Potential Clinical Utility of a New IRMA for Parathyroid Hormone in Postmenopausal Patients with Primary Hyperparathyroidism

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Abstract
Background: A new commercially available (so-called second-generation) IRMA for parathyroid hormone (PTH) separately detects intact PTH and its N-truncated fragments; however, no studies have compared the first- and second-generation IRMAs for PTH in patients with primary hyperparathyroidism (PHPT) to assess their respective diagnostic accuracies. Methods: We concomitantly investigated 39 postmenopausal patients with PHPT and a control group of 70 healthy postmenopausal women matched for age, renal function, and vitamin D status. In all individuals, PTH was measured with a classic IRMA (PTH-S; DiaSorin Inc.), which uses antibodies directed against epitopes 1–34 and 39–84, and a new method (Scantibodies Laboratory. Inc.), which uses antibodies against epitopes 1–4 and 39–84 (PTH-W) and epitopes 7–34 and 39–84 (PTH-T). We also assayed serum PTH in 10 PHPT patients every 24 h for 5 days after successful surgery. Results: The different assays gave serum PTH values that were >2 SD higher than values for the control population in 59% (PTH-S), 77% (PTH-W), and 82% (PTH-T) of patients with PHPT. However, ROC curve analysis showed no significant differences among the three PTH assays, demonstrating overlapping diagnostic sensitivities. In PHPT patients, the correlation among the assays was highly significant (r = 0.91–0.92; P <0.001). The ratio PTH-W:PTH-T × 100 showed a gaussian distribution in both PHPT patients and controls, whose mean (SD) values [63.4 (13.3)% vs 64.5 (9.5)%, respectively] did not differ significantly. After parathyroidectomy, the mean percentages of variation in PTH detected with all of the assays were quite similar. Conclusions: The distribution of the PTH-W:PTH-T ratio in patients and controls suggests that PHPT does not markedly influence the rate at which biologically inactive fragments are generated by central or peripheral cleavage of PTH. The similar postoperative curves seem to contradict the hypothesized effect of acute hypocalcemia in modulating the central secretion of hormonal fragments. Our results indicate that the three investigated assays have similar diagnostic sensitivities in PHPT.