Factors influencing isoprotein dependency of polyclonal apolipoprotein(a) assays

Hyperbolic relationships can be expected when comparing data obtained by apolipoprotein(a) [apo(a)] isoprotein-sensitive assays against data obtained by an isoprotein-independent assay ( x axis), and almost all polyclonal antibody-based apo(a) assays are isoprotein sensitive (1). We have developed a dual polyclonal antibody ELISA for quantification of plasma apo(a) and attempted to standardize it with regard to its accuracy in nmol/L while calibrating on the IFCC PRM2 calibrator. We also followed the traditional approach and compared the results with other established methods that express the results in mass or molar units. We found that our assay gave linear relationships with two other assays, of which at least one was isoprotein-independent. We confirmed the apo(a) isoprotein dependency by testing the IFCC PRM-2 calibrator and material differing in apo(a) molecular weight. Our results suggest that polyclonal antibody-based apo(a) assays a priori are not isoprotein-dependent. The work also gives insight into the suitability and comparativity of three different calibrators with target values in molar or mass units. Here we discuss our experiences in the hope that they are helpful for others who also want to standardize apo(a) assays. apo(a) is a glycoprotein bound to LDLs; a small amount may also circulate in free form (1)(2). There is no doubt that the standardization of total apo(a) measurements in routine laboratories world-wide needs improvement (1). The main problem in standardization is the specificity of the detecting …