Equations for the Calculation of the Protein Catabolic Rate from Predialysis and Postdialysis Urea Concentrations and Residual Renal Clearance in Stable Hemodialysis Patients

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Abstract
Several simple equations exist for the calculation of Kt/V from predialysis (Cpre) and postdialysis (Cpost) measurements of urea concentration. Analogous equations are needed for precise determination of patient protein catabolic rate (nPCR) from Cpre and Cpost. In this study we develop three simple nPCR equations from urea mass balance theory. The equations, which include a term for residual function, may be applied to any session of the week for patients dialyzed three times weekly who are in steady state with respect to dialysis dose and protein catabolism. The precision of each equation was tested with Cpre, Cpost data obtained from steady state simulations of 540 patients without residual renal clearance (KR) and 972 simulated patients with significant residual Kr. The simplest equation has the form: nPCR = a[kt/V+ Kr/V](Cpre+ Cpost)+0.17 where V is urea distribution volume, and a and d are constants varying with session of the week. When compared to nPCR values calculated from formal urea kinetic modelling, the error determined with this formula never exceeded 5 % for the midweek or final session. A more complicated equation of the form: nPCR =a{[1-bCpost/Cpre][1-Cpost/Cpre+ΔBW/V]Cpre/(1-0.0003t)+d KR/V(Cpre+ Cpost)}+0.17 provided nPCR estimates with a maximum error pre, Cpost, length of dialysis session, volume ultrafiltered (ΔBW), and an approximate value of the patient’s urea distribution volume. The proposed equations permit nPCR to be calculated simply and accurately for stable patients dialyzed three times a week.

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