EFFECT OF DIFFERENT STORAGE TEMPERATURES, SAMPLE COLLECTION PROCEDURES AND IMMUNOASSAY METHODS ON OSTEOCALCIN MEASUREMENT

The apparent instability of measured osteocalcin has been reported as method-dependent and related to preanalytical variables such as storag e temperature, and the use of anticoagulants and protease inhibitors. The aim of this study was to determine a sample collection procedure w hich minimised osteocalcin degradation. Blood samples from five normal individuals were collected with or without anticoagulants and proteas e inhibitors (heparin, EDTA, or heparin and aprotinin) and stored at 4 degrees C, -20 degrees C Or -70 degrees C for up to 7 days, 28 days a nd 90 days respectively. Osteocalcin was measured by both a monoclonal EIA specific for intact osteocalcin and a bovine polyclonal RIA. Oste ocalcin concentrations in serum and EDTA-treated samples significantly decreased by 40% (P < 0.001) with the ELISA and 72% (P < 0.001) with the RIA after 7 days storage at 4 degrees C. Similar falls were docume nted in these samples when stored at -20 degrees C and -70 degrees C a nd measured by the ELISA. Minimal changes in osteocalcin immunoreactiv ity were observed in either assay when heparin-treated plasma with or without aprotinin was stored at -20 degrees C or -70 degrees C for up to 90 days. The apparent instability of measured osteocalcin can be mi nimised using these conditions.