Determinants of diagnostic discordance for non-diabetic hyperglycaemia and Type 2 diabetes using paired glycated haemoglobin measurements in a large English primary care population: cross-sectional study

Aim: To investigate factors influencing diagnostic discordance for non-diabetic hyperglycaemia and Type 2 diabetes. Methods: Some 10 000 adults at increased risk of diabetes were screened with HbA1c and fasting plasma glucose (FPG). The 2208 participants with initial HbA1c ≥ 42 mmol/mol (≥ 6.0%) or FPG ≥ 6.1 mmol/l were retested after a median 40 days. We compared the first and second HbA1c results, and consequent diagnoses of non-diabetic hyperglycaemia and Type 2 diabetes, and investigated predictors of discordant diagnoses. Results: Of 1463 participants with non-diabetic hyperglycaemia and 394 with Type 2 diabetes on first testing, 28.4% and 21.1% respectively had discordant diagnoses on repeated testing. Initial diagnosis of non-diabetic hyperglycaemia and/or impaired fasting glucose according to both HbA1c and FPG criteria, or to FPG only, made reclassification as Type 2 diabetes more likely than initial classification according to HbA1c alone. Initial diagnosis of Type 2 diabetes according to both HbA1c and FPG criteria made reclassification much less likely than initial classification according to HbA1c alone. Age, and anthropometric and biological measurements independently but inconsistently predicted discordant diagnoses and changes in HbA1c. Conclusions: Diagnosis of non-diabetic hyperglycaemia or Type 2 diabetes with a single measurement of HbA1c in a screening programme for entry to diabetes prevention trials is unreliable. Diagnosis of non-diabetic hyperglycaemia and Type 2 diabetes should be confirmed by repeat testing. FPG results could help prioritise retesting. These findings do not apply to people classified as normal on a single test, who were not retested.