Gonzalez A, Deng Y, Lane A, et al. - Researchers examined if errors could occur in the diagnostic classification of diabetes and prediabetes, based only on HbA_1c levels, in relation to HbA_1c 'mismatches' (HbA_1c levels that are higher or lower than expected for the average glucose levels in different individuals). They conducted a cross-sectional study including 3,106 participants without known diabetes. The participants 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. More frequent higher mismatches were noted among those who were black, men, older, and have higher BMI. Using oral glucose tolerance test criteria, mismatch tertiles had similar distribution of normal glucose metabolism, prediabetes and diabetes; however, using HbA_1c criteria, the participants with low mismatches were classified as 97% normal glucose metabolism, 3% prediabetes and 0% diabetes, ie mostly normal, while those with high mismatches were classified as 13% normal glucose metabolism, 77% prediabetes and 10% diabetes, ie mostly abnormal. Findings thereby suggest that in people with low mismatches, under-diagnosis could occur and in those with high mismatches, over-diagnosis could occur in correlation to measuring only HbA_1c. They recommend performing additional oral glucose tolerance tests and/or fasting glucose testing to complement HbA_1c in diagnostic classification in most individuals.