Differences between "intact" PTH and 1-84 PTH assays in chronic renal failure and dialysis.

"Intact" parathyroid hormone (iPTH) assays overestimate actual PTH as they cross-react with non (1-84) PTH fragments (C-PTH) that accumulate in renal failure. New assays that measure just 1-84 PTH (CAP-PTH) are now available. It has been suggested that there is a linear relationship between the two assays; however, increased C-PTH levels are found as glomerular filtration rate (GFR) declines and in patients on dialysis. We investigated the relationship between iPTH and CAP-PTH in children with chronic renal failure (CRF) managed conservatively and on dialysis. We investigated 241 children, 156 with a GFR <60 ml/min per 1.73 m(2) managed conservatively, 49 post renal transplant (and GFR <60 ml/min per 1.73 m(2)) and 36 on dialysis, by measuring PTH levels by iPTH and CAP-PTH assays. Multiple regression analysis comparing differences between PTH levels in each patient group was performed. Correlation slopes between iPTH and CAP-PTH assays differed between CRF and dialysis patients (P=0.001). These assays perform differently in CRF and dialysis patient groups. Studies investigating the correlation between newer assays and bone histology are required to determine whether these more-specific PTH assays are superior surrogate markers of bone turnover.