Normal A1C Levels
For adults without diabetes, a normal A1C is below 5.7%, while values of 5.7-6.4% indicate prediabetes, and 6.5% or higher on two separate occasions confirms diabetes. 1
Defining Normal A1C Ranges
The American Diabetes Association establishes clear thresholds for A1C interpretation:
- Normal glycemic range: A1C below 5.7% (39 mmol/mol) 1
- Prediabetes: A1C 5.7-6.4% (39-47 mmol/mol) 1
- Diabetes diagnosis: A1C ≥6.5% (48 mmol/mol) on two separate occasions 1
An A1C of 5.3% falls within the normal range and represents good glycemic control, corresponding to an estimated average glucose of approximately 105 mg/dL 2. Linear regression data from NHANES indicates that an A1C of 5.4% corresponds to a fasting plasma glucose of approximately 100 mg/dL 2.
Understanding Risk Across the A1C Spectrum
Risk for developing diabetes exists on a continuum, even within ranges traditionally considered "normal." 3
Progressive Risk Stratification:
- A1C <5.0%: Annualized diabetes incidence of 0.1%, representing the lowest risk category 4
- A1C 5.0-5.5%: Increased incidence relative to those with A1C <5%, but absolute incidence remains less than 9% over 5 years 4
- A1C 5.5-6.0%: Moderately increased risk with 9-25% incidence over 5 years 1, 4
- A1C 6.0-6.5%: Highly increased risk with 25-50% incidence over 5 years, representing a 20-fold higher relative risk compared to A1C of 5.0% 1, 4
Clinical Implications for "Normal" Values:
Even individuals with A1C levels below 5.7% may carry some diabetes risk depending on additional factors such as obesity, family history, and ethnicity 2, 3. The risk is continuous and extends below the lower limit of the prediabetes range 1.
Screening Intervals Based on A1C
For individuals with A1C <6.0%, rescreening at intervals shorter than 3 years identifies very few individuals (≤1%) who progress to diabetes (A1C ≥6.5%). 5
The American Diabetes Association recommends repeat testing at minimum 3-year intervals for those with normal results, or more frequently if BMI is increasing or risk factor profile is deteriorating 1.
Important Caveats
Testing Methodology:
A1C measurements for diagnosis should be performed by a clinical laboratory rather than point-of-care testing due to lack of standardization of the latter 1. The test is now widely standardized in the United States 1.
Limitations to Consider:
- Low sensitivity: A1C may miss some cases of diabetes, particularly when used as a single screening tool 6
- Racial disparities: A1C may detect different proportions of at-risk individuals across ethnic groups 3
- Interference: Anemia and certain medications can affect A1C accuracy 1
- Fasting plasma glucose comparison: In one study of African Americans, fasting plasma glucose ≥126 mg/dL detected less than 1 in 5 cases with type 2 diabetes, while A1C ≥6.5% was more effective 6
The "Normal" A1C Paradox:
An A1C ≤5.6%, proposed to indicate normal glucose tolerance, was present in only 28 patients in one study, yet 64.3% of these individuals actually had either impaired glucose tolerance or type 2 diabetes by oral glucose tolerance test 6. This underscores that A1C ≤5.6% does not definitively rule out glucose abnormalities 6.