Intact or N-MID osteocalcin assays for assessment of bone formation in hemodialysis patients?

Abstract

Background: Osteocalcin (OC) is a non-collagen bone matrix protein that is used as a noninvasive marker of bone formation. It is present in the circulation as an intact molecule and as fragments. The two known methods for OC determination in serum samples differ in the detection antibodies for an intact OC molecule and its N-terminal fragment. The aim of our study was to find out if these two methods give comparable results. Methods: 102 serum samples of hemodialysis patients were analyzed. While 26 patients were receiving calcitriol treatment, the remaining 76 patients were not. The average age of patients was 60 years. The causes of chronic renal failure were: diabetic nephropathy (27), chronic glomerulonephritis (31), polycystic kidney (26), and other (18). OC levels, β-CrossLaps (CTx) and parathyroid hormone (PTH) were measured. Serum levels of OC were determined by intact-OC and N-MID-OC methods. Following the comparison of results, we assessed the relationships between OC, CTx, and PTH. SPSS 12.1 for Windows was used for statistical analysis. Results: Serum levels of N-MID-OC were 10.6- fold higher than intact-OC. N-MID-OC and intact-OC levels did not differ between patients with and without calcitriol therapy. The results of both methods correlated well (r = 0.702, p < 0.001). Conclusions: The differences between osteocalcin serum levels assessed by intact-OC and NMID-OC methods were statistically significant. However, their correlation was good, so we can conclude that their clinical application is comparable.