What is the best time to confirm diabetes with a second test after the initial test?

Best Time to Confirm Diabetes by Second Test

For patients with an initial abnormal test result suggesting diabetes, the second confirmatory test should be performed without delay unless there is a clear clinical diagnosis of diabetes.1

Timing of Confirmatory Testing

• If using two separate test samples, the second test (either a repeat of the initial test or a different test) should be performed without delay after the first abnormal result 1
• For patients with test results near the margins of the diagnostic threshold, healthcare professionals should follow the patient closely and repeat the test in 3-6 months 1
• In cases of unequivocal hyperglycemia (patient with classic symptoms of hyperglycemia or hyperglycemic crisis and random plasma glucose ≥200 mg/dL), confirmation with a second test is not necessary 1

Confirmation Methods

• It is preferable to repeat the same test for confirmation, as there will be a greater likelihood of concurrence 1
• If two different tests (such as A1C and FPG) are available and both are above the diagnostic threshold, this confirms the diagnosis without need for further testing 1
• If a patient has discordant results from two different tests, the test result that is above the diagnostic cut point should be repeated 1

Diagnostic Criteria Requiring Confirmation

• Fasting plasma glucose (FPG) ≥126 mg/dL (7.0 mmol/L) after at least 8 hours of fasting 1
• 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during an oral glucose tolerance test (OGTT) 1
• A1C ≥6.5% (48 mmol/mol) performed in a laboratory using a method that is NGSP certified and standardized to the DCCT assay 1

Special Considerations

• Preanalytic and analytic variability exists for all tests, with A1C having the least variability, followed by FPG, and 2-h PG having the most variability 1
• For plasma glucose samples, it is critical that samples be spun and separated immediately after they are drawn to prevent preanalytic variability 1
• In conditions with abnormal red blood cell turnover (e.g., pregnancy, hemoglobinopathies, recent blood loss), only plasma glucose criteria should be used for diagnosis 1

Clinical Examples

• Example 1: If the A1C is 7.0% and a repeat result is 6.8%, the diagnosis of diabetes is confirmed 1
• Example 2: If a patient meets the diabetes criterion of A1C (two results ≥6.5%) but not FPG (<126 mg/dL), that person should nevertheless be considered to have diabetes 1

Common Pitfalls to Avoid

• Delaying confirmatory testing can lead to delayed diagnosis and treatment 1, 2
• Failing to consider factors that may affect A1C reliability (hemoglobinopathies, pregnancy, HIV treated with certain drugs, iron-deficient anemia) 1
• Not properly handling plasma glucose samples (should be spun and separated immediately) can lead to falsely lower results 1