The Glucose Tolerance Test, But Not HbA 1c, Remains the Gold Standard in Identifying Unrecognized Diabetes Mellitus and Impaired Glucose Tolerance in Hypertensive Subjects

Abstract

The objective of this study was to compare the value of the oral glucose tolerance test (GTT), glycated hemoglobin concentration (HbA1c), and fasting plasma glucose (FPG) for identifying unrecognized diabetes mellitus (DM) and impaired glucose tolerance (IGT) in hypertensive subjects. One hundred forty-four consecutive subjects who were not known to have DM and who were attending the Hypertension Clinic underwent 24-hour ambulatory blood pressure (BP) monitoring. A GTT and an HbA1c measurement were also carried out. Abnormal results from GTT were found in 94 patients (65%). Results from FPG were not different between those with DM and IGT but were significantly higher than in the euglycemic subjects. The FPG was between 110-125 mg/dL (6.1-6.9 mmol/L) in 31% (n=20) of patients with IGT and in 53% (n=16) of those with DM. With use of the previously published criteria to diagnose DM of FPG ≥103 mg/dL (5.7 mmol/L) and HbA1c ≥5.9%, 33% of our diabetic subjects and 75% of those with IGT would have been misclassified as euglycemic. The previously reported cut-off point for HbA1c of >6.1% to diagnose DM was present in 77% of our patients with DM and in 14% (n=9) of the patients with IGT. Multiple regression analysis showed that an abnormal result from GTT was independent of the level of clinical or ambulatory BP, nocturnal BP dip, cholesterol level, smoking history, race, or class of antihypertensive medication taken. FPG levels or HbA1c, or their combination, are not accurate enough to identify DM or IGT in patients attending a hospital Hypertension Clinic. A GTT may be required in these patients to reliably identify those with DM or IGT.