The post: pre-dialysis plasma urea nitrogen ratio to estimate K·t/V and NPCR: mathematical modeling

The mathematical basis of the relationship between K· t/V and the ratio of the postdialysis (C _t ) to predialysis (C _o ) plasma urea nitrogen levels (C _t /C _o = R) is the urea kinetic model. The R vs. K· t/V relationship is modulated by the patient's normalized protein catabol-ic rate (NPCR), the dialysis session length, and the predialysis plasma urea nitrogen level, due to urea generated during the dialysis session; the latter increases C, and hence raises R. The relationship between R and K·t/V is also affected by the amount of ultrafiltrate removed during the dialysis session, because convective urea removal, which is a part of K, does not result in a lowering of the (C _t ). In stable adult maintenance dialysis patients receiving 3 treatments/week, with an NPCR of ≤ 1.1 g/kg/day and zero residual renal function, the target K· t/V is 1.05 and the target R will be about 0.41. In patients who require different K· t/V values, corresponding values of R can be computed. Based on an empiric examination of the urea kinetic equations, several formulas are proposed for estimating K· t/V from R and vice versa, which depend only on the dialysis session length t, the amount of ultrafiltrate UF, and the postdialysis weight Wor V; e.g., K· t/V = -In (R - 0.008·1 - UF/W).

After K· t/V has been estimated from R, t, UF and W, one can then estimate the NPCR if the residual renal urea clearance (Kru) has also been measured. From the estimated K· t/V, the Kru, and an estimated V, the total urea clearance over time corrected for V ("KT") is computed. Once "KT" is known, then NPCR can be derived from a nomogram using the midweek predialysis plasma urea nitrogen level. (Int J Artif Organs 1989; 12:411-9)