STANDARDIZATION OF LP(A) MEASUREMENTS

Two direct binding ELISAs were developed in our laboratory for measuring Lp(a) in human plasma. The first ELISA was performed by using a monoclonal antibody to apo(a) bound to the solid phase and a second monoclonal antibody to apo(a) as detecting antibody. The second ELISA was performed by using the same monoclonal antibody bound to the solid phase and an anti-ape B polyclonal antibody as detecting antibody. The immunoreactivity of Lp(a) particles of different size, isolated from subjects with F, B or S2 isoform, were evaluated by the two ELISA methods. Superimposable standard curves were obtained by the three Lp(a) preparations when using the apo(a) detection system, but the Lp(a) containing the largest apo(a) isoform S2 had a significantly reduced slope by the apo B detection method. Lp(a) concentration was determined in plasma samples by the two ELISA methods. When Lp(a) with apo(a) isoform S2 was used to calibrate the assays, similar Lp(a) values were obtained by the two different detecting systems on samples from subjects with isoforms S2, S3 or S4, while values in the samples with B and S1 isoforms were significantly higher. When Lp(a) with isoform B was used as calibrator, comparable Lp(a) values were obtained by the two methods on samples with B isoform, while the values were lower in the samples with the higher molecular weight isoforms when measured by the apo B detecion method. A pilot study was conducted to evaluate Lp(a) values obtained by different methods calibrated with a common fresh-frozen serum with a defined apo(a) isoform. Nine different methods were evaluated, including four different ELISAs using apo(a) detection, two ELISAs using apo B detection and three different immunoturbidimetric assays. Similar Lp(a) values were obtained on most of the serum samples by the seven different apo(a) detection methods. The Lp(a) values obtained by the two apo B detection methods agreed reasonably well with those obtained by the apo(a) detection method, except that the samples with low molecular weight apo(a) isoforms were significantly higher by the apo B detection method. The apo B detection assay is sensitive to the size of Lp(a) used as calibrator. Differences between the values obtained by the apo(a) and apo B detection methods are dependent on the size of the apo(a) isoform in the sample. Selection of a common Lp(a) calibrator can reduce the difference in Lp(a) values obtained by the different methods.