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 deter mined both by an electroimmunodiffusion assay (EIA), using the Hydrage l Lp(a) kit (Sebia, France) and by two immunonephelometric assays, one on a Behring Nephelometer Analyzer (BNA), using antiserum from Immuno france, and the other on a Beckman analyzer (Array), using antiserum f rom 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 t he 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 ar e caused by nonspecific precipitation. Analysis of the precipitate ind icated the presence of Lipoprotein X, an abnormal lipoprotein that app ears in the serum of patients with obstructive jaundice or with lecith in-cholesterol acyltransferase deficiency. The precipitant seems to be polyethyleneglycol (PEG) that was added to the reaction medium in bot h the BNA and the Array assays to stabilize the Lp(a)-anti Lp(a) immun e complex. In the Array assay, interference by this nonspecific precip itation is eliminated by preliminary centrifugation of the diluted sam ple. However, coprecipitation of Lp(a) could occur during this step. C onsequently, the results of Lp(a) measurement in serum from patients w ith hepatobiliary diseases should be interpreted with caution when imm unonephelometric assays are used with a medium containing PEG.