G6PD Deficiency Diagnosis
G6PD deficiency is diagnosed primarily through quantitative enzyme activity assays measuring G6PD levels in red blood cells, with genetic testing reserved for inconclusive cases or when identifying specific variants is clinically necessary. 1
Primary Diagnostic Approach
Initial Testing Strategy
- Quantitative G6PD enzyme activity assay is the gold standard, measuring enzyme activity in units per gram of hemoglobin (IU/gHb) 1
- Calculate the percentage of normal G6PD activity by dividing the patient's G6PD level by the laboratory upper limit of normal and multiplying by 100 1
- Qualitative fluorescent spot tests are appropriate only for detecting severe G6PD deficiency (enzyme activity <10% of normal) in males and homozygous females, but are insufficient for heterozygous females 1
Patient-Specific Testing Selection
The type of assay matters significantly based on patient characteristics:
- Males and homozygous females with severe deficiency: Qualitative tests are adequate 1
- Heterozygous females: Quantitative assays or flow cytometry are necessary 1
- Patients with concurrent hematological conditions: Quantitative testing is required 1
Critical Timing Considerations
When NOT to Test
Avoid testing during or immediately after acute hemolytic episodes, as reticulocytes and young red blood cells contain near-normal enzyme levels, potentially masking the deficiency 1, 2
- Wait at least 50 days after RBC transfusion before performing enzyme assays, as donor RBC contamination of 6-12% causes false-negative results 1
- Testing 120 days post-transfusion is ideal but often impractical in frequently transfused patients 1
- If testing must occur during recovery from hemolysis, consider calculating the PK/G6PD ratio or comparing to controls with similar reticulocyte counts when interpreting borderline results 1
Sample Handling Requirements
- Buffy coat removal or filtration is essential for accurate RBC enzyme measurement—testing whole blood without removing white cells and platelets prevents detection of deficiency in all cases 1
- Whole blood samples remain relatively stable at -20°C and 6°C for up to 14 days, losing only about 20% activity over 48 hours 2
Clinical Context for Testing
Who Should Be Screened
Screen before starting oxidant drugs (dapsone, primaquine, sulfonamides) in patients of Mediterranean, African, Indian, or Southeast Asian descent 1, 3
Neonatal Hyperbilirubinemia
Quantitative G6PD testing is indicated when:
- TSB is rapidly rising and approaching exchange transfusion levels 1
- Infant is receiving phototherapy with TSB rising rapidly and crossing percentiles unexpectedly 1
- Jaundice appears in the first 24 hours of life 1
- Elevated indirect bilirubin (>1.0 mg/dL when total bilirubin is ≤5 mg/dL) suggests hemolysis 2
G6PD deficiency was identified as the cause in 31.5% of infants who developed kernicterus, highlighting the critical importance of testing in this population 1
Genetic Testing Indications
When Molecular Analysis Is Required
- Heterozygous females with inconclusive enzymatic testing 1, 2
- Borderline enzymatic activity (30-70% of normal) to determine specific variant and predict severity 2
- Distinguishing Mediterranean variant (Gdmed) from African variant (GdA-), which determines hemolytic risk severity and drug contraindications 1
Clinical Significance of Variants
- Mediterranean variant (Gdmed): Causes life-threatening hemolysis requiring strict oxidant drug avoidance 1
- African variant (GdA-): Produces milder, self-limited hemolysis 1
- Molecular genetic analysis can detect up to 100% of mutations in homogeneous populations 2
Interpretation of Results
Activity Levels and Clinical Implications
- <10% activity: Severe deficiency, qualitative tests sufficient for detection 1
- 30-70% activity: Borderline cases requiring quantitative testing; patients with non-Mediterranean variants can receive modified primaquine regimens with close monitoring 1
- ≥70% activity: Required for tafenoquine use in P. vivax radical cure 1