Conventional and new diagnostic applications of a two-site immunochemiluminometric assay for parathyroid hormone

This investigation was carried out to evaluate the clinical utility and diagnostic value of serum intact PTH measurement using a recently introduced immunochemiluminometric assay (ICMA). Studies were carried out in 42 normal subjects, 24 patients with primary hyperparathyroidism, 21 patients on chronic maintenance hemodialysis, 8 patients with postsurgical hypoparathyroidism, 7 patients with cancer hypercalcemia and 6 patients with osteomalacia. A good correlation was found in normal subjects between serum ICMA PTH levels and both intact PTH measured by a two-site immunoradiometric assay (n = 42, r = 0.67, p < 0.001) and a widely used midmolecule radioimmunoassay (n = 21, r = 0.78; p < 0.001). Similar good correlations were found in primary hyperparathyroidism patients (ICMA vs immunoradiometric assay r = 0.74; p < 0.001; ICMA vs midmolecule assay r = 0.77; p < 0.001). As far as the hypercalcemic conditions were concerned, in 5 patients with mild primary hyperparathyroidism, ICMA PTH levels were in the upper range of those found in normal subjects, even though they were inappropriately high in respect to serum calcium values. However, serum ICMA PTH levels were clearly suppressed or undetectable in the majority of patients with cancer hypercalcemia or postsurgical hypoparathyroidism. Following calcium and EDTA infusions in patients with primary hyperparathyroidism, the behaviour of ICMA PTH levels in general parallelled that of immunoradiometric PTH assay, thus indirectly suggesting the ability of the method to measure the intact molecule. Finally, shortening incubation periods so that overall assay time was limited to as little as 30 minutes gave the possibility of providing good discrimination between high PTH levels typical of severe primary hyperparathyroidism and normal values or the suppressed levels found in hypercalcemic cancer patients. In conclusion, the measurement of PTH levels using an ICMA method seems to provide a valuable means of investigating parathyroid physiology and mineral metabolism disorders.