What are the diagnosis criteria for Glucose-6-phosphate dehydrogenase (G6PD) deficiency?

Diagnosis Criteria for Glucose-6-phosphate dehydrogenase (G6PD) Deficiency

The diagnosis of G6PD deficiency should be confirmed through quantitative enzyme activity testing, followed by molecular genetic testing to identify specific mutations. 1

Initial Laboratory Evaluation

First-line Testing

• Quantitative G6PD enzyme activity measurement:
  • Gold standard for diagnosis
  • Measures the rate of NADPH production from NADP+
  • Results reported as units per gram of hemoglobin (U/g Hb) or per red blood cell
  • Indications include:
    • Unexplained hemolysis
    • Before administering high-risk medications (e.g., primaquine)
    • Individuals from high-prevalence regions 1

Screening Tests

• Fluorescent Spot Test (FST):

  • Widely used screening method
  • Results interpretation:
    • Absent fluorescence: Severe deficiency
    • Intermediate fluorescence: Partial deficiency
    • Bright fluorescence: Normal activity 1

• Point-of-care rapid diagnostic tests:

  • ~100% sensitivity for severe/moderate deficiency
  • Useful for field screening
  • Similar performance to FST 1

Confirmatory Testing

Molecular Genetic Testing

• Full gene sequencing of G6PC (GSD Ia) and SLC37A4 (GSD Ib) genes
• Can detect up to 100% of affected individuals in some homogeneous populations
• May detect only one mutation in mixed populations 2

Testing Strategy

1. Complete G6PC sequencing first (unless neutropenia is present)
2. If neutropenia is present, consider SLC37A4 sequencing
3. If liver biopsy tissue is available, analyze for G6Pase enzymatic activity 2

Special Considerations for Heterozygous Females

• May have intermediate deficiency that can still cause clinical symptoms
• Flow cytometry can detect mosaicism due to X-chromosome inactivation
• Timing of testing is important - avoid testing during acute hemolytic episodes 1, 3

Diagnostic Challenges

False Negatives

• Recent blood transfusion (higher proportion of normal donor cells)
• Testing during acute hemolytic episode (younger RBCs have higher enzyme activity)
• Heterozygous females with normal or subnormal enzyme activity 3

Complicating Conditions

• Sickle cell disease may cause false negative results with flow cytometry
• Other hematological disorders may affect test interpretation 3

Laboratory Findings Consistent with G6PD Deficiency

• Hypoglycemia
• Lactic acidosis
• Hypercholesterolemia
• Hypertriglyceridemia
• Hyperuricemia
• Neutropenia (suggests GSD Ib, but can also be seen in GSD Ia) 2

Histopathology (If Liver Biopsy Performed)

• Fat and glycogen in hepatocytes without fibrosis
• Glycogen content mildly increased compared to other liver GSDs
• Normal G6Pase enzyme activity level in liver is 3.50 ± 0.8 μmol/min/g tissue
• In most individuals with GSD Ia, G6Pase enzyme activity is <10% of normal 2

Diagnostic Algorithm

1. Initial suspicion based on:

   • Clinical presentation (hemolytic anemia, jaundice)
   • Exposure to triggers (medications, fava beans)
   • Family history or high-risk ethnicity

2. Screening test:

   • Fluorescent spot test or point-of-care rapid test
   • If positive or equivocal, proceed to quantitative testing

3. Quantitative enzyme activity testing:

   • Definitive test for diagnosis
   • Interpret results based on reference ranges for age, sex, and laboratory

4. Molecular genetic testing:

   • Full gene sequencing to confirm diagnosis
   • Identifies specific mutations
   • Essential for genetic counseling and family screening

5. Document G6PD status prominently in medical records to prevent future hemolytic episodes 1

Pitfalls to Avoid

• Testing timing: Avoid testing during or immediately after hemolytic episodes
• Recent transfusions: May lead to falsely normal results
• Heterozygous females: May be missed by standard testing; consider flow cytometry
• Neonatal screening: Requires specialized interpretation due to higher enzyme activity in newborns
• G6Pase enzyme activity on liver biopsy: Will not detect GSD Ib 2, 3