G6PD

Glucose-6-Phosphate Dehydrogenase

Q: Why is G6PD deficiency more common in certain ethnic groups?

G6PD deficiency evolved as a survival advantage against Plasmodium falciparum malaria. Red blood cells with reduced G6PD activity create an inhospitable environment for the malaria parasite, which depends on the host cell's antioxidant pathways. This "balanced polymorphism" means the gene persisted in populations where malaria was endemic—sub-Saharan Africa (10–25% prevalence), the Mediterranean (up to 20%), the Middle East, and Southeast Asia. The trade-off is susceptibility to oxidative hemolysis.

Q: Can women have G6PD deficiency?

Yes, though it is less common and often less severe. G6PD is X-linked, so males (with one X chromosome) are either fully deficient or normal. Females have two X chromosomes, and due to random X-inactivation (lyonization), heterozygous females are genetic mosaics with a mixture of G6PD-normal and G6PD-deficient red blood cells. Most heterozygous females have sufficient enzyme activity to prevent significant hemolysis, but some—depending on the pattern of X-inactivation—can be as severely affected as hemizygous males. Homozygous females are fully deficient.

Q: Why should I not be tested during a hemolytic crisis?

During acute hemolysis, the oldest red blood cells (with the lowest G6PD activity) are preferentially destroyed, leaving younger reticulocytes with higher G6PD activity. This can produce a falsely normal test result—the very cells that would show deficiency have been destroyed. For accurate results, wait at least 2–3 months after an acute hemolytic episode so that the red blood cell population returns to a steady state with a normal age distribution. If urgent results are needed, molecular genetic testing can confirm the diagnosis regardless of timing.

Complete Blood Count

What is Glucose-6-Phosphate Dehydrogenase?

Glucose-6-phosphate dehydrogenase (G6PD) is a cytoplasmic enzyme present in all cells but particularly critical in red blood cells. It catalyzes the first and rate-limiting step of the pentose phosphate pathway (hexose monophosphate shunt), converting glucose-6-phosphate to 6-phosphogluconate while reducing NADP+ to NADPH. NADPH is essential for regenerating reduced glutathione (GSH), the primary antioxidant defense in red blood cells. Unlike most cells, mature red blood cells lack mitochondria and a nucleus, making the pentose phosphate pathway their sole source of NADPH. Without adequate G6PD activity, red blood cells cannot neutralize oxidative stress, leading to oxidative damage to hemoglobin (forming Heinz bodies), membrane lipid peroxidation, and ultimately hemolysis.

G6PD deficiency is the most common human enzyme deficiency, affecting approximately 400–500 million people worldwide. It is X-linked recessive, primarily affecting males, though heterozygous females can also be symptomatic due to X-inactivation (lyonization). The deficiency is most prevalent in populations from malaria-endemic regions—Africa, the Mediterranean, the Middle East, and Southeast Asia—because G6PD deficiency confers partial resistance to Plasmodium falciparum malaria, a classic example of balanced polymorphism. Over 200 G6PD variants have been identified, with varying degrees of enzyme deficiency and clinical severity.

Why It Matters

G6PD deficiency is a major cause of drug-induced hemolytic anemia, favism (hemolysis triggered by fava beans), and neonatal jaundice. The clinical significance lies in prevention: identifying G6PD-deficient individuals before prescribing oxidative medications—including primaquine, dapsone, rasburicase, nitrofurantoin, and certain sulfonamides—can prevent potentially life-threatening hemolytic crises. G6PD testing is also recommended before prescribing rasburicase for tumor lysis syndrome, as methemoglobinemia and hemolysis can occur. In neonates, unrecognized G6PD deficiency is a leading cause of severe hyperbilirubinemia and kernicterus in endemic regions.

Normal Reference Ranges

Group	Range	Unit
Adults (quantitative)	4.6–13.5	U/g Hb
Qualitative (fluorescent spot test)	Normal fluorescence
Neonates	May be higher than adult values	U/g Hb

Reference ranges may vary by laboratory. Always compare results to the ranges provided by your testing facility.

What High G6PD Levels Mean

Common Causes

Reticulocytosis (young red blood cells have higher G6PD activity)
Recent hemolytic episode (testing during recovery may show falsely normal/elevated levels)
Chronic myeloproliferative disorders
Hyperthyroidism
Megaloblastic anemia after treatment (reticulocyte surge)

Possible Symptoms

Elevated G6PD activity itself does not cause symptoms
May mask underlying G6PD deficiency if tested during reticulocytosis

What to do: Elevated G6PD activity is not clinically concerning and usually reflects reticulocytosis. However, if G6PD deficiency is suspected and the patient has recently had a hemolytic episode, the high reticulocyte count can falsely normalize the G6PD level. Repeat testing 2–3 months after the acute episode when the reticulocyte count has normalized to obtain an accurate baseline G6PD level.

What Low G6PD Levels Mean

Common Causes

G6PD deficiency (X-linked genetic condition)
WHO Class I: chronic non-spherocytic hemolytic anemia (severe, <10% activity)
WHO Class II: Mediterranean and Asian variants (severe, <10% activity, intermittent hemolysis)
WHO Class III: African variant (A-) (moderate, 10–60% activity, usually mild)

Possible Symptoms

Acute hemolytic anemia triggered by oxidative stress (medications, fava beans, infections)
Dark urine (hemoglobinuria)
Jaundice and pallor
Fatigue and tachycardia
Back or abdominal pain during acute hemolysis
Neonatal jaundice (in severe variants)
Chronic hemolytic anemia (rare, severe variants)

What to do: G6PD deficiency has no cure, but hemolytic episodes are preventable. Provide the patient with a list of medications and substances to avoid (primaquine, dapsone, rasburicase, nitrofurantoin, fava beans, naphthalene mothballs). Treat acute hemolysis with hydration, monitoring, and blood transfusion if severe. Screen neonates at risk for severe jaundice. Educate the patient and family, and ensure the deficiency is documented in medical records. Genetic counseling may be appropriate for family planning.

When Is G6PD Testing Recommended?

Before prescribing oxidant medications (primaquine, dapsone, rasburicase, nitrofurantoin)
In neonates with unexplained or severe jaundice, especially in at-risk ethnic groups
When evaluating acute hemolytic anemia with Heinz bodies or bite cells on blood smear
In patients with recurrent episodes of hemolysis triggered by drugs, infections, or foods
In males of African, Mediterranean, Middle Eastern, or Southeast Asian descent before certain medications

Frequently Asked Questions

G6PD deficiency evolved as a survival advantage against Plasmodium falciparum malaria. Red blood cells with reduced G6PD activity create an inhospitable environment for the malaria parasite, which depends on the host cell's antioxidant pathways. This "balanced polymorphism" means the gene persisted in populations where malaria was endemic—sub-Saharan Africa (10–25% prevalence), the Mediterranean (up to 20%), the Middle East, and Southeast Asia. The trade-off is susceptibility to oxidative hemolysis.

Yes, though it is less common and often less severe. G6PD is X-linked, so males (with one X chromosome) are either fully deficient or normal. Females have two X chromosomes, and due to random X-inactivation (lyonization), heterozygous females are genetic mosaics with a mixture of G6PD-normal and G6PD-deficient red blood cells. Most heterozygous females have sufficient enzyme activity to prevent significant hemolysis, but some—depending on the pattern of X-inactivation—can be as severely affected as hemizygous males. Homozygous females are fully deficient.

During acute hemolysis, the oldest red blood cells (with the lowest G6PD activity) are preferentially destroyed, leaving younger reticulocytes with higher G6PD activity. This can produce a falsely normal test result—the very cells that would show deficiency have been destroyed. For accurate results, wait at least 2–3 months after an acute hemolytic episode so that the red blood cell population returns to a steady state with a normal age distribution. If urgent results are needed, molecular genetic testing can confirm the diagnosis regardless of timing.

Related Biomarkers

HgbHemoglobin TBILBilirubin HpHaptoglobin LDHLactate Dehydrogenase

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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges may vary between laboratories. Always consult your healthcare provider for interpretation of your specific test results.