6 OSTEOCALCIN ASSAYS COMPARED

We evaluated comparatively six commercially available osteocalcin kits : ELSA-OST-NAT IRMA (CIS), ELSA-Osteo IRMA (CIS), Osteocalcin IRMA (Ni chols), OSTK-PR RIA (CIS), OSCA Test Osteocalcin RIA (Henning), and Os teocalcin RIA (Nichols). All of them presented acceptable values of va riance, 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 import ant differences in standardization. After treating samples with antibo dy against amino acids 43-49 of human osteocalcin, only the ELSA-OST-N AT IRMA detected no residual osteocalcin. Stability of serum values at room temperature and at 4 degrees C is good in the ELSA-Osteo and Ost eocalcin IRMAs and in the Henning RIA if serum samples are collected i n special tubes available with the kit. Differences in stability and a fter antibody treatment can be due to the degradation of intact osteoc aicin molecules in serum and to osteocalcin fragments detected by assa ys. Results by different assays were well correlated for samples from controls and postmenopausal osteoporotic women but not for patients wi th impaired renal function or Paget disease of bone, probably because of osteocalcin fragments accumulated in serum of these patients and di fferences in specificity of the assays.