Diagnostic Accuracy of Body Mass Index, Bioelectrical Impedance Analysis, and Waist-to-Hip Ratio Against DEXA for Obesity in Saudi Arabia: A Cross-Sectional Study

Background/Objectives: Body mass index (BMI) is the most widely used measure for obesity classification, yet it does not directly measure body fat and may substantially underestimate obesity prevalence. Simultaneous evaluation of the diagnostic accuracy of bioelectrical impedance analysis (BIA), BMI, and waist-to-hip ratio (WHR) against dual-energy X-ray absorptiometry (DEXA) for obesity classification is lacking in Saudi Arabia. We aimed to quantify the diagnostic performance of these three surrogate measures against DEXA in Saudi adults. Methods: We conducted a cross-sectional diagnostic accuracy study reported according to the STARD guideline at Prince Sultan Military Medical City, Riyadh, Saudi Arabia. We enrolled a convenience sample of 399 adults aged 18–65 years recruited from primary health care and outpatient clinic waiting areas. Each participant underwent InBody 770 BIA, anthropometric measurements (BMI, WHR), and DEXA (Lunar iDXA, GE) on the same day. We evaluated BIA body fat percentage against DEXA body fat percentage, BMI ≥ 30 kg/m2 against DEXA body fat percentage and fat mass index (FMI)-defined obesity, and WHR against the DEXA android-to-gynoid ratio. We calculated sensitivity, specificity, positive and negative predictive values, likelihood ratios, and area under the receiver operating characteristic curve (AUC), overall and stratified by sex. Results: DEXA classified 86.0% of participants as having obesity, compared with 62.9% by BIA and 24.8% by BMI; against DEXA body fat percentage, BMI ≥ 30 kg/m2 had a sensitivity of 28.9% and specificity of 100%. When FMI replaced body fat percentage as the reference standard, BMI sensitivity increased to 68.5% (specificity 97.3%, AUC 0.95). BIA achieved a sensitivity of 72.3%, specificity of 94.4%, and AUC of 0.86. WHR had the lowest sensitivity for central obesity (54.0%, specificity 94.8%, AUC 0.80). BIA showed the strongest correlation with DEXA body fat percentage (Spearman ρ = 0.82), followed by BMI (ρ = 0.60). Conclusions: BMI alone missed two-thirds of participants with DEXA-defined obesity in this Saudi adult cohort. Although BIA was more sensitive, it still missed 27.7% of individuals with DEXA-defined obesity. These findings support integrating body composition assessment into clinical practice and suggest that national obesity prevalence estimates based on BMI alone substantially underestimate the true burden.