Glucose-6-phosphate dehydrogenase (G6PD) is a protein that protects red blood cells from oxidative damage. When G6PD is deficient, exposure to oxidizing agents (including several medications) results in a precipitation of the hemoglobin contained in the red blood cells, causing them to rupture (hemolysis with fatigue, jaundice, etc.). Many individuals have a hereditary G6PD deficiency, including a variant more prevalent in Africa, Asia and the Mediterranean area accompanied by hemolysis attacks following the ingestion of beans (favism). The level of G6PD is expressed in units of enzyme per gram of hemoglobin (U/g Hb).
A below-normal level of G6PD indicates a higher probability of a hemolytic reaction when the person is exposed to an oxidizing agent. A normal level in male individuals makes a G6PD deficiency unlikely and, in the presence of anemia, indicates that G6PD deficiency is probably not the cause. The test should be repeated if the sample was taken during an episode of hemolytic anemia. Women with the mutation have one of two copies of the gene in abnormal form and can have normal or low levels of G6PD. Rare cases of women who are homozygous for the mutation (i.e. who have two copies of the defective gene) should have significantly lower than normal levels of G6PD.