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Glucose 6 Phosphate Dehydrogenase Screen (G6PD)

Glucose 6 phosphate dehydrogenase (G6PD) is an enzyme necessary for the production of NADPH through the pentose phosphate pathway in the red blood cell. NADPH is required in the red cell to reduce glutathione, which protects the cell from oxidative damage.

G6PD deficiency is inherited as an X-linked disorder that causes hemolytic anemia. The G6PD gene maps to the subtelomeric region of the long arm of the X chromosome. Nearly 200 mutations in the G6PD gene have been identified. Almost all of these mutations cause severe deficiency. Prevalence is 13% in African American males and 3% of African American females. G6PD deficiency is also commonly found in Sardinians, Greeks and Sephardic Jews. It is rare among Caucasians. Males who inherit an abnormal gene are invariably affected. Females who are heterozygous usually have approximately 50% of the normal G6PD enzyme activity.  Random Lyonisation of X chromosomes rarely results in heterozygous females who are severely affected.

In G6PD deficiency, decreased NADPH levels results in oxidative injury of cell membranes, which may lead to hemolytic anemia. Hemoglobin is susceptible to oxidant damage, and denatured globin precipitates in the RBC forming Heinz bodies. This leads to membrane rigidity and premature removal of red cells by the liver and spleen. G6PD deficiency is the most common cause of drug‑induced hemolytic anemia. Clinically significant hemolysis may occur with infections or after exposure to medications including antimalarials, sulfonamides, ascorbic acid and analgesics.

G6PD deficiency can also be precipitated by ingestion of uncooked fava beans (Vicia faba). Favism occurs commonly where the frequency of G6PD deficiency is relatively high and where fava beans are a popular food. Both factors coexist in southern Europe, the Middle East and Southeast Asia. Favism can cause severe life-threatening acute hemolytic anemia.

Methods of screening for G6PD deficiency rely on the conversion of glucose-6-phosphate to 6-phosphgluconate (6PG) by G6PD. If G6PD is present, NADP will be converted to NADPH, which then reduces a nonfluorescent substrate (G6P) to a fluorescent product (6PG). In G6PD deficiency, no colored or fluorescent product is detected.

In qualitative assays, patient blood is mixed with reagent and incubated for 5 minutes. Drops are spotted on filter paper and then viewed under long wavelength ultraviolet light. Normal patients exhibit fluorescence, while blood from patients with G6PD does not. Results are reported as normal or deficient. The reference value is normal.

In quantitative assays, the rate of formation of NADPH is measured as an increase in absorbance at a wavelength of 340 nm. Results are expressed as units per gram of hemoglobin. Abnormal results are usually 0 to 20% of the normal mean.

Specimen requirement is one lavender top (EDTA) tube of blood.

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