BACKGROUND Adequate delivered dose of solute removal (as assessed by urea reduction and calculation of Kt/V) is an important determinant of clinical outcome in chronic hemodialysis (HD) patients. This requires both prescription of an adequate dose of HD and regular assessment that the delivered treatments are also adequate. Online conductivity monitoring (OCM) using sodium flux as a surrogate for urea allows for the repeated non-invasive measurement of Kt/V on each HD treatment. METHODS We prospectively studied 17 (9 males, 8 females) established chronic HD patients over an eight-week period (408 treatments). A pre- and post-dialyzer measurement of the conductivity is performed by two mutually independent temperature-compensated conductivity cells equipped with Fresenius 4008 S(R) dialysis machines. Urea reduction was measured once a week by a single-pool calculation using immediate post-treatment sampling. No changes were made to any of the dialysis prescriptions over the study period. Values of calculated Kt/V and simultaneously obtained online Kt/V were compared. RESULTS There was a statistically significant difference between calculated Kt/V and online Kt/V over the study period. The mean calculated Kt/V was 1.37 +/- 0.09, and mean online Kt/V 1.02 +/- 0.15 (p = 0.000). Calculated Kt/V >or= 1.2 was achieved in all our patients, while online Kt/V >or= 1.2 was achieved in only 17.64%. Yet there was moderate correlation between calculated Kt/V and online Kt/V (r(2) = 0.48). CONCLUSIONS Online conductivity monitoring (OCM) results underestimates dialysis efficiency compared to calculated Kt/V readings. This difference has to be considered when applying Kt/V to clinical practice.