New PTH assays and renal osteodystrophy.

Parathyroid hormone (PTH) levels have been used instead of bone histomorphometric analysis in renal failure, but the assessment of tetracycline-labeled bone biopsy remains the most reliable method to diagnose the different subtypes of renal osteodystrophy. The availability of the first-generation immunometric PTH assay (1(st) PTH-IMA) allowed the distinction between the different types of renal bone diseases. However, 1(st) PTH-IMA not only detects the intact hormone PTH(1-84), but also additional PTH truncated fragments. A second-generation immunometric PTH assay (2(nd) PTH-IMA) recognizes only PTH(1-84) and possible PTH fragments that are truncated at the carboxyl-terminus, but not PTH(7-84). In addition, whether assessment of the ratio PTH(1-84) and amino-terminally truncated PTH(1-84) fragments is a better predictor of bone turnover remains controversial. An initial study using the 2(nd) PTH-IMA suggested that the ratio between PTH(1-84) and amino-terminally truncated PTH(1-84) fragments more accurately predicts bone turnover in adult patients treated with hemodialysis. However, subsequent studies using the Scantibodies assay have failed to better predict the underlying bone disease in adults undergoing maintenance hemodialysis. Furthermore, a different 2(nd) PTH-IMA (Immutopics) with similar, but not identical, in vitro characteristics did not show a superior predictive value of the ratio in pediatric patients treated with peritoneal dialysis. Although the 2(nd) PTH-IMA may provide important new insights into the physiology of parathyroid gland function, at present, measurement of PTH using either 1(st) or 2(nd) PTH-IMAs provides similar accuracy for predicting bone turnover in patients treated with dialysis. Thus, the current data do not yet support the claim that 2(nd) PTH-IMAs provide an advantage over 1(st) PTH-IMAs for the diagnosis of the different subtypes of renal bone diseases.