Comparison of total alkaline phosphatase and three assays for bone-specific alkaline phosphatase in childhood and adolescence

We compared serum levels of total alkaline phosphatase (TAP) and bone-specific alkaline phosphatase (BAP) as determined by three different assays (lectin affinity electrophoresis, immunoradiometric assay, enzyme-linked immunosorbent assay) in subjects aged 5-20 years suffering from X-linked hypophosphatemic rickets (n = 14), chronic renal failure (n = 10) and chronic cholestatic liver disease (n = 16). Results were compared to controls of the same age and were expressed as standard deviation scores (SDS). TAP correlated significantly with BAP (r > 0.9 for each assay; p < 0.001) in controls. In children with cholestatic diseases, TAP (median SDS + 2.0) was elevated, but BAP, as measured by the electrophoretic assay, was within the reference range for most patients (median SDS: -0.4; p = 0.003 for the difference between the median SDS of TAP and BAP). In contrast, results for BAP as determined by the two immunoassays were not significantly different from TAP in any of the three patient groups (p > 0.05 in each group for both assays). In this study, the two immunoassays did not have a detectable advantage over lectin affinity electrophoresis in the determination of BAP.