Do I need to repeat Hemoglobin A1c (HbA1c) to establish the diagnosis of diabetes?

From the Guidelines

No, you do not need to repeat an HbA1c test to establish a diagnosis of diabetes if the initial HbA1c result is 6.5% or higher, as a single HbA1c test result of 6.5% or above is sufficient for diagnosing diabetes in most clinical situations, according to the most recent guidelines 1. However, if the HbA1c value is close to the diagnostic threshold (around 6.5%), or if there are inconsistencies with other clinical findings, a repeat test may be considered for confirmation. Some key points to consider when diagnosing diabetes include:

• A single HbA1c test result of 6.5% or above is sufficient for diagnosing diabetes in most clinical situations.
• If the initial HbA1c result is close to the diagnostic threshold, or if there are inconsistencies with other clinical findings, a repeat test may be considered for confirmation.
• Diabetes can also be diagnosed using other criteria, such as a fasting plasma glucose of 126 mg/dL or higher, a 2-hour plasma glucose of 200 mg/dL or higher during an oral glucose tolerance test, or a random plasma glucose of 200 mg/dL or higher in a patient with classic symptoms of hyperglycemia.
• The HbA1c test measures the percentage of hemoglobin that has glucose attached to it, reflecting average blood glucose levels over the past 2-3 months, making it a reliable diagnostic tool that is not affected by daily fluctuations in blood glucose levels or recent food intake. It's worth noting that the guidelines for diagnosing diabetes have been updated over time, with the most recent guidelines recommending the use of HbA1c for diagnosis, in addition to other criteria such as fasting plasma glucose and oral glucose tolerance tests 1. In clinical practice, the diagnosis of diabetes should be based on a combination of clinical judgment, patient symptoms, and laboratory test results, with consideration of the potential for laboratory error and the need for confirmation of diagnosis in certain cases 1.

From the Research

Diagnosis of Diabetes using HbA1c

• 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.
• The American Diabetes Association has recommended glycated hemoglobin (HbA1c ≥6.5%) as an alternate to fasting plasma glucose (FPG ≥7.0 mmol/L) for diagnosis of diabetes 3.
• However, studies have shown that HbA1c ≥ 6.5% detects less than 50% of diabetic patients defined by FPG and less than 30% of diabetic patients defined by 2hPG 4.

Repeat HbA1c Testing

• The requirement for a second positive test in asymptomatic individuals is retained, with an HbA1c greater than or equal to 50 mmol/mol (repeated on a second occasion in asymptomatic patients) being diagnostic of diabetes 5.
• If an HbA1c test result is discordant with the clinical picture or equivocal, plasma glucose testing should be performed 2.
• For patients with an initial HbA1c result of 41-49 mmol/mol, cardiovascular risk assessment and lifestyle interventions are recommended with repeat HbA1c screening in 6-12 months 5.

Limitations of HbA1c Testing

• HbA1c testing has the potential for systematic error, and factors that may not be clinically evident can impact HbA1c test results and may systematically raise or lower the value relative to the true level of glycemia 2.
• HbA1c ≥ 6.5% may miss a substantial number of people with type 2 diabetes, including some with fasting hyperglycemia, and may miss most people with impaired glucose tolerance 2.
• There were 484 patients below the diagnostic threshold (HbA1c <6.5%), resulting in 3.78% false negative predictions, suggesting that individuals with HbA1c between 6.0% and 6.5% may be considered as "probable diabetic" and their status should be verified by combined FPG and HbA1c criteria 3.