What are the hemoglobin (Hb) A1c levels for diagnosing diabetes?

Hemoglobin A1c Diagnostic Criteria for Diabetes

An HbA1c level of ≥6.5% (≥48 mmol/mol) is diagnostic for diabetes mellitus when measured using an NGSP-certified laboratory method. 1, 2, 3

Diagnostic Thresholds

Diabetes Diagnosis

• HbA1c ≥6.5% (≥48 mmol/mol) establishes the diagnosis of diabetes 1, 2, 3
• The test must be performed in a laboratory using an NGSP-certified method standardized to the DCCT assay 1
• Point-of-care HbA1c devices should NOT be used for diagnostic purposes due to lack of standardization and proficiency testing requirements 1

Prediabetes (High Risk for Diabetes)

• HbA1c 5.7-6.4% (39-46 mmol/mol) identifies individuals with prediabetes or high risk for developing diabetes 1, 2, 3
• This range carries a 5-year diabetes incidence of 25-50% for the upper end (6.0-6.4%) and 9-25% for the mid-range (5.5-6.0%) 1

Confirmation Requirements

In the absence of unequivocal hyperglycemia, a second confirmatory test is required. 1, 3

You can confirm the diagnosis by:

• Repeating the same HbA1c test on a different day (e.g., if initial A1C is 7.0% and repeat is 6.8%, diabetes is confirmed) 1
• Using a different diagnostic test above threshold (e.g., HbA1c ≥6.5% plus fasting glucose ≥126 mg/dL) 1
• Measuring two different tests on the same day if both are above diagnostic thresholds 3

Exception: Confirmation is not needed in patients with classic hyperglycemic symptoms and random plasma glucose ≥200 mg/dL 1, 3

Critical Limitations and When NOT to Use HbA1c

Conditions Requiring Glucose-Based Testing Only

Do not use HbA1c for diagnosis in these situations—use only plasma glucose criteria: 1

• Sickle cell disease or other hemoglobinopathies affecting red cell turnover 1
• Pregnancy (second and third trimesters) 1
• Recent blood loss or transfusion 1
• Hemodialysis 1
• Erythropoietin therapy 1
• Hemolytic anemia 1

Hemoglobin Variants

• Most common hemoglobin variants do not interfere with U.S. assays, but marked discordance between HbA1c and plasma glucose should raise suspicion of assay interference 1
• For patients with sickle cell trait (abnormal hemoglobin but normal red cell turnover), use an HbA1c assay without interference from abnormal hemoglobins 1
• Check www.ngsp.org/interf.asp for updated interference information 1

Age Considerations

• The diagnostic HbA1c threshold of ≥6.5% was validated only in adult populations 1
• It remains unclear whether the same cutoff should apply to children and adolescents for diagnosis 1

Advantages of HbA1c Testing

HbA1c offers several practical benefits over glucose-based tests: 1, 2

• No fasting required, increasing patient convenience and adherence 1, 2
• Greater preanalytical stability (sample doesn't degrade quickly) 1, 2
• Less day-to-day variability during stress and acute illness 1
• Reflects average glucose over 60-90 days rather than a single point in time 2

Common Pitfalls to Avoid

1. Never use point-of-care devices for diagnosis—only NGSP-certified laboratory methods are acceptable 1

2. Don't ignore discordant results—if HbA1c and plasma glucose don't match, consider hemoglobin variants or conditions affecting red cell turnover 1

3. Don't forget confirmation—unless the patient has clear hyperglycemic crisis, always confirm with a second test 1, 3

4. Don't use HbA1c in high red cell turnover states—this is a hard contraindication, not just a caution 1

5. Race/ethnicity may affect HbA1c independently of glycemia—be aware this can impact interpretation 1