Serum alkaline phosphatase (AP), the bone fraction of which is secreted by osteoblasts, is elevated in rickets. Both normal and elevated levels of serum osteocalcin (OC), a bone-specific marker secreted by osteoblasts, have been reported in rickets. Expression of the OC gene is enhanced by 1,25-dihydroxyvitamin D (1,25(OH)2D) in experimental models. This study assessed serum OC levels in 14 controls and 41 patients with active rickets divided into a phosphopenic (n = 20) and a calciopenic (n = 21) group. Phosphopenic subjects were older (9.5 versus 5.7 years, P = 0.03) with higher median serum calcium level (2.35 versus 2.16 mmol/l, P = 0.0002) and serum 25-hydroxyvitamin D level (15.4 versus 10.4 ng/l, P = 0.003); and lower serum phosphate (0.80 versus 1.51 mmol/l, P = 0.0001), serum 1,25(OH)2D (43.0 versus 95.6 pg/ml, P = 0.0001) and intact serum parathyroid hormone level (45.0 versus 141.5 ng/l, P = 0.01) than calciopenic subjects. There were no differences in median serum AP (774 versus 1430 IU/l, P = 0.17) and OC (14.5 versus 13.4 ng/ml, P = 0.6) between the two groups. The mean OC value for the 41 rickets subjects was 15.1 +/- 6.2 ng/ml and 17.4 +/- 7.8 ng/ml for the 14 control subjects. In the face of markedly elevated serum AP levels in the rickets subjects, all of the serum OC values in the study fell within two standard deviations of the mean for normals. There was no association between serum OC and 1,25-(OH)2D in either the phosphopenic or the calciopenic group.
Conclusion: These results show that serum osteocalcin levels are not elevated in all forms of active rickets and that, unlike serum alkaline phosphatase, serum osteocalcin cannot be used in the diagnosis of rickets.