ObjectivesTo assess the influence of abnormalities in fluid status and body composition on agreement between multifrequency bioimpedance analysis (MF-BIA), segmental BIA (ΣBIA), the Watson formula, and tracer dilution techniques.DesignCross-sectional.SettingMulticenter.Patients40 patients (29 males, 11 females) on peritoneal dialysis (PD).Main Outcome MeasuresAgreement between the various techniques used to assess total body water (TBW) [MF-BIA, deuterium oxide (D₂O), and the Watson formula] and extracellular water (ECW) [MF-BIA, bromide dilution (NaBr), and ΣBIA], also in relation to the relative magnitude of the body water compartments [ECW (NaBr):body weight (BW) and TBW (D₂O):BW] and body composition (DEXA). Second, the relation between body water compartments with echocardiographic parameters.ResultsWide limits of agreement were observed between tracer dilution techniques and MF-BIA [TBW (D₂O – MF-BIA) 2.0 ± 3.9 L; ECW (NaBr – MF-BIA) –2.8 ± 3.9 L], which were related to the relative magnitude of the body water compartments: r = 0.70 for ECW and r = 0.40 for TBW. ΣBIA did not improve the agreement [ECW (NaBr – ΣBIA): 3.7 ± 2.9 L]. Also, wide limits of agreement were observed between D₂O and the Watson formula (–2.3 ± 3.3 L). The difference between D₂O and Watson was related to hydration state and to percentage of fat mass ( r = 0.70 and r = –0.53, p < 0.05). Both ECW and TBW as assessed by BIA and tracer dilution were related to echocardiographic parameters.ConclusionWide limits of agreement were found between MF-BIA and ΣBIA with dilution methods in PD patients, which were related to hydration state itself. The disagreement between the Watson formula and dilution methods was related to both hydration state and body composition.