IRMA (whole PTH) is a more useful assay for the effect of PTH on bone than the Allegro intact PTH assay in CAPD patients with low bone turnover marker

Abstract

The common intact parathyroid hormone (i-PTH) assay detects not only PTH (1-84) but also the PTH (7-84) fragment. Recently, it was reported that the PTH (7-84) fragment is an antagonist to the biological action of PTH (1-84). It was also reported that the accumulation of the PTH (7-84) fragment plays a role in skeletal resistance in haemodialysis (HD) patients. However, the role of accumulation of the PTH (7-84) fragment in continuous ambulatory peritoneal dialysis (CAPD) patients, with a different clearance rate from that of HD patients, is still unclear. Therefore, we have measured only the active form of PTH (1-84) using a new method of whole PTH (w-PTH) assay in 20 CAPD patients (15 male and five female; mean age 51.0+/-13.0 years). The mean w-PTH value was 88.5 +/-14.2 pg/ml in CAPD patients, which was 42.1% of i-PTH (152.6+/-23.6 pg/ml). The approximate value of w-PTH was calculated using the following formula (w-PTH=0.58 x iPTH-0.4, R(2)=0.94). PTH (7-84) fragment was calculated by the formula i-PTH-w-PTH. The PTH (7-84) fragment/w-PTH ratio as an index of skeletal resistance, and serum alkaline phosphatase activity as an osteoblastic marker were negatively correlated (P=0.02). From these results, we concluded that the i-PTH level as calculated using the common assay method might lead to an overestimation of parathyroid function and bone turnover in CAPD patients similarly to HD patients. The w-PTH assay may be useful for more precise evaluation of PTH activity in end-stage renal disease patients.