A two-site immunoradiometric assay for PTH(1-84) using N and C terminal specific monoclonal antibodies

We report the development of a two-site immunoradiometric assay for PTH(1-84) based on two site-specific monoclonal antibodies--3B3 (radiolabelled antibody) specific for PTH(1-34) and ESQ1 (on solid phase) specific for PTH(74-84). Antibody 3B3 is sensitive to the oxidation of the methionine residues in PTH(1-34) therefore hydrogen peroxide (0.1 M) is added to the incubation mixture. Validation studies confirm quantitative recovery of both oxidized and reduced PTH(1-84). The assay has a minimum detection limit of 0.5 pmol/L and a range of 1.5-250 pmol/L with an intra-assay CV of less than 10% (2.8-250 pmol/L less than 5% CV). Studies on clinical samples indicate good discrimination between normal subjects (mean 2.21; range 1.0-5.0 pmol/L) and patients with primary hyperparathyroidism (mean 21.0; range 5.8-100 pmol/L) who in turn are well separated from patients with hypercalcaemia of malignancy (14/18 less than 0.5 pmol/L).