Changes in parathyroid hormone during haemodialysis sessions with two different dialysis membranes: specific adsorption of intact parathyroid hormone

In patients dialysed with two different membranes (cuprophan and high-flux polysulphone) and using three radioimmunoassays recognizing either intact molecule (iPTH), C terminal (cPTH) or median fragments (mPTH), we compared parathyroid hormone levels at the beginning of and during dialysis. At the beginning of dialysis, cPTH and mPTH levels were always increased but iPTH levels were sometimes within the normal range; during dialysis only iPTH distinctly decreased whichever membrane was used. The behaviour of iPTH cannot be explained by crossing through the membrane because its molecular weight is too high; it cannot be explained by the variation of calcium and phosphate parameters during the session because no correlation exists between the variations in plasma levels of iPIH and the variations in phosphorous and calcium levels. In vitro experimentation with radiolabelled intact iPTH was performed to confirm the specific behaviour of iPTHthese experimental data agree with our in vivo results and suggest an adhesion to the dialysis membrane. Our study shows a specific singular property of iPTH during dialysis and demonstrates the necessity of considering the time of sampling during dialysis and the moiety of parathyroid hormone being measured in patients undergoing dialysis in order to assess osteodystrophy.