How far apart should A1C (Hemoglobin A1C) levels be drawn to confirm a diagnosis of diabetes in a patient?

Timing of Confirmatory A1C Testing for Diabetes Diagnosis

The second confirmatory A1C test should be performed without delay—meaning as soon as practically possible, not waiting weeks or months—unless there is a clear clinical diagnosis of diabetes with classic symptoms and random glucose ≥200 mg/dL. 1, 2

Immediate Confirmation Protocol

• Perform the repeat test immediately using the same laboratory method that is NGSP-certified and standardized to the DCCT assay 1
• The same test should be repeated for confirmation since there will be greater likelihood of concurrence 1, 2
• If the initial A1C is 7.0% (53 mmol/mol) and the repeat result is 6.8% (51 mmol/mol), diabetes is confirmed even though both tests were done without delay 1

Alternative Confirmation Approaches

• Two different tests from the same sample (such as A1C and fasting plasma glucose) that are both above diagnostic thresholds confirm diabetes immediately 1
• Two different tests from separate samples analyzed without delay also confirm the diagnosis if both are elevated 1
• If using A1C plus fasting glucose, and both meet diagnostic criteria (A1C ≥6.5% and FPG ≥126 mg/dL), no further testing is needed 1

When to Wait 3-6 Months

The only scenario where waiting 3-6 months is appropriate is when:

• A test result was initially above the diagnostic threshold but the immediate repeat falls below the diagnostic cutpoint 1, 2
• This suggests the patient has values near the margins of the diagnostic threshold 1
• In this borderline situation, close follow-up with repeat testing in 3-6 months is recommended 1, 2

Critical Considerations for A1C Reliability

A1C has the least preanalytic and analytic variability compared to fasting glucose and 2-hour glucose, making it the most reliable test for repeat confirmation 1, 2

However, do not use A1C for confirmation in these conditions:

• Sickle cell disease, pregnancy (second/third trimesters), hemodialysis, recent blood loss or transfusion, or erythropoietin therapy 1
• Any condition with increased red blood cell turnover 1
• In these cases, use only plasma glucose criteria with immediate repeat testing 1

Handling Discordant Results

• If A1C meets diabetes criteria (≥6.5%) but fasting glucose does not (<126 mg/dL), repeat the A1C immediately 1, 2
• If both A1C results are ≥6.5%, diabetes is confirmed regardless of the glucose result 1
• Consider A1C assay interference from hemoglobin variants if marked discrepancy exists between A1C and plasma glucose 1

Common Pitfalls to Avoid

• Never wait weeks or months for routine confirmatory testing when the initial result suggests diabetes—this delays critical treatment 2
• Never diagnose based on a single test unless there is unequivocal hyperglycemia with classic symptoms 1
• Never use point-of-care A1C for diagnostic decisions due to lack of standardization 3
• Ensure plasma glucose samples are spun and separated immediately after drawing to prevent preanalytic variability that falsely lowers results 1, 2