Physiologic clotting of a dialysis circuit was achieved using an in vitro method. The closed loop method used a circuit filled with defibrinated bovine blood. To achieve clotting, a slow infusion (5-15 ml/hr) of ACD bovine plasma was performed. The rate and amount of infusion of the plasma allowed for careful control of the amount of clotting in the circuit and dialyzer. By varying these, a range of clotting (10-40% dialyzer volume reduction) was achieved in a timeframe of approximately 100 minutes. The results of the technique closely mimicked clinical examples of dialysis circuits in appearance. Residual blood clots were present in the venous drip chamber filter, in nonstreamlined areas of the bloodline (e.g., bond joint gaps), and throughout the dialyzer fibers and header space. As an additional check of clotting distribution, the performance of the dialyzers compared favorably with previously reported effects of reuse. Solute clearances decreased with the fiber bundle volumes (FBV) of the devices. Correlations of small molecular weight (MW) (urea, creatinine) clearances with FBV were linear, whereas middle MW (vitamin B12) clearances and the ultrafiltration coefficient (kUF) had nonlinear correlations. The results indicate that this clotting method can be used as a valuable tool for a qualitative in vitro assessment of a hemodialysis extracorporeal circuit.