Clearance of small molecules in different dialyzer flow configurations

To overcome problems of insufficient clearance, multiple dialyzers may be placed in series or in parallel. The present study aimed to investigate in vitro the overall clearance of small molecules in different dialyzer configurations in which mutual flow directions were changed. Single pass tests were performed with low flux Fresenius F6HPS dialyzers placed in series (12 tests), in parallel (6) and in single use (2). As blood substitute, either high concentrated (45 mS) bicarbonate dialysate (AB solution – MW20‐180) or a trisodiumphosphate (Na₃PO₄– MW395) concentration (30 mS) was used. Standard blood and dialysate flows of 250 and 500 mL/min, respectively, were applied. Furthermore, clearance was derived from conductivity measurements in the inlet and outlet bloodline, correcting for the overall ultrafiltration rate of 0.5 L/h (AB) and 0.1 L/h (Na₃PO₄). Compared to the standard setup using a single dialyzer with counter current flows, clearance increases by 3 to 8%(AB) and by 15 to 18%(Na₃PO₄) using two dialyzers in parallel and in series, respectively. With co‐current flows in a serial dialyzer set up, clearance increases by 16%(AB) and 22%(Na₃PO₄) compared to the single dialyzer use. Changing subsequently the counter current flows to co‐current in one and both dialyzers in series, the overall clearance decreases by 2 to 9%, respectively, for the AB solution, and by 8 to 15% for the Na₃PO₄ concentration. With respect to the parallel dialyzer setup, a split dialysate flow (250 mL/min in each dialyzer) counter current to the blood flow, increases the clearance by 4 and 12%, respectively. In conclusion, overall clearance is most ameliorated using two dialyzers in series with counter current flows.