Agreement of the Watson and Hume–Weyers Equations with Total Body Water Derived from a Bioimpedance Scale Across Adiposity Levels in an Andean–Amazonian Population

Anthropometric equations can estimate total body water (TBW) when bioelectrical impedance analysis (BIA) is unavailable, but their performance may vary by population and adiposity. We conducted a cross-sectional method-comparison study of 3655 adults in Chachapoyas, Peru, using sampling weights. TBW estimated by the Watson and Hume–Weyers equations was compared with estimates from the Tanita BC-730F scale, an indirect BIA comparator rather than an isotope-dilution reference method. Agreement was assessed using bias, limits of agreement, mean absolute error (MAE), root mean square error (RMSE), concordance, and bootstrap confidence intervals. Watson showed a bias of −0.04 L, an MAE of 1.17 L, an RMSE of 1.48 L, and limits of agreement from −2.94 to 2.87 L. Corresponding Hume–Weyers values were −1.05 L, 1.82 L, 2.24 L, and from −4.93 to 2.84 L. The paired MAE difference was 0.64 L (95% CI: 0.60–0.69) in favor of Watson. Performance varied by body mass index (p < 0.001), with the largest difference in class II–III obesity; hypertension did not independently modify error after adjustment. Recalibration did not meaningfully improve Watson. Watson showed closer and more consistent agreement with BIA-derived TBW than Hume–Weyers, supporting its use as a simple anthropometric approximation in population research and field settings.