When do we perform quantitative analysis for Glucose-6-phosphate dehydrogenase (G6PD) deficiency?

When to Perform Quantitative G6PD Analysis

Quantitative G6PD testing should be performed when you need to determine the precise degree of enzyme deficiency, particularly before prescribing tafenoquine (requires >70% activity), when evaluating heterozygous females or patients with borderline enzyme activity (30-70% of normal), and when qualitative screening results are inconclusive. 1, 2

Primary Indications for Quantitative Testing

Before Specific Drug Therapy

• Tafenoquine administration requires quantitative testing - do not use this drug if G6PD activity is <70% of normal 2, 3
• Before primaquine therapy when considering modified dosing regimens - patients with 30-70% activity and non-Mediterranean variants may receive weekly primaquine with close monitoring 2, 3
• When qualitative screening is insufficient to guide treatment decisions 1

When Qualitative Testing is Inadequate

• Heterozygous females require quantitative assays or flow cytometry because qualitative tests only reliably detect severe deficiency (<10% activity) in males and homozygous females 2
• Patients with concurrent hematological conditions that may interfere with qualitative screening 2
• When borderline results need clarification to determine specific variant type and predict hemolytic risk severity 1, 2

Clinical Scenarios Requiring Quantitative Assessment

• Infants with rapidly rising TSB approaching exchange transfusion levels or not responding to phototherapy - quantitative G6PD testing is recommended as part of the evaluation 4
• Infants receiving phototherapy with TSB rising rapidly and crossing percentiles unexpectedly 4
• When determining whether a patient can safely receive oxidant medications that have dose-dependent risks 1, 2

Critical Timing Considerations

When NOT to Perform Testing

• Avoid testing during or immediately after acute hemolytic episodes - reticulocytes and young RBCs contain near-normal enzyme levels, potentially masking deficiency and giving falsely elevated results 1, 2, 5
• Wait at least 50 days after RBC transfusion before performing enzyme assays, as donor RBC contamination of 6-12% can cause false-negative results 2
• Testing 120 days post-transfusion is ideal but often impractical in frequently transfused patients 2
• Do not test during transfusion or immediately post-transfusion 1

Optimal Testing Windows

• Repeat testing after 3 months may be necessary if initial testing occurred during acute hemolysis 1
• For infants with jaundice in the first 24 hours or excessive jaundice for age, measure G6PD as part of the laboratory evaluation 4

Specific Clinical Algorithms

For Neonatal Hyperbilirubinemia

• Measure quantitative G6PD when TSB is approaching exchange levels 4
• Consider G6PD testing when jaundice appears in the first 24 hours of life 4
• G6PD deficiency was identified as the cause in 31.5% of infants who developed kernicterus in one series 4

For Malaria Treatment Planning

• Quantitative testing is mandatory before tafenoquine - the drug is contraindicated if activity <70% 2, 3
• For primaquine in P. vivax/P. ovale radical cure, quantitative testing helps determine if weekly dosing with monitoring is feasible versus standard daily dosing 2, 3
• Patients with Mediterranean variants require stricter avoidance regardless of quantitative level due to life-threatening hemolysis risk 2, 3

For Patients with Borderline Results

• Calculate the PK/G6PD ratio or compare to controls with similar reticulocyte counts when interpreting borderline quantitative results 2
• Consider molecular genetic analysis when enzymatic activity is borderline (30-70% of normal) to determine specific variant and predict severity 1

Sample Handling Requirements

• 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 1
• Buffy coat removal or filtration is essential - testing whole blood without removing white cells and platelets prevents accurate detection of deficiency 2
• Snap-frozen samples in liquid nitrogen are required for enzymatic activity testing in liver tissue 1

Common Pitfalls to Avoid

• Do not rely solely on qualitative screening for heterozygous females - they require quantitative assessment 2
• Do not test during active hemolysis or recent transfusion - results will be falsely elevated 1, 2, 5
• Do not assume neutropenia indicates G6PD deficiency - it is not a reliable screening marker 2
• Recognize that G6PD levels may be falsely elevated during acute hemolysis because young reticulocytes have higher enzyme activity 1