Kinetics of phosphorus during hemodialysis and the calculation of its effective 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. Estimation of K of Pi is associated with some problems because its kinetics during HD cannot be expressed by a simple mathematical function. The purpose of this study was to determine to what extent the calculation of K of Pi is affected by the calculation method employed. In six patients, the kinetics of Pi in serum and effluent dialysis fluid was repeatedly monitored during HD. The first stage of HD, associated with a fairly rapid decrease in serum Pi concentrations, was followed by their stabilization at a constant level. The average predialysis serum Pi was 2.69 (+/-0.41) mmol/l. Three hours after the start of HD, the average serum Pi concentration was 1.33 (+/-0.33) mmol/l and, four hours after the start of HD, it was 1.34 (+/-0.30) mmol/l in average. The following procedures were used for K calculation: (a) the amount of Pi removed during the time interval at which serum Pi concentration was stabilized at a constant level (and calculated per one minute) was divided by the recorded stabilized serum value, (b) the amount of Pi removed 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 serum Pi during HD, (d) the total amount of Pi removed during HD was divided 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 divided 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 average 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 with the classic method for calculating the renal clearance of various substances and used generally in clinical nephrology. We therefore used this method as a reference one. The results obtained suggest that, of the other methods used for K calculation, only method (d) based on the calculation of the D/AUCs ratio gives values which do not differ significantly from those obtained using method (a).
Schück
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Institute for Clinical and Experimental Medicine, NMC Hemodialysis Center; Prague-Krc, Czech Republic.
Kaslikov J
D003404:Creatinine / Q000097:blood; D005260:Female; D006801:Humans; D007676:Kidney Failure, Chronic / Q000378:metabolism / Q000628:therapy*; D007700:Kinetics; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D010758:Phosphorus / Q000097:blood / Q000378:metabolism*; D006435:Renal Dialysis; D014508:Urea / Q000097:blood
D010758:Phosphorus; D014508:Urea; D003404:Creatinine