Six osteocalcin assays compared

We evaluated comparatively six commercially available osteocalcin kits: ELSA-OST-NAT IRMA (CIS), ELSA-Osteo IRMA (CIS), Osteocalcin IRMA (Nichols), OSTK-PR RIA (CIS), OSCA Test Osteocalcin RIA (Henning), and Osteocalcin RIA (Nichols). All of them presented acceptable values of variance, sensitivity, and serial dilutions. However, only the ELSA-OST-NAT, ELSA-Osteo, and OSCA Test kits had analytical recoveries near 100% for added purified human osteocalcin; the other assays showed important differences in standardization. After treating samples with antibody against amino acids 43-49 of human osteocalcin, only the ELSA-OST-NAT IRMA detected no residual osteocalcin. Stability of serum values at room temperature and at 4 degrees C is good in the ELSA-Osteo and Osteocalcin IRMAs and in the Henning RIA if serum samples are collected in special tubes available with the kit. Differences in stability and after antibody treatment can be due to the degradation of intact osteocalcin molecules in serum and to osteocalcin fragments detected by assays. Results by different assays were well correlated for samples from controls and postmenopausal osteoporotic women but not for patients with impaired renal function or Paget disease of bone, probably because of osteocalcin fragments accumulated in serum of these patients and differences in specificity of the assays.