Immunoradiometric assay for intact human osteocalcin(1-49) without cross-reactivity to breakdown products

Background: Osteocalcin (Oc), a serum marker of bone turnover, circulates i n several forms. We developed an assay for intact human Oc and investigated its Clinical features. Methods: We generated goat antibodies and N- and C-terminal Oc. The former was used on solid phase (polystyrene beads), and the latter was used as the tracer in an IRMA. Results: The Essay was linear with:no cross-reactivity to Oc(1-43), total i mprecision (CV) of <10%, and recovery of 100% +/- 10%. Assay: values for in tact Oc in EDTA plasma samples were unchanged at 18-25 degrees C for 6 h. V alues for intact-Oc-in serum, EDTA plasma, and heparin plasma samples did n ot change after storage on ice for 8 h. Serum samples from patients with va rious conditions were stored at -70 or -135 degrees C for up to 5 years and yielded z-scores comparable to an Oc(1-43): IRMA for all conditions except for renal failure. In renal failure, the Oc(1-43) assay values were increa sed, whereas the intact assay values;were in the reference interval. Conclusion: Decreases in Oc assay values are inhibited by calcium chelation , and slowed by reduced temperatures. The described assay for intact Oc all ows improved specificity for bone compared with an assay for Oc(1-43). (C) 1999 American Association for Clinical Chemistry.