Misclassification Bias in the Waist-To-Hip Ratio: Implications for Obesity-Related Risk of Incident Type 2 Diabetes

Waist-to-hip ratio (WHR) directly reflects body fat distribution compared to other obesity indicators such as body mass index (BMI), waist circumference (WC), and waist-to-height ratio (WHtR). However, WHR may misclassify Type 2 diabetes mellitus (T2DM) risk by failing to account for the individual contribution of WC and hip circumference (HC). The study aimed to evaluate the discriminatory ability of WHR for T2DM and to identify its misclassification bias.

This prospective cohort study included 15,211 Chinese participants aged 50+ years without T2DM at baseline (2003-08), with follow-up conducted from 2008 to 2015. The Cox proportional hazard regression model was used to examine associations between obesity indicators and T2DM. Discriminatory ability was evaluated using the C-statistic, net reclassification index (NRI), and integrated discrimination improvement (IDI).

During a median follow-up of 5.4 years, 2238 participants (14.71%) developed T2DM. Participants with large WC and large HC had a higher T2DM risk compared to those with small WC and small HC in the low or high WHR category (adjusted hazard ratios (95% confidence intervals [CIs]): 2.33 [1.98, 2.74], 1.73 [1.47, 2.03]). WHR showed lower discrimination (C-statistic 0.632, 95% CI 0.619, 0.645) than WC (0.645, 95% CI 0.632, 0.658) and WHtR (0.657, 95% CI 0.644, 0.670). Combining WHR with BMI significantly improved discrimination compared to WHR alone (△C-statistic, NRI, IDI, and 95% CIs were 0.028 [0.020, 0.035], 0.015 [0.012, 0.020], and 0.141 [0.115, 0.165]), and it showed better discriminatory ability compared to the combination of BMI and WC (△C-statistic 0.007, 95% CI 0.004, 0.011).

WHR alone underestimates T2DM risk in individuals with large WC and large HC. Combining WHR with BMI addresses misclassification bias.