Comparison of Rapid Tests Used to Detect Antibiotic Residues in Milk ¹

Five rapid methods for detection of antibiotics in milk were compared. The Bacillus stearothermophilus var. calidolactis disc assay was also performed on the same samples. The rapid methods were: Angenics Spot Test, Charm II, Delvotest P, Penzyme Farm, and Penzyme Lab III. Ten antibiotics (penicillin G, cephapirin, cloxacillin, ampicillin, streptomycin, chloramphenicol, erythromycin, novobiocin, tetracycline, and gentamicin) were used individually to spike eight raw milk samples at five levels of antibiotic. Antibiotic levels were chosen that would result in zones of <16 mm, 16 mm, and >16 mm on the disc assay. Only the disc assay, Charm II and Delvotest P were compared on non-beta-lactam antibiotics. A small percentage of milks with no antibiotic added tested positive with the Charm II and Penzyme Lab III. On combined data for penicillin G, cephapirin, and cloxacillin, for which all methods were compared, the percent correctly categorized as pass (below actionable) for the <16 mm zone spiked level, reject or caution at the 16 mm zone level, and reject or caution at the >16 mm zone level were: Angenics 79, 83, 100; Charm II 66, 92, 100; Delvotest P 74, 93, 100; Disc Assay 100, 74, 100; Penzyme Farm 93, 61, 92; Penzyme Lab III 81,78, 100 respectively. In most cases, the rapid methods showed greater apparent sensitivity than the disc assay and did not fail to reject milks spiked with antibiotic in excess of the 16 mm zone level.