one G6PD-deficient — and X-inactivation skewing creates a spectrum from normal to fully G6PD-deficient phenotype in heterozygous females.

Gene locus: G6PD (Xq28)

Enzyme activity assays measure G6PD activity but not the specific variant or WHO activity class. Variant-level data from whole genome sequencing is required to determine activity class, predict hemolysis severity, and guide drug avoidance recommendations.

Enzyme activity assays give false-normal results in females and during hemolytic episodes

G6PD enzyme activity assays — the standard point-of-care G6PD test — have two major limitations in clinical practice. First, in heterozygous females with X-inactivation skewing toward the normal allele, measured enzyme activity is normal despite having a G6PD-deficient allele that could cause significant hemolysis if X-inactivation shifts during illness or medications. Second, during an acute hemolytic episode, the most severely G6PD-deficient red cells are selectively destroyed — the residual red cell population is enriched for younger, less deficient cells, and the measured enzyme activity may be falsely elevated (false normal). Molecular genotyping from whole genome sequencing identifies the specific G6PD variant regardless of measured enzyme activity — providing the correct classification even when enzyme assays give misleading results.

The specific G6PD variant determines which drugs are safe — not all G6PD deficiency is equivalent

G6PD A- (Class III, common in African ancestry) produces moderate enzyme deficiency with hemolysis only from strong oxidant triggers like high-dose primaquine — most affected individuals tolerate malaria prophylaxis doses with careful monitoring. G6PD Mediterranean (Class II) produces severe deficiency with hemolysis from lower oxidant stress, including therapeutic doses of drugs that would be tolerated by G6PD A- patients. Rasburicase (used for tumor lysis syndrome management) is absolutely contraindicated in any G6PD-deficient patient due to hydrogen peroxide generation. These variant-specific clinical distinctions require molecular genotyping to implement — an enzyme activity result does not identify which G6PD variant is present or which risk category the patient falls into.

G6PD Deficiency — the most common human enzymopathy affecting 500 million people, causing acute hemolytic crises from common medications including antimalarials, antibiotics, and chemotherapy agents.

G6PD Deficiency

Enzyme activity assays measure G6PD activity but not the specific variant or WHO activity class. Variant-level data from whole genome sequencing is required to determine activity class, predict hemolysis severity, and guide drug avoidance recommendations.

Enzyme activity assays give false-normal results in females and during hemolytic episodes

The specific G6PD variant determines which drugs are safe — not all G6PD deficiency is equivalent

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