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Glucose-6-phosphate dehydrogenase (G6PD) is an enzyme involved in energy production. It is found in all cells, including red blood cells (RBCs) and helps protect them from certain toxic by-products of cellular metabolism. A deficiency in G6PD causes RBCs to become more vulnerable to breaking apart (hemolysis) under certain conditions. This test measures the amount of G6PD in RBCs to help diagnose a deficiency.

G6PD deficiency is a genetic disorder. When individuals who have inherited this condition are exposed to a trigger such as stress, an infection, certain drugs or other substance(s), significant changes occur in the structure of the outer layer (cell membrane) of their red blood cells. Hemoglobin, the life-sustaining, oxygen-transporting protein within RBCs, forms deposits (precipitates) called Heinz bodies. Some individuals may experience these reactions when exposed to fava beans, a condition called "favism." With these changes, RBCs can break apart more readily, causing a decrease in the number of RBCs. When the body cannot produce sufficient RBCs to replace those destroyed, hemolytic anemia results and the individual may develop jaundice, weakness, fatigue, and/or shortness of breath.

G6PD deficiency is the most common enzyme deficiency in the world, affecting more than 400 million people. It may be seen in up to 10% of African-American males and 20% of African males. It is also commonly found in people from the Mediterranean and Southeast Asia.

G6PD deficiency is inherited, passed from parent to child, due to mutations or changes in the G6PD gene that cause decreased enzyme activity. There are over 440 variants of G6PD deficiency. The G6PD gene is located on the sex-linked X chromosome. Since men have one X and one Y sex chromosome, their single X chromosome carries the G6PD gene. This may result in a G6PD deficiency if a male inherits the single X chromosome with an altered gene.

Since women have two X sex chromosomes, they inherit two copies of the G6PD gene. Women with only one mutated gene (heterozygous) produce enough G6PD that they usually do not experience any symptoms (i.e., asymptomatic), but under situations of stress, they may demonstrate a mild form of the deficiency. In addition, a mother may pass the single mutated gene to any male children. Rarely do women have two mutated gene copies (homozygous), which could result in G6PD deficiency.