1. Regular dialysis treatment is reported to remove inorganic sulphate, but not to restore its level to normal. To evaluate the adequacy of modern dialysis techniques in maintaining the sulphate balance, intra- and inter-dialysis plasma profiles and removal rates of sulphate were studied in 20 stable patients on maintenance haemodialysis. The influence of sulphate levels on the distribution of calcium-complex species was also investigated.

2. Sulphate was determined by ion-exchange chromatography of both serum ultrafiltrates, taken at the start of, during, at the end of, and at 24 h and 48 h after a dialysis session, and whole diffusate collections. Dialyser clearances of sulphate were assessed by two independent procedures and compared with those of urea and creatinine, on two different methods of dialysis, i.e. traditional haemodialysis with Cuprophan hollow fibre filters, and haemodiafiltration with high-flux Polysulphone or polyacrylonitrile dialysers. Concentrations of the main serum ions were determined before and after dialysis and used to solve a multiple mass balance equation system by which concentrations of the calcium-complex species were calculated.

3. Before dialysis, sulphate levels were eight times those determined in 17 control subjects and remained higher than normal at the end of dialysis. These changes were independent of the dialysis procedure. There was a close correlation between serum levels of sulphate and creatinine. Dialyser clearances of sulphate were comparable with those of creatinine, but lower than those of urea. Clearances of all solutes were higher on haemodiafiltration.

4. The mean removal of sulphate during a single session was about 30 mmol; the estimated removal rate averaged 15 mmol/day and was closely related to the protein catabolic rate as derived by urea kinetic modelling. Removal rates were slightly lower than rates of sulphate excretion in 40 control subjects. Because serum sulphate levels were quite stable over time this decrease was accounted for by a reduction in the net generation of sulphate, due to either altered catabolism of sulphur-containing amino acids or reduced intakes of animal protein.

5. The distribution of calcium-complex species demonstrated that the calcium-sulphate complex concentration in pre-dialysis samples was similar to that of calcium-phosphate complex and both were markedly increased as compared with normal subjects. Dialysis minimized but did not cancel out these abnormalities, which were responsible for significant decreases in free calcium concentration.

6. It is concluded that, although dialysis efficiently removes newly generated inorganic sulphate, it does not prevent chronic increases in this anion. Inadequate control of plasma sulphate levels by dialysis or through the diet may adversely affect the clinical state of uraemic patients and induce pervasive changes in calcium homoeostasis.

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