Comparison of Hemoglobin A(1c) with Fasting and 2-h Plasma Glucose Tests for Diagnosis of Diabetes and Prediabetes among High-risk South Indians

BACKGROUND: Glycosylated hemoglobin (HbA1c) has not been evaluated extensively for diabetes and prediabetes diagnosis and short-term variability of fasting plasma glucose (FPG), 2-h PG post-75 g glucose load (2 hPG) and HbA1c has not been studied among Indians. OBJECTIVES: The study aimed to compare the sensitivity of HbA1c, FPG and 2 hPG for diabetes and prediabetes diagnosis as per the American Diabetes Association criteria, assess short-term variability of three tests and determine optimal HbA1c cutoffs for diabetes and prediabetes diagnosis among high-risk south Indians. METHODS: This diagnostic accuracy study, conducted at a tertiary care teaching hospital located in South India, enrolled 332 adults at high risk for diabetes and subjected them to testing (FPG, 2 hPG, and HbA1c) twice at 2–3 weeks interval. Sensitivity of three tests for diagnosing diabetes and prediabetes was determined based on the final diagnosis of normoglycemia/prediabetes/diabetes made with six test results for each participant. Optimal HbA1c cutoffs for diabetes and prediabetes were determined based on the final diagnosis of glycemic status made with four test results of FPG and 2 hPG. RESULTS: FPG, 2 hPG, and HbA1c, at American Diabetes Association recommended values, had sensitivity of 84.4%, 97%, and 93.8% respectively for diabetes diagnosis. HbA1c had lowest short-term variability (CVw = 1.6%). Receiver operating characteristic curve plotted with mean (of two values) HbA1c for each participant showed optimal HbA1c cutoffs of 6.5% for diabetes (area under curve [AUC] =0.990, sensitivity = 95.8%, specificity = 96.2%, accuracy = 95.2%) and 5.9% for prediabetes (AUC = 0.893, sensitivity = 84.3%, specificity = 80%, accuracy = 75.6%) diagnosis respectively. HbA1c <5.6% had 100% negative predictive value to exclude prediabetes/diabetes. CONCLUSIONS: HbA1c ≥6.5% is a convenient and reliable alternative to plasma glucose tests to diagnose diabetes among high-risk South Indians. HbA1c ≥5.9% is optimal for prediabetes diagnosis and value <5.6% excludes prediabetes/diabetes. Abbreviations used in the manuscript: ADA: American Diabetes Association, AUC: Area under curve, CVw: Within-person coefficient of variation, FPG: Fasting plasma glucose, 2 hPG: Two-hour plasma glucose post-75 g oral glucose load, HbA1c: Glycosylated haemoglobin, IFG: Impaired fasting glucose, IGT: Impaired glucose tolerance, NPV: Negative predictive value, PPV: Positive predictive value; PG: Plasma glucose, ROC: Receiver operating characteristic