The Clear Space Index.

BACKGROUND: Kt/V for urea (Kt/V) is a dimensionless clinical index of dialysis dose; however, it does not directly reflect the removal capacity of the solute. Since the total amount of solute removal, M, is directly proportional to the initial solute concentration CB(0), it is reasonable to introduce an index by dividing M by CB(0). This index was termed 'clear space' (CS) because it was later found that it reflects the body fluid volume, the concentration of which becomes zero by the treatment.

SUMMARY: Although CS originally requires measurement of M, storing all the effluent of dialysis fluid, it may be more easily calculated by measuring concentrations in blood, using a simpler formula without storing any effluent of dialysis fluid. Moreover, this technique is applied not only for urea, but also for much larger solutes. The concept of CS is also applied to peritoneal dialysis, identifying the maximum CS relative to the total effluent volume of the dialysis fluid. It was shown by CS analysis that the 1-compartment model may not work well, even for small solutes like urea, under higher levels of solute removal. It was also shown that divergence from the 1-compartment model was greater in patients with regenerated cellulose membranes than in those with synthetic polymeric membranes. Key Messages:CS is a simple yet useful solute removal index that avoids the effect of CB(0) and may be used for comparison of dose among two or more treatment modalities. Unlike Kt/V, CS can be measured for all kinds of solute, giving the average for two or more compartments in which the concentration of solute of interest becomes zero.