Introduction: Parathyroid hormone (PTH) shows a strong correlation with histomorphometric and biochemical parameters of bone turnover, however its measurement presents limitations due to inter-method variability. Circulating PTH is a mixture of peptides, but only on its whole form (1-84 PTH) is responsible of PTH biological activity. Carboxyl-terminal fragments exhibit antagonist actions and their proportion differs at each stage of chronic kidney disease, as consequence of differences on their renal clearance. The aim of this study is to evaluate possible differences in the proportion of these fragments according to dialysis type: haemodialysis (HD) or peritoneal dialysis (PD).
Material and methods: Serum total (Ca) and ionized calcium (iCa), phosphate (P), carboxyl-terminal telopeptides of collagen type I (BCTx) were measured in 73 patients on PD (46 men and 27 women with an age between 22 and 82 years). PTH was quantified by six second generation assays (one isotopic and five chemiluminescence assays) and by one third generation PTH method.
Results: Mean serum levels of Ca, iCa, P and BCTx were 9.03, 4.76, 4.73 mg/dl and 1181 pmol/l, respectively. Significant differences were observed in PTH values according to the method used. Adjustment of PTH results to PTH Allegro (Nichols) range of 150-300 nmol/l in PD patients showed higher values than those assessed previously for HD population. The percentage of biologically active 1-84 PTH as the 1-84 PTH/ 7-84 PTH ratio in PD were significantly lower than in HD patients, reflecting the higher proportion of 7-84 PTH circulating fragments for a given intact PTH result in PD.
Conclusions: PD patients have a higher proportion of 7-84 PTH circulating fragments. Consequently, the inter-method adjustment algorithms proposed for HD patients are not useful for PD patients. This study proposes alternative algorithms for PTH inter-method adjustment to be applied in PD.