QUANTITATING DIALYSIS USING 2 DIALYSATE SAMPLES - A SIMPLE, PRACTICALAND 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 f or the same set of blood urea values the calculated Kt/V can range fro m 1.0 to 1.5. We have developed a new dialysate-based method (2DSM) to estimate the urea kinetic parameters using dialysate and blood sample s taken at the beginning and at the end of dialysis. The total urea re moved (TUR) was calculated from the geometric mean of the two dialysat e samples, dialysate flow rate and the duration of dialysis. The Watso n formula was used to determine the volume of distribution of urea. A comparison of the 2DSM and the direct dialysate quantification (DDQ) m ethod showed the following results (mean +/- sd, n = 52): for total ur ea removal (TUR) 697 +/- 32 vs 722 +/- 37 mmol (p = 0.6, r(2) = 0.928, y = 101 + 0.83 x, mean difference 25 +/- 76 mmol, see Bland-Altman pl ot), dialysate urea concentration (Durea) 5.55 +/- 0.25 vs 5.75 +/- 0. 29 mmol/l (p = 0.6, r(2) = 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/d ay. We have developed a simple method of determining dialysate-based u rea kinetics which requires two dialysate samples, one at the beginnin g 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 parame ters can be calculated.