Bioassay of parathyrin: Analytical characteristics and clinical performance in patients with hypercalcemia

This new bioassay for parathyrin (PTH) in plasma (bio-PTH) combines immunoextraction on affinity columns [goat anti-hPTH (1-44) conjugated to Sepharose 4B] and a receptor assay involving an osteosarcoma cell line. The mean extraction efficacy ranges from 87% (as determined with immunopurified ¹²⁵I-labeled PTH) to 62% for hPTH bioactivity. The assay is standardized with synthetic hPTH (1-84) and can detect as little as 0.9 pmol/L of PTH in 2 mL of plasma. In 100 healthy adults, the 95% reference interval for bio-PTH was <0.9 to 6.1 pmol/L (median, 2.0 pmol/L) In 185 patients with surgically confirmed hyperparathyroidism, bio-PTH concentrations ranged from 1.0 to >120 pmol/L (median 12.9 pmol/L); 80% of values were >6.1 pmol/L. In 50 patients with both preoperative and postoperative determinations, the mean (± SD) concentrations of calcium in serum were 113 ± 10 and 89 ±6 mg/L, respectively; the median bio-PTH concentrations were 13.6 and 2.0 pmol/L, respectively. In 22 patients with nonparathyroid-mediated hypercalcemia, the concentration of bio-PTH ranged from <0.9 to 5.3 pmol/L (median, 1.8 pmol/L). This bio-PTH assay is slightly less sensitive than our GP235 immunoreactive PTH (iPTH) immunoassay for detecting hyperparathyroidism (Clin Chem 1982;28:69-74); however, the bioassay is more specific and detected some cases missed by the iPTH assay. Overall, 95% of the hyperparathyroid patients had an increased test result for either the bio-PTH or the iPTH assay.