Calf Resistivity Values in Chronic Kidney Disease in a Caucasian Population

Background: Continuous intradialytic calf bioimpedance spectroscopy (cBIS) allows assessment of changes in calf extracellular fluid volume (ECV) to determine dry weight (DW) in hemodialysis patients. During dialysis, calf ECV decreases until excessive ECV has been removed and normalized resistivity ρ_N,5 rises to values comparable to those of a normal population (cBIS-DW). It is not clear whether chronic kidney disease (CKD) itself influences ρ_N,5 and whether normal values depend on race and ethnicity. Methods: Therefore, we determined ρ_N,5 in 2 populations consisting of 35 healthy Caucasians and 37 with CKD in KDIGO stages G2-G4. Calf resistivity was determined using a bioimpedance spectrum analyzer (Xitron Technologies, San Diego, CA, USA) and was normalized for body mass index. Results: ρ_N,5 was significantly higher in healthy subjects than in CKD patients (males: 18.2 ± 2.2 vs. 15.0 ± 2.8 × 10^-2·Ωm³kg^-1, p < 0.001; females: 19.7 ± 3.2 vs. 16.4 ± 3.3 × 10^-2·Ωm³kg^-1, p = 0.009). ρ_N,5 in Caucasians was significantly lower than in previously examined North American healthy subjects with prevailing African American race or Hispanic ethnicity (males: 18.2 ± 2.2 vs. 20.5 ± 2.0 × 10^-2·Ωm³kg^-1, p < 0.001; females: 19.7 ± 3.2 vs. 21.7 ± 2.6 × 10^-2·Ωm³kg^-1, p = 0.026). Conclusion: We present the first determination of ρ_N,5 values in a Caucasian healthy as well as a non-dialysis dependent CKD population. Both groups differ significantly. Due to higher amounts of extracellular water, subclinical fluid overload already occurs in pre-dialysis stages of CKD. ρ_N,5 in Caucasians differs significantly from previously established normal ranges in other races/ethnicities. Population-based reference ranges should be established and used in the future to determine DW by means of cBIS.