Discrepancies between lipoprotein(a) concentrations in icteric sera measured by immunonephelometry and electroimmunodiffusion

We compared the lipoprotein(a) [Lp(a)] levels in 32 icteric sera determined both ba an electroimmunodiffusion assay (EIA), using the Hydragel Lp(a) kit (Sebia, France) and by two immunonephelometric assays, one on a Behring Nephelometer Analyzer (BNA), using antiserum from Immunofrance, and the other on a Beckman analyzer (Array), using antiserum from, Dako (Denmark). With the EIA assay, the Lp(a) level was 0.09 ± 0.09 (mean ± SD in g/L), with the BNA assay, 1.01 ± 1.51 and with the Array assay, 0.05 ± 0.05. Sample blanks values (0.76 ± 1.28 g/L) demonstrated that the high Lp(a) levels obtained in the BNa assay are caused by nonspecific precipitation. Analysis of the precipitate indicated the presence of Lipoprotein X, and abnormal lipoprotein that appears in the serum of patients with obstructive jaundice or with lecithin-cholesterol acyltransferase deficiency. The precipitant seems to be polyethyleneglycol (PEG) that was added to the reaction medium in both the BNA and the Array assays to stabilize the Lp(a)-anti Lp(a) immune complex. In the Array assay, interference by this nonspecific precipitation is eliminated by preliminary centrifugation of the diluted sample. However, coprecipitation of Lp(a) could occur during this step. Consequently, the results of Lp(a) measurement in serum from patients with hepatobiliary diseases should be interpreted with caution when immunonephelometric assays are used with a medium containing PEG.