KINETICS OF PHOSPHORUS DURING HEMODIALYSIS AND THE CALCULATION OF ITSEFFECTIVE DIALYSIS CLEARANCE

Removal of Pi from the body during hemodialysis (HD) is more difficult than that of urea or creatinine. Therefore, the precise evaluation of its effective dialysis clearance (K) is of practical significance. Es timation of K of Pi is associated with some problems because its kinet ics during HD cannot be expressed by a simple mathematical function. T he purpose of this study was to determine to what extent the calculati on of K of Pi is affected by the calculation method employed. In six p atients, the kinetics of Pi in serum and effluent dialysis fluid was r epeatedly monitored during HD. The first stage of HD, associated with a fairly rapid decrease in serum Pi concentrations, was followed by th eir stabilization at a constant level. The average predialysis serum P i was 2.69 (+/- 0.41) mmol/l. Three hours after the start of HD: the a verage serum Pi concentration was 1.33 (+/- 0.33) mmol/l and, four hou rs after the start of HD, it was 1.34 (+/- 0.30) mmol/l in average. Th e following procedures were used for K calculation: (a) the amount of Pi removed during the time interval at which serum Pi concentration wa s stabilized at a constant level (and calculated per one minute) was d ivided by the recorded stabilized serum value, (b) the amount of Pi re moved from the body during HD and calculated per one minute (D/t) was divided by the average value of pre- and post-HD serum concentrations, (c) K calculation assumed exponential function of the decrease in ser um Pi during HD, (d) the total amount of Pi removed during HD was divi ded by the area under the curve of serum concentrations (D/AUCs), (e) the amount removed during HD and calculated per one minute (D/t) was d ivided by the serum concentration recorded at the midpoint of HD. The average value of K of Pi calculated on the basis of method (a) was 122 .5 (+/- 33.6) ml/min and was significantly higher (p < 0.01) than that calculated on the basis of methods (b) (95.7 +/- 31.2 mmol/l) and (c) (103.2 +/- 31.9 mmol/l), but was significantly lower (p < 0.05) than that calculated according to method (e) (142.8 +/- 46.3 ml/min). The a verage value of K calculated on the basis of method (a) did not differ significantly from that calculated according to method (d) (124.7 +/- 35.2 ml/min). Calculation of K according to method (a) is identical w ith the classic method for calculating the renal clearance of various substances and used generally in clinical nephrology. We therefore use d this method as a reference one. The results obtained suggest that, o f the other methods used for K calculation, only method (d) based on t he calculation of the D/AUCs ratio gives values which do not differ si gnificantly from those obtained using method (a).