Influence of dialyser clearance measurement accuracy on haemodialysis prescription based on Kt/V.

Kt/V urea (u) has been used as a measure of adequacy of haemodialysis (HD). However, the accurate assessment of its components is difficult and subject to error in a clinical setting. This study was designed to evaluate different forms of dialyser clearance (K) measurements and their influence on Kt/V. Sixteen patients on high-flux HD were studied at blood flow (Qb) rates of 250 and 350 ml/min and at constant dialysate flow rates. K of urea was measured by the arteriovenous blood sampling technique (Kbu), corrected for access recirculation (Kbru) and compared with K as determined by dialysate collection (Kdu) using a new sampling device. At Qb 250 and 350 ml/min, Kbu as based on dialysate collection was significantly lower than Kbru and Kbu as based on arteriovenous blood sampling: at Qb 250, Kdu 169.0 +/- 13.3, Kbru 191.2 +/- 11.5, and Kbu 203.0 +/- 9.3 ml/min (P < 0.0005); at Qb 350, Kdu 196.5 +/- 17.3, Kbru 227.7 +/- 15.5, and Kbu 243.6 +/- 12.7 ml/min (P < 0.0005). At Qb 250 ml/min Kbu t/V (1.33 +/- 0.17) overestimated Kdu t/V (1.11 +/- 0.13) by 16:8%, at Qb 350 ml/min by 19.3% (1.58 +/- 0.19 versus 1.27 +/- 0.15). Dialyser clearances based on arteriovenous differences in blood overestimate true clearances (and therefore Kt/V) as measured by dialysate collection. This overestimation is more marked with higher blood flow rates.