Detectability and Precision of the AOAC Bacillus stearothermophilus Disc Assay Demonstrated in the 1981 FDA Split Milk Sample Testing Program

Abstract
Four blind duplicate milk samples containing β-lactam residues were analyzed by 149 analysts at 47 laboratories. Four official variations were used: (a) antibiotic 4 (A4) agar and 55°C incubation, (b) A4 agar and 64°C incubation, (c) PM indicator (PMI) agar and 55°C incubation, and (d) PMI agar and 64°C incubation. A β-lactam concentration of 0.008 IU/ml was detectable 100% of the time by variations a, b, and c and 98% of the time by variation d, with average zone diameters of 18.7, 17.7, 16.4 and 17.6 mm, respectively. The 0.005 IU/ml concentration was detected 92% of the time by variation a, 99% by b, 57% by c and 89% by d, with average zone diameters of positive units of 16.3, 15.5, 14.9 and 15.2 mm, respectively.

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