iPTH values during hemodialysis: Role of ionized Ca, dialysis membranes and iPTH assays

iPTH values during hemodialysis: Role of ionized Ca, dialysis membranes and iPTH assays. The evolution of serum iPTH concentration during hemodialysis was studied in eight patients who were dialyzed with cuprophane (Cu) and polyacrylonitrile membranes (PAN) during two four-hour sessions. Ca++ concentration in the dialysate was 1.37 mM/liter. iPTH was measured with an intact hormone immunoradiometric assay (I), with two late (L1, L2) and one mid (M) carboxylterminal immunoassays at the beginning and end of hemodialysis, from the arterial and venous sides of the extracorporeal unit. Results are means ± SD. Serum Ca++ increased during dialysis with Cu (1.26 ± 0.08 vs. 1.33 ± 0.03 mmol/liter, P < 0.05), without any change in the concentration of iPTH measured with L1, L2 or M, but with a 50% decrease in iPTH measured with I (21.8 ± 19.2 vs. 10.3 ± 9.0 pmol/liter, P < 0.05). Serum Ca++ increased similarly with PAN (1.25 ± 0.10 vs. 1.34 ± 0.04 mmol/liter, P < 0.01), but there was a greater than 50% decrease in iPTH concentration measurements for all four assays (I: 17.2 ± 17 vs. 7.6 ± 8.3 pmol/liter, P < 0.05; L1: 92 ± 75 vs. 36 ± 32 pmol/liter, P < 0.05; L2; 312 ± 289 vs. 126 ± 128 pmol/liter, P < 0.01; M: 926 ± 1074 vs. 373 ± 422 pmol/liter, P < 0.05). iPTH clearance was minimal through Cu and significantly higher through PAN with I (124 ± 33ml/min, P < 0.005), L1 (79 ± 17ml/min, P < 0.0005), L2 (83 ± 19ml/min, P < 0.0005) and M (52 ± 25ml/min, P < 0.05); since intact hormone was cleared more readily than large fragments and the latter more readily than small fragments, hydrophobic adherence to the PAN was first postulated and then demonstrated during in vitro dialysis of 131I-bPTH(1–84) and 125I-bPTH(41–84). Thus, increasing serum Ca++ concentration during hemodialysis reduced the circulating level of intact PTH by half without influencing the concentration of carboxylterminal fragments. The concentration of the latter could be reduced by more than 50%, and that of intact hormone by a further 25% by selection of a PAN which permitted extracorporeal clearance of iPTH. These results suggest that the predialysis concentration of iPTH, and mostly of intact iPTH, is best to evaluate the parathyroid status of hemodialyzed patients, since various factors will influence iPTH concentration during and immediately after hemodialysis.