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The ratio Kt/V (K is patient clearance, t dialysis time, V urea space) has become the standard measure of dialysis adequacy. In this article simple Kt/V equations are developed theoretically from the urea mass balance equation. Two approximations lead to the most precise equation:Kt/V=-In R[1+1/1-R δBW/V]/[1-0.071786 t] where R is the post to pre dialysis urea ratio, BW/V is the amount of fluid removed during dialysis (ABW) expressed as a fraction of urea distribution space (V) at dry body weight (BW), and t is dialysis length in hours. A second equation arises with V approximated as 58% of BW. One further approximation leads to a simpler but slightly less precise Kt/V formula:These and earlier published equations were tested with two sets of data: 1) 49 sessions involving 17 patients on maintenance dialysis and 2) 540 computer simulations spanning all likely values of Kt/V (0.6-1.6), protein catabolic rate (0.6-1.6), interdialytic weight gain (0-4% of BW per day) and dialysis session length (2-4 hr). The most precise formula (upper equation above) had a maximum error of 0.031 and 0.035 Kt/V units for the clinical and simulated data, respectively, whereas the lower equation was slightly less accurate with maximum Kt/V errors of 0.079 and 0.081, respectively. The proposed Kt/V equations are considerably more accurate than previously published formulas.