The Test Sample
What is being tested?
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 by-products of cellular metabolism. A deficiency in G6PD causes RBCs to become more vulnerable to breaking apart (hemolysis) when exposed to certain substances. This test measures the amount of G6PD enzyme in RBCs to help diagnose a G6PD 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), changes occur in the structure of the outer layer (cell membrane) of their red blood cells. Hemoglobin, the 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 RBCs. When the body cannot produce sufficient RBCs to replace those that have been destroyed, then 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 about 400 million people, according to the Nemours Foundation. It may be seen in up to 10% of African-American males in the U.S. and 20% of the male population in Africa. Depending on the specific location, it may occur in as low as 3% or more than 35% of the male population in the Mediterranean area and Southeast Asia.
G6PD deficiency is inherited, passed from parent to child. Inherited mutations or changes in the G6PD gene lead to 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.
Women have two X sex chromosomes; they thus inherit two copies of the G6PD gene. Women with only one altered gene (heterozygous) produce enough normal G6PD that they usually do not experience any symptoms, but under situations of stress they may demonstrate a mild form of the deficiency. In addition, the mother may pass the single altered gene to any male children. Rarely, a woman may have two altered gene copies (homozygous) and will experience G6PD deficiency.
G6PD deficiency is a common cause of persistent jaundice in newborns. Persistent jaundice, if left untreated, can lead to brain damage and mental retardation.
Most people with G6PD deficiency can lead fairly normal lives but must be cautious and avoid certain medications such as aspirin, antibiotics with "sulf" in the name, the anti-malarial drug quinine, foods such as fava beans, and chemical substances such as naphthalene (found in moth balls). Infections can also initiate episodes of hemolytic anemia.
With hemolytic anemia, RBCs are destroyed at an accelerated rate and the person affected becomes pale and fatigued (anemic) as their capacity for providing oxygen to their body decreases. In severe cases of red cell destruction, jaundice can also be present during episodes of hemolysis. Most of these episodes are self-limiting, but if a large number of RBCs are destroyed and the body cannot replace them fast enough, then the affected person may require a blood transfusion. A small percentage of those affected with G6PD deficiency may experience chronic anemia.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm, by fingerstick, or by heelstick (newborns).
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Generally, no test preparation is needed. However, if the individual is experiencing an episode in which symptoms of hemolytic anemia are acute, it is advised to postpone testing until the crisis has passed.