Monitoring of osteoblast activity with an immunoradiometric assay for determination of bone alkaline phosphatase mass concentration in patients receiving renal transplants.

We examined the diagnostic validity of an immunoradiometric assay for determination of mass concentration of bone alkaline phosphatase (EC 3.1.3.1) in 134 sera from 35 patients receiving renal transplants. Comparison between bone alkaline phosphatase concentration and total alkaline phosphatase activity yielded a strong correlation (r = +0.860; P < 0.001). Nine (17%) of 54 sera which were characterized by a total alkaline phosphatase activity between 100 units/l and the upper reference limit (178 units/l (males) and 160 units/l (females), respectively) showed an increased bone alkaline phosphatase concentration (> 21.3 micrograms/l (males) and > 15.0 micrograms/l (females), respectively). There was also a correlation between bone alkaline phosphatase values and parathyroid hormone levels both before (r = +0.640 (n = 23), P < 0.001) and after renal transplantation (r = +0.528 (n = 111), P < 0.0001). A follow-up of 15 patients after renal transplantation revealed that the median of bone alkaline phosphatase values increased from 5.5 micrograms/l before transplantation to 14.9 micrograms/l 3 months after transplantation (P < 0.0001). Nevertheless no correlation could be observed between parathyroid hormone concentrations and bone alkaline phosphatase values at any time following renal transplantation in these 15 patients (P > 0.1). Rise of bone alkaline phosphatase concentration following renal transplantation is most probably due to an activating effect of cyclosporin A upon osteoblasts.