Simplified equations for monitoring Kt/V, PCRn, eKt/V, and ePCRn

Abstract

Although computer solution of the differential equations used in urea kinetic modeling has its advantages, simplified formulas that are actually approximate algebraic solutions to the same equations in the clinically useful range are also useful. The Kt/V can be resolved from the predialysis to postdialysis urea nitrogen ratio (R), the weight loss (UF), session length in hours (t), and anthropometric or modeled volume (V) using the equation: KtV = In (R - 0.008 x t) + (4 - 3.5 x R) x 0.55 UF/V. The equilibrated Kt/V (eKTV) can be estimated from the single-pool arterial Kt/V (spaKTV) or the single-pool venous Kt/V (spmvKTV) using a rate equation based on the regional blood flow model of urea kinetics: eKTV = spaKTV - (0.60)(spaKTV)/t + 0.03 = spmvKTV - (0.46) (spmvKTV)/t + 0.02. The normalized protein catabolic rate (PCRn) can be determined from either the single-pool or equilibrated Kt/V based on the predialysis urea nitrogen level (C0) and the Kt/V (KTV) using the generalized equation: PCRn = C0/(a + bKTV + c/ + KTV) + 0.168, where the constants a, b, and c vary depending on the dialysis schedule and the time of the week that the predialysis blood specimen has been drawn. Such equations can be used either for retrospective surveys, or for quality assurance purposes, as well as for bedside guidance in an individual patient. The actual modeled urea distribution volume can also be easily computed if some effort is made to estimate the dialyzer urea clearance.