Various methods for calculation of Kt/V: a clinical comparison

Analysis of the National Cooperative Dialysis Study (NCDS) showed that dialysis morbidity and mortality was related to Kt/V of urea. Various formulas have been derived to make calculation of Kt/V simpler. We compared the kinetically measured Kt/V (A) to estimated Kt/V derived from approximate total body water (0.6 body weight) and uniform dialyzer clearance (B), Kt/V = 0.04 PRU-1.2 where PRU is % reduction in BUN (C) and Kt/V = In(R-0.03-UF/W) where R is post/pre BUN, UF is ultrafiltration L/HD and W is postdialysis weight (D) in 26 patients. The correlation coefficient (r) was 0.14 (p less than 0.05) for A vs. B; 0.75 (p less than 0.01) A vs. C; 0.77 (p less than 0.01) A vs. D; and 0.97 for C vs. D. In an additional 51 patients, Kt/V was calculated using formulas B, C, and D. The r was 0.91 C vs. D, 0.70 B vs. D, and 0.68 B vs. C. With B, greater than 30% of the patients could have been misclassified using NCDS criteria for adequacy of dialysis. The study suggests that use of formulas C and D, but not B, is appropriate and accurate for Kt/V calculation. Formula C may be simpler and easier than D, as UF and W are not required. Serial Kt/V could be easily and accurately calculated using C and D for quality assurance and quantitation of dialysis.