Diagnosis of Diabetes with A1C Testing
A single A1C ≥6.5% is NOT sufficient to diagnose diabetes in most cases—you need confirmatory testing with a repeat test unless the patient has unequivocal hyperglycemia or classic symptoms of hyperglycemic crisis. 1, 2
When Confirmation is Required
In the absence of unequivocal hyperglycemia, any diagnostic test result (including A1C ≥6.5%) must be repeated to rule out laboratory error before confirming the diagnosis of diabetes. 1, 3
The American Diabetes Association explicitly states that a test result diagnostic of diabetes should be repeated unless the diagnosis is clear on clinical grounds, such as a patient with classic symptoms of hyperglycemia or hyperglycemic crisis. 1, 2
For patients without clear clinical symptoms, the second confirmatory test should be performed without delay. 2
How to Confirm the Diagnosis
It is preferable to repeat the same test (A1C) for confirmation, since there will be a greater likelihood of concurrence. 1, 2
For example, if the initial A1C is 7.0% and a repeat result is 6.8%, the diagnosis of diabetes is confirmed—both values are above the 6.5% threshold. 1, 2
If two different tests are available (e.g., A1C and fasting plasma glucose) and both are above their respective diagnostic thresholds, the diagnosis is confirmed without need for a third test. 1, 2
When Discordant Results Occur
If you have discordant results from two different tests (one above and one below the diagnostic threshold):
The test whose result is above the diagnostic cut point should be repeated, and the diagnosis is made on the basis of the confirmed test. 1
If a patient meets the diabetes criterion by A1C (two results ≥6.5%) but not by fasting plasma glucose (<126 mg/dL), that person should be considered to have diabetes. 1
Important Caveats and Pitfalls
A1C has the least preanalytic and analytic variability compared to fasting plasma glucose and 2-hour post-challenge glucose, making it the most reliable test when repeated. 1, 2
However, A1C should NOT be used for diagnosis in conditions with abnormal red blood cell turnover (hemolytic anemia, iron deficiency anemia, hemoglobin variants, recent blood loss or transfusion, erythropoietin therapy). 1, 3
In rapidly evolving type 1 diabetes (particularly in children), A1C may not be significantly elevated despite frank diabetes, so glucose-based criteria should be used. 1
A1C testing for diagnosis must be performed in a certified laboratory using an NGSP-certified method standardized to the DCCT assay—point-of-care A1C testing is NOT acceptable for diagnosis. 1, 3
When a Single Test IS Sufficient
The only exception where a single test suffices is when there is unequivocal hyperglycemia (random plasma glucose ≥200 mg/dL) with classic symptoms of hyperglycemia or hyperglycemic crisis. 1, 3, 4
Borderline Cases
For patients whose repeat test falls below the diagnostic threshold after an initial elevated result: