Development and validation of an immunoradiometric assay of parathyrin-related protein in unextracted plasma

This two-site immunoradiometric assay for human parathyrin-related protein 1-86 (PTHRP1-86) in plasma uses a mouse monoclonal antibody to PTHRP1-34 coupled to cellulose particles for immunoextraction of N-terminal immunoreactivity, and a rabbit antiserum to PTHRP37-67 that is indirectly labeled with 125I-labeled PTHRP37-67 for quantifying the bound analyte. The detection limit of the assay is 0.23 pmol/L, corresponding to 0.4 pg (0.04 fmol) per tube, for a sample volume of 200 microL. Recovery of PTHRP1-86 added to serum is essentially quantitative, and within- and between-batch precision is 4.4% and 11.1%, respectively. PTH1-84, PTHRP18-34, PTHRP9-34, PTHRP1-34, and PTHRP37-67 do not cross-react in the assay at concentrations up to 2 nmol/L. Plasma concentrations of PTHRP1-86 were below or close to the detection limit of the assay in normal subjects and in patients with primary hyperparathyroidism, hypoparathyroidism, chronic renal failure, and normocalcemic malignancy. In 37 hypercalcemic patients with various malignancies, we found detectable PTHRP1-86 concentrations in 35 (95%, mean 7.4 pmol/L, range 0.46-24.7). The data support the proposed humoral role of PTHRP in cancer-associated hypercalcemia and suggest that the assay has clinical utility in the differential diagnosis of hypercalcemia.