EVALUATION OF AN IMMUNORADIOMETRIC ASSAY FOR BONE ALKALINE-PHOSPHATASE MASS CONCENTRATION IN HUMAN SERA

The performance characteristics of an immunoradiometric assay for bone alkaline phosphatase mass concentration in human sera are reported. Within-run imprecision (n = 20) was 12.1% (($) over bar x = 7.8 eta g/l) and 3.6% (($) over bar x = 22.8 mu g/l), between-day imprecision (n = 8) was 10.1% (($) over bar x = 20.3 mu g/l) and 2.8% (($) over bar x = 84.3 mu g/l). There was a linear relationship between the concentrations of the standards employed and the counts per minute up to 120 mu g/l. The detection limit was 0.3 mu g/l. In 102 apparently healthy persons (51 males and 51 females; range of age: 18-56 years) the following reference intervals were established: 3.8-21.3 mu g/l (males) and 3.4-15.0 mu g/l (females). We compared the values obtained using the immunoassay with those obtained by precipitating of bone alkaline phosphatase with wheat-germ lectin (alkaline phosphatase activity concentration was determined at + 25 degrees C by the optimized standard method according to the Recommendations of the German Society for Clinical Chemistry). For the reference individuals the relationship between the results of the two methods is given by the following regression equation: Bone alkaline phosphatase activity concentration [U/l] = 14.81 + 3.28 x bone alkaline phosphatase mass concentration [mu g/l] (r = + 0.783). In 89 sera from 32 patients before and after renal transplantation (range of bone alkaline phosphatase mass concentration: 2-39 mu g/l) comparison between the two methods yielded a linear correlation coefficient of r = + 0.886. Of 20 sera taken from patients suffering from various hepatobiliary diseases (range of total alkaline phosphatase activity concentration: 217-3270 U/l) 18 (90%) showed a bone alkaline phosphatase mass concentration above the upper reference limit (range of bone alkaline phosphatase mass concentration: 16-206 mu g/l). This is probably due to a cross-reactivity of the antibodies employed for the immunoassay of bone alkaline phosphatase with liver alkaline phosphatase in plasma. It is concluded that an increased release of liver alkaline phosphatase into serum leads to falsely high values for bone alkaline phosphatase mass concentration, severely limiting the diagnostic validity of the test in such cases.