COMPARISON OF 2 POLYCLONAL ANTIBODIES FOR THE DETECTION OF 19-NORTESTOSTERONE IN BOVINE BILE BY ELISA

19-Nortestosterone (beta-NT) is banned for use as a growth promoter in food animals within the European Union. For regulatory control purpos es, urine and bile samples are routinely screened by immunoassay. The aim of the present study was to compare the ability of two immunoassay s, using two rabbit polyclonal antibodies raised against two different NT derivatives, to detect NT residues in bovine bile. One antiserum c ross-reacted with both alpha-NT and beta-NT (alpha/beta-NT), whereas t he other was specific for alpha-NT. Bile samples from 266 slaughtered cattle were deconjugated and analyzed using both antibodies, with all screening positives (>2 ng ml(-1)) confirmed by high resolution gas ch romatography mass spectrometry. The alpha/beta-NT and alpha-NT antibod y-based ELISAs screened 39 and 44 samples positive, respectively, with NT confirmed in 22 and 39, respectively. The alpha/beta-NT antibody-b ased ELISA produced a false-negative rate of 44% compared to 0% for th e alpha-NT antibody-based ELISA. Supplementary investigations conclude d that a matrix effect was a major cause of the marked differences in false-negative rates. This result underlines the necessity to validate immunoassays in the sample matrix.