What is the method for testing Glucose-6-phosphate dehydrogenase (G6PD) deficiency?

Testing for G6PD Deficiency

Qualitative screening tests are recommended as the initial method for detecting G6PD deficiency, particularly before starting therapy with oxidant drugs in patients with predisposing racial or ethnic backgrounds. 1, 2

Recommended Testing Approach

Initial Screening

• Qualitative screening tests are the first-line approach for G6PD deficiency detection and should be performed upon entry into care or before starting therapy with oxidant drugs in patients with predisposing racial or ethnic backgrounds (such as African, Mediterranean, Indian, or Southeast Asian heritage) 1, 2
• Fluorescent spot test (FST) is a widely used qualitative screening method, though it has limitations in detecting heterozygous females and patients with mild deficiency 3, 4
• Point-of-care rapid tests (such as CareStart G6PD) have shown non-inferiority to the laboratory fluorescent spot test, with 96% sensitivity at G6PD activity ≤40% of normal 4

Confirmatory Testing

• Quantitative spectrophotometric enzyme assay is the gold standard for confirming G6PD deficiency and determining the degree of enzyme deficiency 3
• The normal G6PD activity in adults is approximately 11 ± 2.5 IU/gHb, with deficiency typically defined as activity <5.7 IU/gHb (60% of normal mean) 3
• Severe deficiency is defined as residual enzyme activity <0.95 IU/gHb (10% of normal mean) 3

Special Considerations

• Flow cytometric assay is particularly useful for detecting heterozygous females and patients with anemia, showing good correlation with spectrophotometric assay (Pearson's r 0.918-0.957) 5
• Cytochemical testing with a cut-off of >20% negative cells has shown the closest correlation to molecular analysis for detecting G6PD deficiency 3

Testing Timing and Interpretation Challenges

Timing Considerations

• Testing should be performed before initiating treatment with oxidant drugs such as dapsone, primaquine, sulfonamides, or methylene blue 1
• In neonates with hyperbilirubinemia, G6PD testing is important as deficient infants may develop sudden increases in bilirubin levels 1

Interpretation Challenges

• Reticulocytosis can affect test results, as young RBCs may contain normal or near-normal enzyme levels even in deficient patients 1
• In patients with hemolysis, G6PD levels can be falsely elevated, which may obscure the diagnosis in the acute setting 1
• If G6PD deficiency is strongly suspected despite normal test results during hemolysis, repeat testing should be performed after the hemolytic episode resolves (approximately 3 months later) 1

Common Pitfalls and Recommendations

• Whole blood testing without RBC purification can lead to false-negative results due to leukocyte and platelet contamination 1
• Semi-quantitative methods may miss partially deficient females (heterozygotes), who can still experience hemolytic episodes 6
• Recent blood transfusions can affect test results; ideally, testing should be delayed for at least 50 days after transfusion 1
• Using a quantitative method with a cut-off of 6.4 U/g Hb is recommended to detect both total and partial G6PD deficiency 6

Clinical Implications

• G6PD deficiency testing is crucial before administering oxidant drugs, particularly in patients with predisposing ethnic backgrounds 1, 2
• Methylene blue should not be used in G6PD-deficient patients as it can worsen hemolysis and is ineffective in treating methemoglobinemia in these patients 1
• Family screening is recommended for first-degree relatives of patients with confirmed G6PD deficiency 2
• Patients with confirmed deficiency should be educated about avoiding precipitating factors including certain foods (particularly fava beans), medications, and infections 7