Racial differences in performance of HbA1c for the classification of diabetes and prediabetes among US adults of non‐Hispanic black and white race

Aim To characterize differences between black and white people in optimal HbA1c thresholds for diagnoses of diabetes and prediabetes. Methods Data were included from the National Health and Nutrition Examination Survey, 2005–2014. Black and white adults (age 18–70 years) who underwent an oral glucose tolerance test and had available fasting plasma glucose, 2‐h plasma glucose and HbA1c measurements were eligible for inclusion. Diabetes or prediabetes status was defined by fasting plasma glucose and 2‐h plasma glucose using American Diabetes Association criteria. Classification of diabetes, prediabetes and dysglycaemia by HbA1c was evaluated for a range of HbA1c thresholds, with optimal thresholds defined as those values that maximized the sum of sensitivity and specificity (Youden's index). Results In 5324 black (32.3%) and white (67.7%) individuals, Youden's index (optimal) thresholds for HbA1c were ≥42 mmol/mol (6.0%) and ≥39 mmol/mol (5.7%) for discriminating diabetes vs non‐diabetes, ≥ 44 mmol/mol (6.2%) and ≥39 mmol/mol (5.7%) for discriminating diabetes vs prediabetes (excluding normoglycaemia), ≥39 mmol/mol (5.7%) and ≥37 mmol/mol (5.5%) for discriminating dysglycaemia vs normoglycaemia, and ≥39 mmol/mol (5.7%) and ≥37 mmol/mol (5.5%) for discriminating prediabetes vs normoglycaemia (excluding diabetes), in black and white people, respectively. Conclusions Consistently higher optimal HbA1c thresholds in black people than in white people suggest a need to individualize HbA1c relative to glucose levels if HbA1c is used to diagnose diabetes and prediabetes. What's new? A recent study using continuous glucose monitoring found that HbA1c levels were 3.3–5.5 mmol/mol (0.3–0.5%) higher among black people than white people at the same plasma glucose concentration. We examined how racial differences in the relationship between HbA1c and plasma glucose levels could influence classification of diabetes, prediabetes and dysglycaemia. Optimal HbA1c thresholds were 2 mmol/mol (0.2%) to 5 mmol/mol (0.5%) higher for black people than for white people. Differences in the relationship between HbA1c and plasma glucose suggest a need to individualize HbA1c interpretation for the diagnosis and treatment of diabetes and prediabetes.