Normally we measure postdialysis serum urea nitrogen concentration "C" and compare this with the initial or predialysis level "C"0. The session length or time is "t" and this is measured by the clock. The dialyzer clearance "K" is usually estimated, based on the urea transfer ability of the dialyzer (a function of its size and membrane permeability), the blood flow rate, and the dialysate flow rate. In some dialysis machines, the urea clearance during dialysis is estimated by testing the ability of the dialyzer to remove a small salt load that is added to the dialysate during dialysis.