Normalized bioimpedance indices are better predictors of outcome in peritoneal dialysis patients

BACKGROUND: While phase angle of bioimpedance analysis (BIA) has great survival-predicting value in dialysis populations, it is known to be higher in male than in female subjects. In this study, we aimed to explore the factors influencing the predictive value of phase angle and to identify the appropriate physics terms for normalizing capacitance (C) and resistance (R).

METHODS: We formulated body capacitive index (BCI), C(BMI) (capacitance × height(2)/weight), body resistive index (BRI), R(BMI) (resistance × weight/height(2)), and CH(2) (capacitance × height(2)). We also studied H(2)/R, R/H, and reactance of a capacitor/height (X(C) /H). There are 3 components in this study design: (1) establishment of normal values in a control Malaysian population, (2) comparison of these with a CAPD population, and (3) prediction of survival within a CAPD population. We initially performed a BIA study in 206 female and 116 male healthy volunteers, followed by a prospective study in a cohort of 128 CAPD patients [47 with diabetes mellitus (DM), 81 non-DM; 59 males, 69 females] for at least 2 years. All the parameters during enrolment, including BIA, serum albumin, peritoneal equilibrium test, age, and DM status, were analyzed. Outcome measurement was survival.

RESULTS: In healthy volunteers, both genders had the same BCI (2.0 nF kg/m(2)). On the contrary, female normal subjects had higher BRI than male normal subjects (median 15 642 vs 13242 Ω kg/m(2), p < 0.001) due to higher fat percentage (35.4% ± 0.4% vs 28.0% ± 0.6%, p < 0.001), resulting in a lower phase angle (mean 5.82 ± 0.04 vs 6.86 ± 0.07 degrees, p < 0.001). Logistic regression showed that BCI was the best risk indicator in 128 CAPD patients versus 322 normal subjects. In age- and body mass index (BMI)-matched head-to-head comparison, BCI had the highest χ(2) value (χ(2) = 102.63), followed by CH(2) (or H(2)/X(C); χ(2) = 81.00), BRI (χ(2) = 20.54), and X(C)/H (χ(2) = 20.48), with p value < 0.001 for these parameters. In comparison, phase angle (χ(2) = 11.42), R/H (χ(2) = 7.19), and H(2)/R (χ(2) = 5.69) had lower χ(2) values. 35 (27.3%) patients died during the study period. Univariate analysis adjusted for DM status and serum albumin level demonstrated that non-surviving patients had significantly higher CH(2) (245 vs 169 nF m(2), p < 0.001) and BCI (4.0 vs 2.9 nF m(2)/kg, p = 0.005) than patients that survived. CH(2) was the best predictor for all-cause mortality in Cox regression analysis, followed by BCI, phase angle, and X(C)/H.

CONCLUSION: Measures that normalize, such as BCI and CH(2), have higher risk discrimination and survival prediction ability than measures that do not normalize, such as phase angle. Unlike phase angle, measurement of BCI overcomes the gender effect. In this study, the best risk indicator for CAPD patients versus the general population is BCI, reflecting deficit in nutritional concentration, while CH(2) reflects total nutritional deficit and thus is the major risk indicator for survival of CAPD patients.