Qualitative ampule and multitest for beta-lactam residues in fluid milk products: collaborative study.

A collaborative study was performed on a rapid Bacillus stearothermophilus agar diffusion ampule method to detect low levels of penicillin G in 7 types of fluid milk products. A multitest technique for processing a large number of samples simultaneously was also studied. Slight modifications were made in the original method to establish more uniformity and to eliminate doubtful responses by specifying a confirmation procedure. Twenty samples spiked with penicillin G (0.000 to 0.008 IU/mL) and tetracycline hydrochloride were frozen and sent to 20 laboratories in the ampule test, and 16 laboratories in the multitest. Each analyst was asked to do a screening run and a confirmation run. Results were reported by color reaction and also as positive or negative for beta-lactam inhibitors. The concentrations (penicillin G) where percent positive results equal 100 or not significantly less than 100 (alpha = 0.05) ranged from 0.005 to 0.007 IU/mL in the ampule test and from 0.004 to 0.007 IU/mL in the multitest. Both techniques have been adopted official first action.