Impact of mismatches in HbA1c vs glucose values on the diagnostic classification of diabetes and prediabetes

Abstract Aims To determine whether HbA 1c mismatches (HbA 1c levels that are higher or lower than expected for the average glucose levels in different individuals) could lead to errors if diagnostic classification is based only on HbA 1c levels. Methods In a cross‐sectional study, 3106 participants without known diabetes underwent a 75‐g oral glucose tolerance test (fasting glucose and 2‐h glucose) and a 50‐g glucose challenge test (1‐h glucose) on separate days. They were classified by oral glucose tolerance test results as having: normal glucose metabolism; prediabetes; or diabetes. Predicted HbA 1c was determined from the linear regression modelling the relationship between observed HbA 1c and average glucose (mean of fasting glucose and 2‐h glucose from the oral glucose tolerance test, and 1‐h glucose from the glucose challenge test) within oral glucose tolerance test groups. The haemoglobin glycation index was calculated as observed – predicted HbA 1c , and divided into low, intermediate and high haemoglobin glycation index mismatch tertiles. Results Those participants with higher mismatches were more likely to be black, to be men, to be older, and to have higher BMI (all P <0.001). Using oral glucose tolerance test criteria, the distribution of normal glucose metabolism, prediabetes and diabetes was similar across mismatch tertiles; however, using HbA 1c criteria, the participants with low mismatches were classified as 97% normal glucose metabolism, 3% prediabetes and 0% diabetes, i.e. mostly normal, while those with high mismatches were classified as 13% normal glucose metabolism, 77% prediabetes and 10% diabetes, i.e. mostly abnormal ( P <0.001). Conclusions Measuring only HbA 1c could lead to under‐diagnosis in people with low mismatches and over‐diagnosis in those with high mismatches. Additional oral glucose tolerance tests and/or fasting glucose testing to complement HbA 1c in diagnostic classification should be performed in most individuals.