Derivation & validation of glycosylated haemoglobin (HbA(1c)) cut-off value as a diagnostic test for type 2 diabetes in south Indian population

BACKGROUND & OBJECTIVES: Glycosylated haemoglobin (HbA(1c)) has been in use for more than a decade, as a diagnostic test for type 2 diabetes. Validity of HbA(1c) needs to be established in the ethnic population in which it is intended to be used. The objective of this study was to derive and validate a HbA(1c) cut-off value for the diagnosis of type 2 diabetes in the ethnic population of Rayalaseema area of south India. METHODS: In this cross-sectional study, consecutive patients suspected to have type 2 diabetes underwent fasting plasma glucose (FPG) and 2 h post-load plasma glucose (2 h-PG) measurements after a 75 g glucose load and HbA(1c) estimation. They were classified as having diabetes as per the American Diabetes Association criteria [(FPG ≥7 mmol/l (≥126 mg/dl) and/or 2 h-PG ≥11.1 mmol/l (≥200 mg/dl)]. In the training data set (n = 342), optimum cut-off value of HbA(1c) for defining type 2 diabetes was derived by receiver-operator characteristic (ROC) curve method using oral glucose tolerance test results as gold standard. This cut-off was validated in a validation data set (n = 341). RESULTS: On applying HbA(1c) cut-off value of >6.3 per cent (45 mmol/mol) to the training data set, sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for diagnosing type 2 diabetes were calculated to be 90.6, 85.2, 80.8 and 93.0 per cent, respectively. When the same cut-off value was applied to the validation data set, sensitivity, specificity, PPV and NPV were 88.8, 81.9, 74.0 and 92.7 per cent, respectively, although the latter were consistently smaller than the proportions for the training data set, the differences being not significant. INTERPRETATION & CONCLUSIONS: HbA(1c) >6.3 per cent (45 mmol/mol) appears to be the optimal cut-off value for the diagnosis of type 2 diabetes applicable to the ethnic population of Rayalaseema area of Andhra Pradesh state in south India.