What A1C (Hemoglobin A1C) level is considered indicative of diabetes?

A1C Diagnostic Threshold for Diabetes

An A1C level of 6.5% or higher is the diagnostic threshold for diabetes. 1

Diagnostic Categories

The American Diabetes Association has established clear A1C-based diagnostic categories:

Diabetes Diagnosis

• A1C ≥6.5% confirms diabetes 1
• This threshold was established based on the glycemic level at which retinopathy prevalence begins to increase in a linear fashion 1
• The diagnostic cut point of 6.5% demonstrates high specificity (98%) for detecting diabetes 2

Prediabetes (Increased Risk Category)

• A1C 5.7-6.4% defines prediabetes 1, 3
• This range identifies individuals at substantially increased risk for future diabetes and cardiovascular disease 1, 3
• Within this range, risk stratification is critical:
  • A1C 6.0-6.4%: Very high risk—25-50% will develop diabetes within 5 years 4
  • A1C 5.5-6.0%: Moderately increased risk—9-25% will develop diabetes within 5 years 4

Normal Range

• A1C <5.7% is considered normal 1, 3
• However, risk exists on a continuum even below 5.7%, particularly with additional risk factors like obesity and family history 1, 5

Clinical Application Considerations

Test Performance

• When using A1C ≥6.5% as the diagnostic criterion, sensitivity is 47% against a single fasting glucose but improves to 67% when compared against repeated fasting glucose measurements 2
• Specificity remains consistently high at 97-98% 2
• A1C performs better as a diagnostic tool when compared to repeated glucose measurements rather than single measurements, which more closely resembles real clinical practice 2

Risk Gradient

The relationship between A1C and diabetes risk is curvilinear, not linear 1:

• Risk increases steeply as A1C rises from 5.0% to 6.5% 4
• Individuals with A1C 6.0-6.4% have a relative risk 20 times higher than those with A1C 5.0% 3
• Annualized diabetes incidence ranges from 0.1% at A1C <5.0% to 54.1% at A1C ≥6.1% 4

Important Caveats

• A1C should be measured using a method certified by the National Glycohemoglobin Standardization Program (NGSP) to ensure accuracy 1
• Certain conditions can affect A1C accuracy (hemoglobinopathies, anemia, recent blood transfusion), though these are not mentioned in the provided guidelines
• For individuals with both elevated fasting glucose (≥126 mg/dL) and A1C ≥6.5%, the 10-year risk of diagnosed diabetes is 88%, compared to only 55% for those with elevated fasting glucose but A1C 5.7-<6.5% 2