Longitudinal changes in bioimpedance phase angle reflect inverse changes in serum IL-6 levels in maintenance hemodialysis patients

Objective: We hypothesize that longitudinal changes in phase angle (PA) have independent associations with changes in inflammatory parameters over time and consequently with long-term survival in patients on maintenance hemodialysis (MHD). The aim of the present study was to determine the effect of change in nutritional and inflammatory parameters over time on change in PA and on subsequent mortality in patients on MHD. Methods: A 2-y prospective longitudinal study was performed on 91 prevalent HD patients (57 men and 34 women), followed by an additional 3 y of clinical observations. Dietary intake, biochemical markers of nutrition, body composition, and interleukin (IL)-6 levels were measured at baseline and at 6, 12, 18, and 24 mo following enrollment. Results: In a linear mixed-effect model adjusted for baseline demographic and clinical parameters, each pg/mL increase in IL-6 over time was associated with a decrease in PA levels of 0.001°/2-y (P=0.003 for IL-6 × time interaction). PA remained associated with the rate of change in IL-6 even after controlling for extracellular water and fat mass. Changes in PA over time were associated with inverse linear changes in IL-6 (adjusted r = -0.32; P = 0.005) and consequently with mortality risk. For each 1° increase in PA, the crude and adjusted mortality hazard ratios using Cox models with effect of time-varying risk were 0.62 (95% confidence interval [CI], 0.54-0.71) and 0.61 (95% CI, 0.53-0.71), respectively. Additionally, longitudinal changes in PA exhibited significant associations with slopes of changes over time in main nutritional markers. Conclusions: Longitudinal changes in PA appear to be reliable in detecting changes in nutritional and inflammatory parameters over time, a combination that may contribute to the understanding of its prognostic utility.