No abstract

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 β-lactam inhibitors. The concentrations (penicillin G) where percent positive results equal 100 or not significantly less than 100 (α = 0.05) ranged from 0.005 to 0.007 IU/mi in the ampule test and from 0.004 to 0.007 IU/mL in the multitest. Both techniques have been adopted official first action.

No abstract

A collaborative study was conducted to determine the reliability of a Bacillus stearothermophilus disc assay method for differentiating various concentrations of penicillin in raw milk. Participating laboratories tested 10 different samples (including one negative) in blind duplicate. Triplicate standards were alternated with triplicate unknowns around the periphery of each of 5 different plates. Zone diameters were measured and the difference in zone size of pairs of adjacent standard and unknown samples were analyzed by a paired t-test. Penicillin concentrations 0.003 IU/mL different from the reference concentrations were consistently distinguishable at a 95% confidence level. Such discriminatory power was determined to be possible with as few as 3 plates (9 replicates) per unknown. The method has been adopted official first action.