DEVELOPMENT AND CLINICAL-APPLICATION OF AN IMMUNOFLUOROMETRIC ASSAY FOR INTACT PARATHYROID-HORMONE

Parathyroid hormone (PTH) is a linear peptide of 84 amino acids that i s found in serum mainly in the form of carboxyl-terminal fragments. Th e biological activity of PTH depends on the presence of the amino-term inal portion and in circulation is limited to the intact molecule. We describe an immunofluorometric assay for the measurement of PTH-(1-84) based on a chicken egg yolk-derived amino-terminal antibody bound to microtiter plates by an anti-chicken Ig monoclonal antibody. As tracer antibody we employed a Europium-labelled carboxyl-terminal specific m onoclonal antibody produced from a mouse immunized with hPTH-(53-84)-B SA conjugate. The assay included an initial overnight incubation of th e sample and the solid phase-bound amino-terminal antibody, followed b y washing and addition of the tracer antibody, and an additional two h ours of incubation prior to fluorescence reading. The least-detectable dose was in the order of 2.5 pg/ml and preliminary studies in 40 norm al adults showed values in the range of 4 to 70 pg/ml; for 12 patients with surgery-proven primary hyperparathyroidism values ranged from 10 9 to 743 pg/ml and for 34 patients with humoral hypercalcemia of malig nancy from 2.5 to 66 pg/ml. We conclude that this assay, with its incr eased sensitivity and specificity, will be a valuable tool in the stud y of PTH secretion in normal and pathological situations.