Performance of a rapid ELISA for penicillin G in milk

A rapid enzyme‐linked immunosorbent assay (ELISA) for the detection of penicillin G in milk at concentrations of 6 ng/ml (0.01 IU/ml) was used to screen 1651 off‐farm milk samples previously reported as containing no detectable levels of antimicrobial substances. Using a single‐well per test format and a percent inhibition cut‐off for the determination of positive/negative endpoint, 3.1% positives were obtained. Comparison with intra‐assay standards lowered the numbers of positives to 0.42%. Analysis of 170 milk samples positive for antimicrobials confirmed 92.4% as containing penicillin G using the prescribed cut‐off level and 87.6% when compared to standards. The ELISA, now available commercially in kit form, analyses between 1 and 48 samples in 15 minutes. It will detect 0.01 IU/ml to a level of confidence of 99% and 0.005 IU/ml with between 75 and 95% confidence.