Comparison of Commercially Available Parathyroid Hormone Immunoassays in the Differential Diagnosis of Hypercalcemia: A Reanalysis

ABSTRACT Aliquots of the same serum sample from 10 proven and 10 probable cases of primary hyperparathyroidism (1°HPT) and 25 of hypercalcemia of malignancy (HCM) were sent to two different laboratories for C-terminal or midmolecule and N-terminal immunoreactive parathyroid hormone (iPTH) assays and total serum calcium measurements. Elevations in iPTH were observed in 70% to 95% of 1°HPT and 13% to 46% of HCM cases. There was a good correlation among the assays in the 1°HPT group. A significant correlation was found only between the C-terminal and N-terminal assays from the same laboratory in the HCM group. Only one (5%) of 20 1°HPT patients had normal iPTH in all assays while only one (4%) of 25 HCM patients had elevated iPTH in all assays. This study shows that currently available assays for iPTH can detect elevations in most patients with 1°ΉPT and can discriminate them from HCM. When renal function is impaired an N-terminal assay can still discriminate.