TECHNICAL AND CLINICAL VALIDATION OF A NEW IMMUNORADIOMETRIC ASSAY FOR HUMAN OSTEOCALCIN

The measurement of circulating osteocalcin or bone GLA protein (BGP) c onstitutes a well established and non-invasive means for evaluating pr eferentially the bone formation rate, but most available commercial as says suffer from several technical constraints, notably a rapid degrad ation of BGP at room temperature or after thawing and the inability to measure subnormal values. We evaluated, from a technical and a clinic al viewpoint, a newly available two-site sandwich immunoradiometric as say (IRMA) using standards of human origin and two different monoclona l antibodies. The theoretical and functional assay detection limit was 0.3 ng/ml. Concentrations of BGP progressively decreased when the ser um was left at 4 degrees C or at room temperature (mean apparent loss of 15% after 24 h). Two cycles of freezing-thawing only slightly reduc ed the BGP concentrations. The mean (+/- SD) BGP concentration was 19. 6 +/- 7.9 ng/ml in healthy subjects (NI, N = 61); the normal range was 8.1-35.6 ng/ml, There was a marked difference between pre- and postme nopausal women: 15.1 +/- 4.4 vs 22.3 +/- 8.4 ng/ml, respectively (p < 0.05). The mean BGP concentration in patients with tumor-induced hyper calcemia (N = 29) was not significantly different from NI, but nine pa tients (31%) had subnormal levels and five (17%) had elevated BGP leve ls. Concentrations of BGP were significantly increased in patients wit h hyperparathyroidism (N = 14) (45.1 +/- 21.0 ng/ml) and significantly lower than NI in patients with hypoparathyroidism (N = 18) (7.3 +/- 4 .6 ng/ml), Concentrations of BGP were also measured by a classical rad ioimmunoassay using bovine standards and tracer; the correlations betw een both sets of measurements were significant in all groups, except i n patients with hypoparathyroidism. In summary, this newly available I RMA for measuring circulating human BGP appears to be quite sensitive, reproducible and robust, It should be especially useful for investiga ting clinical conditions characterized by a low bone formation rate.