Development and evaluation of an osteocalcin chemiluminoimmunoassay.

We developed a competitive chemiluminoimmunoassay of osteocalcin that is similar to radioimmunoassay but uses acridinium-ester-labeled antigen instead of 125I-labeled osteocalcin; a second antibody immobilized on plastic beads is used to separate free and bound fractions. There was good correlation of the new chemiluminoimmunoassay (y) with a polyclonal antiserum (R102) radioimmunoassay (x) used in many previous clinical studies (r = 0.96, y = 0.968x + 2.69, Sy/x = 0.029, n = 86). The new assay recognized both the intact and the small fragment of osteocalcin in plasma and detected decreases of them (total, approximately 47%) after a 24-h infusion of parathyroid hormone. Patients with primary hyperparathyroidism had increased concentrations of intact osteocalcin. Children had higher concentrations of osteocalcin than adults did. Healthy women had greater osteocalcin concentrations at ages 50-70 years than earlier. Inverse correlations of bone mineral density and osteocalcin were found in healthy women and in women with osteoporosis.