How many occasions should A1c and Fasting Blood Sugar levels be measured to diagnose diabetes, and what is the recommended interval between these tests?

Diagnosis of Diabetes: A1C and FBS Testing Requirements

For diagnosing diabetes, two abnormal test results are required, either from the same sample or from two separate test samples, with A1C ≥6.5% or fasting blood sugar ≥126 mg/dL on two occasions being diagnostic. 1

Diagnostic Criteria for Diabetes

A1C Testing

• Threshold: A1C ≥6.5% (48 mmol/mol)
• Requirements:
  • Must be performed in a laboratory using NGSP-certified method standardized to DCCT assay
  • Two separate abnormal results required for diagnosis
  • No specific time interval mandated between tests, but should be performed without delay 1

Fasting Blood Sugar (FBS) Testing

• Threshold: FBS ≥126 mg/dL (7.0 mmol/L)
• Requirements:
  • Fasting defined as no caloric intake for at least 8 hours
  • Two separate abnormal results required for diagnosis
  • Samples must be spun and separated immediately after collection to prevent preanalytic variability 1

Alternative Diagnostic Methods

• 2-hour plasma glucose: ≥200 mg/dL (11.1 mmol/L) during OGTT using 75g glucose load
• Random plasma glucose: ≥200 mg/dL (11.1 mmol/L) with classic symptoms of hyperglycemia (only requires one measurement in this scenario) 1

Confirmation Process

1. Two tests of the same type: If initial A1C is 7.0% and repeat A1C is 6.8%, diabetes is confirmed
2. Two different test types: If both A1C and FBS are above diagnostic thresholds from same sample or separate samples, diabetes is confirmed
3. Discordant results: If results from two different tests are discordant, repeat the test that was above diagnostic threshold 1

Special Considerations

When A1C is Unreliable

• In conditions with increased red blood cell turnover:
  • Pregnancy (second and third trimesters)
  • Sickle cell disease
  • Hemodialysis
  • Recent blood loss or transfusion
  • Erythropoietin therapy
  • HIV treated with certain drugs
  • Iron-deficient anemia 1

Timing Between Tests

• If initial test is abnormal but near diagnostic threshold, repeat testing in 3-6 months is recommended 1
• For patients with normal results and no risk factors, repeat testing at minimum 3-year intervals 1
• For patients with prediabetes (A1C 5.7-6.4%), yearly testing is recommended 1

Common Pitfalls to Avoid

1. Preanalytic variability: FBS samples must be spun and separated immediately after collection to prevent falsely low results 1
2. Point-of-care A1C testing: Not standardized for diagnostic purposes; clinical laboratory testing required 1
3. Single test diagnosis: Except in cases of unequivocal hyperglycemia with classic symptoms, a single abnormal test result is insufficient for diagnosis 1
4. Ignoring hemoglobinopathies: Can interfere with A1C measurement; use glucose-based criteria in affected individuals 1

Algorithm for Diabetes Diagnosis

1. Initial screening: Perform either A1C or FBS test
2. If result abnormal:
   • A1C ≥6.5% or FBS ≥126 mg/dL: Perform second test (same type or different)
   • If second test confirms: Diagnose diabetes
   • If second test discordant: Repeat the test that was above threshold
3. If initial result normal but patient has risk factors:
   • Retest at appropriate interval (yearly for prediabetes, every 3 years otherwise)

By following this evidence-based approach to diabetes diagnosis, clinicians can ensure accurate identification of patients with diabetes, allowing for timely intervention to reduce morbidity and mortality associated with this chronic disease.