Comparison of lipoprotein(a) assay methods in serum and in a plasminogen-free fraction.

We compared five immunoassays for lipoprotein(a) (Lp(a)) determination (end-point immunonephelometry, two-site IRMA and three different ELISA methods) in order to verify their agreement. Since it has been demonstrated that human apo(a) structure is closely similar to that of plasminogen, cross-reactivity of apo(a) antibodies with plasminogen represents an important technical problem in serum Lp(a) quantification. On this account we used a plasminogen-free fraction obtained by one-step ultracentrifugation at a density of 1.125 g/ml, in which no plasminogen activity was found. A satisfactory correlation between serum and plasminogen-free fraction Lp(a) values was found for all the methods; the limits of agreement were too high, however, to use serum and fraction interchangeably. Furthermore, it emerged that the different assays were more comparable and individual Lp(a) differences between methods were less spread when plasminogen-free fraction was used instead of serum.