Using HbA1c and OGTT to identify prediabetes in hypertensive patients

Abstract

Introduction: Identifying individuals at risk for diabetes is important for preventing both diabetes and cardiovascular diseases, which is strongly associated with prediabetes. The American Diabetes Association has incorporated the use of glycated haemoglobin (HbA1c) as an additional test in the criteria for the identification of prediabetes (5,7%–6,4%). Because HbA1c testing is simple and can be performed regardless of prandial status, it is tempting to rely on HbA1c alone in finding such patients. But this has not been introduced into Europe. We aim to evaluate the performance of HbA1c compared to an oral glucose tolerance test (OGTT) in a primary care population. Materials and methods: Seventy three patients (57,6±7,6 years, females 41 (55%) males 32 (44%)) with hypertension were examined. They were further investigated by OGTT with HbA1c taken simultaneously, HbA1c 6,30±1,91%, fasting glucose 5,42±1,34 mmol/L, 2-hour glucose 6,41±1,17 mmol/L. Results: OGTT identified 34 (46,58%) with prediabetes (12 with impaired glucose tolerance, 22 with isolated impaired fasting glycaemia) and 39 (53,42%) with normoglycaemia. Using HbA1c values to classify these categories would identify 47,05% (16/34) of those with diabetes, 5,90% (2/34) with prediabetes and 47,05% (16/34) with normoglycaemia and 46,16% (18/39) of those with prediabetes and 53,84% (21/39) with normoglycaemia. HbA1c provides a sensitivity and specificity 52,95% for identifying prediabetes. 16 subjects were misclassified as not having prediabetes and 18 as prediabetes although they were either normoglycaemic. Conclusion: HbA1c alone is not accurate enough to screen individuals for prediabetes. Many at risk for diabetes will be missed by HbA1c values in the ADA-specified range. Screening should include fasting glucose and OGTT.