Concordance between fasting plasma glucose and HbA_1c in the diagnosis of diabetes in black South African adults: a cross-sectional study.

Abstract

We investigated concordance between haemoglobin A1c (HbA₁c)-defined diabetes and fasting plasma glucose (FPG)-defined diabetes in a black South African population with a high prevalence of obesity. Cross-sectional study. Rural South African population-based cohort. 765 black individuals aged 40-70 years and with no history of diabetes. The primary outcome measure was concordance between HbA_1c-defined diabetes and FPG-defined diabetes. Secondary outcome measures were differences in anthropometric characteristics, fat distribution and insulin resistance (measured using Homoeostatic Model Assessment of Insulin Resistance (HOMA-IR)) between those with concordant and discordant HbA_1c/FPG classifications and predictors of HbA_1c variance. The prevalence of HbA_1c-defined diabetes was four times the prevalence of FPG-defined diabetes (17.5% vs 4.2%). Classification was discordant in 15.7% of participants, with 111 individuals (14.5%) having HbA_1c-only diabetes (kappa 0.23; 95% CI 0.14 to 0.31). Median body mass index, waist and hip circumference, waist-to-hip ratio, subcutaneous adipose tissue and HOMA-IR in participants with HbA_1c-only diabetes were similar to those in participants who were normoglycaemic by both biomarkers and significantly lower than in participants with diabetes by both biomarkers (p<0.05). HOMA-IR and fat distribution explained additional HbA_1c variance beyond glucose and age only in women. Concordance was poor between HbA_1c and FPG in diagnosis of diabetes in black South Africans, and participants with HbA_1c-only diabetes phenotypically resembled normoglycaemic participants. Further work is necessary to determine which of these parameters better predicts diabetes-related morbidities in this population and whether a population-specific HbA_1c threshold is necessary.