Glucose-6-phosphate dehydrogenase deficiency laboratory findings
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-In-Chief: Priyamvada Singh, M.D. [2]
Overview
Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked recessive hereditary disease featuring abnormally low levels of the G6PD enzyme, which plays an important role in red blood cell function. Individuals with the disease may exhibit non-immune hemolytic anemia in response to a number of causes. It is closely linked to favism, a disorder characterized by a hemolytic reaction to consumption of broad beans, with a name derived from the Italian name of the broad bean (fava). Sometimes the name, favism, is alternatively used to refer to the enzyme deficiency as a whole.
Laboratory diagnosis
The diagnosis is generally suspected when patients from certain ethnic groups develop anemia, jaundice and symptoms of hemolysis after challenge to any of the above causes, especially when there is a positive family history. The hemolysis spontaneously resolves in approximately 1 week as the older enzyme-depleted cells are replaced by new cells with sufficient G6PD to prevent further hemolysis.
Generally, tests will include:
- Complete blood count - Hemoglobin decreases by 3-4 g/dL
- Increased reticulocyte count
- Haptoglobin (decreased in hemolysis)
- Liver enzymes (to exclude other causes of jaundice)
- TSH measurement.
- A "direct antiglobulin test" (Coombs' test) - this should be negative, as hemolysis in G6PD is not immune-mediated
- Since, reticulocytes can have normal G6PD levels, measuring G6PD levels during an acute episode may produce a false-negative result.
Specific tests for G6PD anemia
- Heinz bodies
- Beutler fluorescent spot test
- The Motulsky dye-decolouration test
- Direct DNA testing and/or sequencing of the G6PD gene.
Heinz bodies: Early diagnosis
When a macrophage in the spleen "sees" an RBC with a Heinz body, it removes the precipitate and a small piece of the membrane, leading to characteristic "bite cells". However, if a large number of Heinz bodies are produced, as in the case of G6PD deficiency, some Heinz bodies will nonetheless be visible when viewing RBCs that have been stained with crystal violet. This easy and inexpensive test can lead to an initial presumption of G6PD deficiency, which can be confirmed with the other tests.
Beutler fluorescent spot test: Late diagosis
The Beutler fluorescent spot test is a rapid and inexpensive test that visually identifies NADPH produced by G6PD under ultraviolet light. When the blood spot does not fluoresce, the test is positive; it can be false-positive in patients who are actively hemolysing. It can therefore only be done several weeks after a hemolytic episode.
Bite or Blister cells
- In glucose-6-phosphate dehydrogenase deficiency, "bite" or "blister" cells are produced when accumulated oxidized hemoglobin remains adherent to the erythrocyte membrane, which creates an adjacent membrane-bound clear zone.