The mathematical basis of the relationship between K.t/V and the ratio of the postdialysis (Ct) to predialysis (Co) plasma urea nitrogen levels (Ct/Co = R) is the urea kinetic model. The R vs. K.t/V relationship is modulated by the patient's normalized protein catabolic rate (NPCR), the dialysis session length, and the predialysis plasma urea nitrogen level, due to urea generated during the dialysis session; the latter increases Ct 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 (Ct). In stable adult maintenance dialysis patients receiving 3 treatments/week, with an NPCR of less than or equal to 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 W or V; e.g., K.t/V = - ln (R - 0.008.t-UF/W). After K.t/V has been estimated from R, t, UF and W, one can then estimate the NPCR in 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.(ABSTRACT TRUNCATED AT 250 WORDS)