A method for determining the clearance competency of dialyzers utilized in renal hemodialysis comprises measuring the conductivity of a dialysate solution of low initial conductivity while circulating a priming solution of greater conductivity on the blood side of the dialyzer, correlating the rate of dialysis of molecules contained in the printing solution to known clearance rates for urea or other targeted blood impurity for the types and compositions of the particular dialysis membranes being utilized, and calculating the precise length of treatment time or other treatment parameters required to cleanse the blood to be dialyzed of impurities and biotoxins to a predicted target value. Failsafe control means which prevents mistakes in connecting unsafe fluid sources to the dialyzer is also provided.