Blood Tests for Diabetes Reassessed for Reliability

The routine oral glucose tolerance test and the glycosylated hemoglobin (HbA1c) blood tests have been compared for their use in prediabetes diagnosis.

The expert recommendation to use HbA1c test in place of fasting blood glucose levels to screen for and diagnose diabetes is not universally accepted and needs to be tested in clinical settings.

Scientists at the Mayo Clinic (Scottsdale, AZ, USA) examined the blood of 242 patients who were entering a diabetes prevention program between January 1, 2007, and September 13, 2009. They analyzed the participant's baseline oral glucose tolerance test and HbA1c results. Glucose determinations and HbA1c levels were performed on a chemistry analyzer. Blood glucose values are determined by the standard hexokinase assay. The HbA1c determination was based on turbidimetric immunoinhibition using hemolyzed whole blood.

Patients were classified as having impaired fasting glucose (IFG), or impaired glucose tolerance (IGT), or combined IFG and IGT based on oral glucose tolerance test results. The scientists calculated the percentage of patients with IFG and IGT who had HbA1c values in the ranges of 5.7% to 6.4% and 6.0% to 6.4%. The mean age with standard deviation (SD) of the patients was 62 ± 11 years; 64.0% were women, and 88.0% were white. The mean with SD for fasting glucose level was 110 ± 8 mg/dL, the two-hour glucose level was 137 ± 35 mg/dL and HbA1c level was 5.8% ± 0.36%. Isolated IFG was detected in about 56.2% of patients and combined IFG and IGT in about 37.2%. Only 28.5% of patients had HbA1c values in the 6.0% to 6.4% range, whereas 65.3% had values in the 5.7% to 6.4% range. The chemistry analyzer used was the Roche Cobas 6000 (Roche Professional Diagnostics; Indianapolis, IN, USA).

The authors concluded that for their purposes, a diabetes prevention program, by relying solely on the HbA1c testing, a substantial number of eligible patients would be overlooked. They deemed it reasonable to continue using fasting glucose levels, at least for their diabetes prevention program, as the initial screening method for determining eligibility for participation. The study was published in May 2011, in the American Journal of Clinical Pathology.

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