Diagnosis Confirmation: Repeat Fasting Glucose

Rationale for Repeat Testing

The American Diabetes Association recommends repeating the fasting glucose test to confirm the diagnosis of diabetes when the initial result exceeds the diagnostic threshold, in this case, ≥7.0 mmol/L (126 mg/dL) 1, 2, 3, 4, 5
The fasting glucose of 7.4 mmol/L exceeds the diabetes diagnostic threshold, while the HbA1c of 6.3% falls in the prediabetes range, indicating the need for repeat testing to confirm the diagnosis 1, 2, 3, 4
In the absence of unequivocal hyperglycemia, any single abnormal test result must be confirmed by repeat testing, as recommended by the American Diabetes Association 1, 2, 3

The 2010 American Diabetes Association guidelines state that if a patient meets the diabetes criterion of the A1C but not the FPG, or vice versa, that person should be considered to have diabetes, but only after repeat testing has been confirmed 1
The healthcare professional should repeat the abnormal test within 3-6 months to establish the diagnosis, according to the American Diabetes Association guidelines 1

Factors that may explain the discordance between fasting glucose and HbA1c results include preanalytical variability in glucose measurement, recent acute illness or stress, racial/ethnic differences in HbA1c levels, and conditions affecting erythrocyte lifespan, as reported in the Annals of Internal Medicine 6
To ensure accurate results, the repeat fasting glucose test should be performed after at least 8 hours of fasting, with blood drawn in the morning, using a tube with a rapidly effective glycolytic inhibitor, and processed within 15-30 minutes, as recommended by the American Diabetes Association 1, 2, 4

If the repeat fasting glucose is ≥7.0 mmol/L, the patient should be diagnosed with diabetes, even if the HbA1c is <6.5%, and initiated on diabetes management, including lifestyle modifications and consideration of metformin, according to the American Diabetes Association guidelines 1
If the repeat fasting glucose is <7.0 mmol/L but ≥5.6 mmol/L, the patient should be diagnosed with impaired fasting glucose (prediabetes), and counseled on lifestyle interventions, with consideration of repeat testing in 3-6 months, as recommended by the American Diabetes Association 1, 10