Evaluation of performance of A1c and FPG tests for screening newly diagnosed diabetes defined by an OGTT among tuberculosis patients-a study from India.

Aim: The methods used for diagnosis of diabetes have limitations particularly in situations associated with stress hyperglycemia. Aim of this study was to evaluate the performance of A1c and fasting plasma glucose (FPG) tests for screening newly diagnosed diabetes (NDD) defined by OGTT among tuberculosis (TB) cases in India.
Methods: A total of 983 subjects aged ≥18 years with TB were selected from 7 TB units - 4 urban, 2 rural and 1 semi-urban areas of Tamil Nadu, India, during August 2010-March 2011. Screening for diabetes was carried out by 2-h 75g OGTT. Classification of glucose intolerance status was based on WHO criteria. HbA1c was measured by high performance liquid chromatography using Bio-Rad turbo machine. HbA1c≥47.5mmol/mol was used for diagnosis of diabetes. FPG was estimated by glucose-oxidase method. Known cases of subjects with diabetes were excluded and final analysis was done using data of 779 individuals. The performance of A1c and FPG tests was evaluated against the results of OGTT using receiver operating characteristic curve analysis.
Results: Prevalence of NDD was 10.8%. The areas under the curve (AUC) were 0.754 [95% confidence interval (CI) 0.68-0.83] (p<0.001) for A1c and 0.662 (95% CI 0.58-0.74) for FPG (p<0.001) in NDD subjects. The HbA1c cut-off point of ≥47.5mmol/mol gave a sensitivity of 59.1% and specificity of 91.7%, and the respective values were 34.8% and 97.5% for FPG in subjects with NDD.
Conclusion: HbA1c performed better than FPG as a screening tool for newly diagnosed diabetes among subjects with TB.
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