Glucose-6-phosphate dehydrogenase (G6PD) enzyme testing is used to screen for and help diagnose G6PD deficiencies. It may be used to screen children who experienced persistent jaundice as a newborn that could not be explained by another cause. Currently, newborns are not routinely screened for G6PD deficiency, but it is one of the thirty disorders that are recommended for screening by several organizations, including the March of Dimes.
G6PD testing may also be used to help establish a diagnosis for people of any age who have had one or more unexplained episodes of hemolytic anemia, presence of jaundice, or dark urine. If the person had a recent viral or bacterial illness or was exposed to a known trigger (such as fava beans, a "sulfa" drug, or naphthalene), followed by a hemolytic episode, then G6PD deficiency may be considered.
Repeat G6PD testing may occasionally be ordered to confirm initial findings. In the most common form of G6PD deficiency seen in persons of African ancestry, the G6PD enzyme level is normal in newly produced cells but levels decrease as the red cells age. Because of this, testing is not recommended until after a hemolytic episode resolves. During the episode, a higher percentage of the older more G6PD deficient cells are typically destroyed, leaving the newer, less deficient cells to be tested, potentially masking a G6PD deficiency.
Genetic testing is not routinely done but can be ordered as follow up to an enzyme test that indicates a deficiency to determine which G6PD mutation(s) are present. So far, more than 440 G6PD gene variations have been identified and can cause enzyme activity deficiencies of varying severity depending on the mutation and on the individual person. However, only the most common G6PD mutations are identified during testing. If a specific mutation is known to be present in a family line, tests to detect that particular mutation can also be conducted.
G6PD enzyme testing is primarily performed when an individual has signs and symptoms associated with hemolytic anemia. Testing may be done when someone has had an episode of increased red cell destruction (hemolytic crisis) but after the crisis has resolved. Some signs and symptoms include:
Red or brown urine (from the presence of blood/hemoglobin)
Testing may also be done when other laboratory test results are consistent with a hemolytic anemia. These may show increased bilirubin concentrations (bulirubinemia), hemoglobin in the urine (hemoglobinuria), a decreased RBC count, an increased reticulocyte count, presence of "bite cells" on a blood smear, and sometimes the presence of Heinz bodies inside the RBCs on a specially stained blood smear.
G6PD activity testing is typically ordered when other causes of anemia and jaundice have been ruled out and is ordered once the acute incident has been resolved.
If available, screening may be performed on a newborn in the first day or so after birth.
A low level of G6PD enzyme indicates a deficiency. An affected person is more likely to experience symptoms when exposed to a trigger. The results, however, cannot be used to predict how an affected person will react in a given set of circumstances. The severity of symptoms will vary from person to person and from episode to episode.
A normal G6PD enzyme level in a male indicates that it is unlikely he has a deficiency, and if anemia is present, it is likely due to another cause. However, if the test was performed during an episode of hemolytic anemia, it should be repeated a few weeks later when the RBC population has had time to replenish and mature.
Women who are carriers, having one altered and one normal gene copy (heterozygous), will have both G6PD-deficient and non-deficient RBCs. These women will usually have normal or near normal G6PD levels and rarely experience symptoms. A carrier will have a normal or low normal G6PD level, thus may not be identified through G6PD screening; however, the rare female who has two altered gene copies (homozygous) will show a significant decrease in G6PD level.
While G6PD deficiency is found throughout the world, it is most common in those of African or Mediterranean descent and is also found in those from Southeast Asia. Its geographical area of increased prevalence mirrors that of malaria. Some researchers think that having a G6PD deficiency has historically offered a survival advantage to those infected with malaria, a parasite that physically invades the RBCs.
Certain laboratory methods, biochemical testing, and electrophoresis can be used to distinguish between different G6PD enzyme variants. This has been used in the past to study the prevalence and levels of different types of the enzymes, both normal and deficient, but this testing is used almost exclusively in the research setting.
G6PD testing should not be done soon after a blood transfusion as the donor red blood cells can mask a G6PD deficiency.
This article was last reviewed on July 28, 2012. | This article was last modified on July 28, 2012.
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