From the Guidelines
A single A1C test is not typically considered diagnostic for diabetes on its own, and healthcare providers generally require either two separate A1C readings of 6.5% or higher, or confirmation with a different test such as fasting plasma glucose or oral glucose tolerance test. This approach is recommended because A1C results can be affected by various factors including certain anemias, hemoglobinopathies, recent blood loss or transfusions, pregnancy, and kidney or liver disease, as noted in the 2021 standards of medical care in diabetes 1. Additionally, A1C measurements can vary slightly between laboratories. The requirement for multiple tests or confirmation with different methods helps reduce the chance of misdiagnosis based on a single potentially misleading result.
Key Considerations
- The 2021 standards of medical care in diabetes recommend confirming the diagnosis with two abnormal test results, either from the same sample or in two separate test samples 1.
- If using two separate test samples, it is recommended that the second test be performed without delay, and if two different tests are both above the diagnostic threshold, this confirms the diagnosis 1.
- The diagnosis is made on the basis of the confirmed test, and if a patient has discordant results on two different tests, then the test result that is above the diagnostic cut point should be repeated 1.
Implications for Practice
- If your A1C is elevated on a single test, your healthcare provider will likely recommend follow-up testing before making a definitive diagnosis of diabetes or prediabetes.
- Healthcare providers should discuss signs and symptoms with the patient and repeat the test in 3–6 months if patients have test results near the margins of the diagnostic threshold 1.
Supporting Evidence
- The 2020 standards of medical care in diabetes also recommend confirming the diagnosis with two abnormal test results, either from the same sample or in two separate test samples 1.
- The 2019 and 2014 standards of medical care in diabetes provide similar recommendations, emphasizing the importance of confirming the diagnosis with multiple tests or different methods 1.
From the Research
Diagnostic Criteria for Diabetes
- The diagnosis of diabetes can be made using various methods, including hemoglobin A1c (HbA1c) testing, fasting plasma glucose, and oral glucose tolerance tests 2.
- The International Expert Committee recommends that the diagnosis of diabetes be made if HbA1c level is greater than or equal to 6.5% and confirmed with a repeat HbA1c test 2.
Limitations of HbA1c Testing
- HbA1c testing has the potential for systematic error, and factors such as patient ethnicity, normal aging, and conditions that impair erythrocyte production or alter the normal process of glycation can affect the results 3.
- A diagnostic cut-off point of HbA1c greater than or equal to 6.5% may miss a substantial number of people with type 2 diabetes, including some with fasting hyperglycemia, and misses most people with impaired glucose tolerance 2.
Combination of HbA1c and Plasma Glucose Measurements
- Combining the use of HbA1c and plasma glucose measurements for the diagnosis of diabetes offers the benefits of each test and reduces the risk of systematic bias inherent in HbA1c testing alone 2.
- Using HbA1c in combination with plasma glucose determinations can help identify individuals with diabetes and those at risk of developing diabetes 4.
Single HbA1c Measurement for Diagnosis
- A single HbA1c measurement may not be sufficient for the diagnosis of diabetes, as it may not accurately reflect the individual's glucose levels over time 2, 3.
- Repeat HbA1c testing and combination with plasma glucose measurements can help confirm the diagnosis of diabetes and reduce the risk of false positives or false negatives 2, 4.