IMMUNORADIOMETRIC METHOD AND ELECTROPHORETIC SYSTEM COMPARED FOR QUANTIFYING BONE ALKALINE-PHOSPHATASE IN SERUM

Agarose electrophoresis (Isopal, Beckman) and an immunoradiometric ass ay (IRMA) involving specific monoclonal antibodies (Ostase, Hybritech) , two methods for the quantification of serum bone alkaline phosphatas e (ALP, EC 3.1.3.1), a marker of osteoblastic activity, were compared in 293 patients: 79 with end-stage renal failure treated with hemodial ysis and 214 with malignant disease. Overall correlation between the t wo methods was good (r = 0.92), except (a) for low values of bone ALP and (b) in some samples with high total liver ALP activity- both due t o considerable cross-reactivity of the anti-bone ALP antibodies of the Ostase kit with fiver ALP. This interference was not constant and was not evenly distributed across all concentrations of bone ALP. Low bon e ALP determined with the IRMA (less than or equal to 5 mu g/L) was co nfirmed by electrophoresis (less than or equal to 21 U/L), but bone AL P activity determined by electrophoresis to be low (less than or equal to 21 U/L) was not correlated with the IRMA results. After standardiz ing our results by computing z-values for bone ALP, Delta z (= z(Ostas e) - z(Isopal)) was significantly correlated with liver ALP activity ( r = 0.73, P < 0.0001). We conclude that the IRMA for quantifying bone ALP is acceptable as a screening method. However, when high values for bone ALP are found with the Ostase method, confirmation by electropho resis remains mandatory to rule out crossreactivity with high amounts of liver ALP. For detecting low bone ALP activities, electrophoresis r emains the method of choice.