Determination of osteocalcin in human serum: results with two kits compared with those by a well-characterized assay

Osteocalcin, the vitamin K-dependent protein in bone, can also be detected in serum and is receiving increased attention as a marker for bone turnover in evaluating patients with metabolic bone disease. We compared results for patients as determined with two commercial radioimmunoassay kits (Immuno Nuclear Corp. and Seragen Inc.) and with our in-house radioimmunoassay (Methods Enzymol 107: 517, 1984). Results by our assay correlated well (r greater than 0.9) with those by both kits, but the values by the Immuno Nuclear and Seragen methods were respectively 40% and 10% lower than by our radioimmunoassay. Within-run variation (CV) for the two kit methods was respectively 7.3% and 9.8%, run-to-run CV was 9.7% and 8.6%. The standard curve was linear from 1 to 25 micrograms/L for the Immuno Nuclear kit, from 4 to 100 micrograms/L for the Seragen equilibrium method, and from 1 to 25 micrograms/L for the Seragen nonequilibrium method. A second freeze-thaw cycle reduced the serum values by 20% to 40% for both kit methods. A third freeze-thaw cycle further reduced values and eliminated any correlation among methods.