What Hemoglobin A1C (HbA1C) level is considered diagnostic of diabetes?

A1C Diagnostic Threshold for Diabetes

An A1C level of ≥6.5% (48 mmol/mol) is diagnostic of diabetes. 1

Diagnostic Criteria

The American Diabetes Association established A1C ≥6.5% as one of the acceptable criteria for diagnosing diabetes in 2010, and this remains the current standard. 1 This threshold was selected based on its relationship to microvascular complications, particularly retinopathy risk.

Confirmation Requirements

• In the absence of unequivocal hyperglycemia (random glucose >200 mg/dL with classic symptoms), diagnosis requires two abnormal test results. 1

• The confirmation can come from:

  • Two separate A1C measurements both ≥6.5% 1
  • The same sample showing both A1C ≥6.5% AND fasting glucose ≥126 mg/dL 1
  • Two different samples with different tests (e.g., A1C and fasting glucose) both above diagnostic thresholds 1

• If discordant results occur between two different tests, repeat the test that exceeded the diagnostic threshold. 1

• If A1C confirms diabetes but glucose does not (or vice versa), the patient should still be considered diabetic based on the confirmed abnormal test. 1

Test Performance Characteristics

A1C ≥6.5% demonstrates high specificity (97-98%) but moderate sensitivity (47-67%) for detecting diabetes compared to glucose-based criteria. 2 This means:

• A1C will miss some individuals who meet glucose-based diabetes criteria (lower sensitivity) 2
• When A1C is elevated ≥6.5%, diabetes is highly likely (high specificity) 2
• Against repeated fasting glucose measurements, A1C sensitivity improves to 67% while maintaining 97% specificity 2

Critical Clinical Caveats

When A1C Should NOT Be Used

A1C is unreliable and should not be used for diagnosis in these conditions: 1

• Sickle cell disease
• Pregnancy (second and third trimesters)
• Glucose-6-phosphate dehydrogenase deficiency
• Hemodialysis
• Recent blood loss or transfusion
• Erythropoietin therapy
• Postpartum state
• HIV treated with certain drugs
• Iron-deficiency anemia

In these situations, use only plasma glucose criteria for diagnosis. 1

Prediabetes Range

For context, A1C of 5.7-6.4% defines prediabetes (increased diabetes risk). 3 Individuals with A1C 6.0-6.4% have particularly high risk, with 25-50% developing diabetes within 5 years. 3

Prognostic Value of Combined Testing

When both A1C ≥6.5% AND fasting glucose ≥126 mg/dL are present, the 10-year risk of diagnosed diabetes is 88%, compared to only 55% when fasting glucose alone is elevated but A1C is 5.7-6.4%. 2 This supports using both tests when feasible for risk stratification.

Practical Algorithm

1. Measure A1C in patients being evaluated for diabetes
2. If A1C ≥6.5%: Repeat A1C or perform fasting glucose to confirm diagnosis 1
3. If A1C 5.7-6.4%: Diagnose prediabetes and initiate prevention strategies 3
4. If conditions affecting red blood cell turnover exist: Skip A1C entirely and use glucose-based testing 1
5. If results are near diagnostic margins: Discuss symptoms with patient and retest in 3-6 months 1