Development of a Novel Immunoradiometric Assay Exclusively for Biologically Active Whole Parathyroid Hormone 1–84: Implications for Improvement of Accurate Assessment of Parathyroid Function

Abstract

We developed a novel immunoradiometric assay (IRMA; whole parathyroid hormone [PTH] IRMA) for PTH, which specifically measures biologically active whole PTH(1-84). The assay is based on a solid phase coated with anti-PTH(39-84) antibody, a tracer of 125 I-labeled antibody with a unique specificity to the first N-terminal amino acid of PTH(1-84), and calibrators of diluted synthetic PTH(1-84). In contrast to the Nichols intact PTH IRMA, this new assay does not detect PTH(7-84) fragments and only detects one immunoreactive peak in chromatographically fractionated patient samples. The assay was shown to have an analytical sensitivity of 1.0 pg/ml with a linear measurement range up to 2300 pg/ml. With this assay, we further identified that the previously described non-(1-84)PTH fragments are aminoterminally truncated with similar hydrophobicity as PTH(7-84), and these PTH fragments are present not only in patients with secondary hyperparathyroidism (2°-HPT) of uremia, but also in patients with primary hyperparathyroidism (1°-HPT) and normal persons. The plasma normal range of the whole PTH(1-84) was 7-36 pg/ml (mean ؎ SD: 22.7 ؎ 7.2 pg/ml, n ‫؍‬ 135), whereas over 93.9% (155/165) of patients with 1°-HPT had whole PTH(1-84) values above the normal cut-off. The percentage of biologically active whole PTH(1-84) (pB%) in the pool of total immunoreactive "intact" PTH is higher in the normal population (median: 67.3%; SD: 15.8%; n ‫؍‬ 56) than in uremic patients (median:53.8%; SD: 15.5%; n ‫؍‬ 318; p < 0.001), although the whole PTH(1-84) values from uremic patients displayed a more significant heterogeneous distribution when compared with that of 1°-HPT patients and normals. Moreover, the pB% displayed a nearly Gaussian distribution pattern from 20% to over 90% in patients with either 1°-HPT or uremia. The specificity of this newly developed whole PTH(1-84) IRMA is the assurance, for the first time, of being able to measure only the biologically active whole PTH(1-84) without cross-reaction to the high concentrations of the aminoterminally truncated PTH fragments found in both normal subjects and patients. Because of the significant variations of pB% in patients, it is necessary to use the whole PTH assay to determine biologically active PTH levels clinically and, thus, to avoid overesti-

Experience with a third-generation parathyroid hormone assay (BIO-PTH) in the diagnosis of primary hyperparathyroidism in a Brazilian population

“…Aligned with findings by other authors, we found a high correlation between the two methods in these patients (23)(24)(25). However, the Passing-Bablok and BlandAltman tests showed a proportional and systematic difference between both methods, indicating that the second and third-generation PTH assays may not be used interchangeably.…”

“…In the literature, there are only a few studies evaluating third-generation PTH assays in patients with primary hyperparathyroidism (8,(23)(24)(25). Silverberg and cols.…”

Bone mass and mineral metabolism in HIV+ postmenopausal women

“…Serum parathyroid hormone (PTH) was measured by a chemiluminescent method (Corning-Nichols Laboratory, San Juan Capistrano, CA, USA) that detects both a large, terminally truncated (7-84) fragment of PTH and the intact 1-84 molecule [23]. Intra-assay and inter-assay variabilities are 3.4% and 5.6%, respectively.…”

Cytochemical bioassays: every dog has its day

“…75 It has been suggested that this discrepancy may be related to the presence of a truncated PTH molecule which is recognized by the standard two-site immunoassays. 76,77 If this circulating PTH fragment were to act as an inhibitor of PTH bioactivity it might explain the previous studies on pseudohypoparathyroidism. Further developments are awaited with interest.…”