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Glucose-6-phosphate dehydrogenase deficiency(G6PD) is an X-linked recessive hereditary disease characterized by abnormally low levels of glucose-6-phosphate dehydrogenase, a metabolic enzyme involved in the pentose phosphate pathway, especially important in red blood cell metabolism.

Individuals with the disease may exhibit nonimmune hemolytic anemia in response to a number of causes, most commonly infection or exposure to certain medications or fava beans.

The World Health Organization classifies G6PD genetic variants into five classes, the first three of which are deficiency states.^[2]

Severe deficiency (<10% activity) with chronic (nonspherocytic) hemolytic anemia
Severe deficiency (<10% activity), with intermittent hemolysis
Mild deficiency (10-60% activity), hemolysis with stressors only
Non-deficient variant, no clinical sequelae
Increased enzyme activity, no clinical

Most individuals with G6PD deficiency are asymptomatic.

Symptomatic patients are almost exclusively male, due to the X-linked pattern of inheritance, but female carriers can be clinically affected due to unfavorable lyonization, where random inactivation of an X-chromosome in certain cells creates a population of G6PD-deficient red blood cells coexisting with normal red cells. A typical female with one affected X chromosome will show the deficiency in approximately half of her red blood cells. However, in rare cases, including double X deficiency, the ratio can be much more than half, making the individual almost as sensitive as a male.

Abnormal red blood cell breakdown (hemolysis) in G6PD deficiency can manifest in a number of ways, including the following:

Prolonged neonatal jaundice, possibly leading to kernicterus (arguably the most serious complication of G6PD deficiency)
Hemolytic crises in response to:
- Illness (especially infections)
- Certain drugs (see below)
- Certain foods, most notably broad beans
- Certain chemicals
- Diabetic ketoacidosis
Very severe crises can cause acute renal failure

Antimalarial drugs that can cause acute hemolysis in people with G6PD deficiency includeprimaquine, pamaquine, and chloroquine. There is evidence that other antimalarials may also exacerbate G6PD deficiency, but only at higher doses. Sulfonamides (such as sulfanilamide, sulfamethoxazole, and mafenide), thiazolesulfone, methylene blue, and naphthalene should also be avoided by people with G6PD deficiency, as should certain analgesics (such as aspirin,phenazopyridine, and acetanilide) and a few non-sulfa antibiotics (nalidixic acid, nitrofurantoin, isoniazid, dapsone, andfurazolidone)