Nutritional and prognostic correlates of bioimpedance indexes in hemodialysis patients

We carried out a cross sectional and longitudinal study to assess whether bioimpedance indexes (resistance, Rz; reactance, Xc; phase angle, PA) reflect the nutritional status of hemodialysis (HD) patients, and bear a significant association with their long-term survival. The bioimpedance data of 131 patients on chronic HD treatment were compared with those of 272 healthy controls matched for age and sex. Nutritional status was assessed by anthropometric variables, serum albumin (SA), normalized protein catabolic rate (nPCR), and subjective global assessment (SGA). All three bioimpedance indexes varied significantly with HD treatment, however, with the exception of Xc in post-HD, they were on average significantly (P <0.016) different from controls either pre- and post-HD. Post-HD PA appeared to be the best index of nutritional status, being significantly correlated with SA, age, mid arm muscle circumference (MAMC), SGA, and nPCR (R(2)=0.44; P <0.01). However, depending on the cut-off levels, PA failed to detect clinically overt malnutrition in one to two thirds of the 12 patients with the worst SGA score. During the follow-up the changes in bioimpedance indexes reflected poorly the changes in dry body weight, only the Delta Rz bore a significant correlation (r=-0.29; P <0.01) with Delta body wt. Patients having baseline phase angle values within the lower quartile had a significantly lower two-year survival rate than patients having higher values (59.3% vs. 91.3%; P <0.01). Cox's analysis proportional hazard model) showed that phase angle as a predictor of death outweighed all other parameters included in the model (age, SG nPCR, MAMC, SGA), with a relative risk of 2.6 (95% CI=1.6 to 4.2). Bioimpedance indexes do not appear to be reliable in detecting clinically overt depletion of lean body mass. However, the strong association of PA with patient survival suggests that this bioimpedance index reflects some dimension of the illness, which is not fully identifiable with the deranged nutritional status.