Quantitating dialysis using two dialysate samples: a simple, practical and accurate approach for evaluating urea kinetics.

Urea kinetics is now widely used to determine the adequacy of dialysis. Several simplified formulae are currently in use but only a few have been accepted into clinical practice because of their simplicity and ease of calculation. A recent analysis of these formulae showed that for the same set of blood urea values the calculated Kt/V can range from 1.0 to 1.5. We have developed a new dialysate-based method (2DSM) to estimate the urea kinetic parameters using dialysate and blood samples taken at the beginning and at the end of dialysis. The total urea removed (TUR) was calculated from the geometric mean of the two dialysate samples, dialysate flow rate and the duration of dialysis. The Watson formula was used to determine the volume of distribution of urea. A comparison of the 2DSM and the direct dialysate quantification (DDQ) method showed the following results (mean +/- sd, n = 52): for total urea removal (TUR) 697 +/- 32 vs 722 +/- 37 mmol (p = 0.6, r2 = 0.928, y = 101 + 0.83 x, mean difference 25 +/- 76 mmol, see Bland-Altman plot), dialysate urea concentration (Durea) 5.55 +/- 0.25 vs 5.75 +/- 0.29 mmol/l (p = 0.6, r2 = 0.928, y = 0.8 + 0.82 x, mean difference 0.2 +/- 0.6 mmol, see Bland-Altman plot), dialyser clearance (K) 232 +/- 4.4 vs 235 +/- 5.6 ml/min (p = 0.54), Kt/V 1.42 +/- 0.04 vs 1.51 +/- 0.04 (p = 0.21), volume of distribution of urea (Vd) 40.14 +/- 1.04 vs 38.74 +/- 1.2 L, (p = 0.38), and PCR 64.6 +/- 2.6 vs 68.1 +/- 3.1 g/day. We have developed a simple method of determining dialysate-based urea kinetics which requires two dialysate samples, one at the beginning and one at the end of dialysis and a blood sample at the midpoint of dialysis. TUR can be calculated using the dialysate flow rate and the dialysis duration and once this is known all the other kinetic parameters can be calculated.