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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 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.
G6PD deficiency is a common cause of persistent jaundice in newborns. If left untreated, this can lead to significant brain damage and mental retardation.
Most people with G6PD deficiency can lead fairly normal lives, but there is no specific treatment apart from prevention. They must be cautious and avoid certain medications such as aspirin, phenazopyridine and rasburicase, antibiotics with "sulf" in the name and dapsone, anti-malarial drugs with "quine" in the name, foods such as fava beans, and chemical substances such as naphthalene (found in moth balls). Note that fava beans, often called broad beans, are commonly grown in the Mediterranean area. Acute viral and bacterial infections can also initiate episodes of hemolytic anemia as well as elevated levels of acid in the blood (i.e., acidosis). Individuals should consult with their healthcare practitioner to get a comprehensive list of these triggers. A good starting point is the list found on the G6PD Deficiency Favism Association website.
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 RBC destruction, jaundice can also be present. 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. This condition can be fatal if not treated. 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 with symptoms of acute hemolytic anemia, it is advised to postpone testing for several weeks after the crisis has passed.