SERUM OSTEOCALCIN AND BONE-MINERAL DENSITY AT VARIOUS SKELETAL SITES - A STUDY PERFORMED WITH 3 DIFFERENT ASSAYS

The purposes of this study were threefold: (1) to compare values obtai ned by three conventional radioimmunoassays for serum bone-gla-protein (BGP) in a population of normal women, (2) to study the relationship between serum BGP and bone mineral density (BMD) measured at four diff erent skeletal sites (lumbar spine, proximal femur, proximal and ultra distal radius), and (3) to compare the results obtained by the three a ssays with conventional markers of bone turnover. Ninety-seven normal women (age range 25 to 75 years, mean +/- 1 SD = 54.3 +/- 10.9 years) were studied. Three independent assays were used to measure serum oste ocalcin levels: a heterologous radioimmunoassay (RIA) (A) (Incstar Co. , Stillwater, Minn.), a homologous RIA (B) (Nichols institute, San Jua n Capistrano, Calif.), and a two-site immunoradiometric assay (C) (Cis Biointernational, Gif-sur-Yvette, France). Mean +/- SD values of seru m osteocalcin in the group as a whole were 4.05 +/- 1.37 mu g/L by ass ay A, 6.03 +/- 2.90 mu g/L by assay B, and 22.67 +/- 7.52 mu g/L by as say C. Serum osteocalcin levels increased linearly with age; however, no correlation between serum BGP (whatever the assay used) and age was observed when only postmenopausal women were taken into account. When the effect of age was held constant by means of partial correlation a nalysis, only serum BGP levels measured by assays B and C were still i nversely related with lumbar spine and ultradistal radius BMD; the lat ter assay was also weakly correlated with Ward's triangle BMD. After a ll the biochemical and clinical variables taken into consideration wer e introduced in a multiple regression equation, serum BGP still repres ented an important predictor of ultradistal radius and lumbar spine BM D only. Regarding relationships with other markers of bone turnover, t he assay C in general showed the highest r values. In conclusion, our results indicate that commercially available BGP assays differ analyti cally and clinically furthermore for the first time they show the exis tence of an inverse correlation between serum osteocalcin levels (whic h reflects bone turnover at the time of examination) and bone mass (wh ich at a given time represents the balance of all previous metabolic e vents), after the influence of aging is excluded.