Quality control of susceptibility tests with 5-micrograms trimethoprim disks

In 1981 and again in 1984, we performed multi-laboratory studies to develop quality control limits for susceptibility tests with 5-micrograms trimethoprim disks. Zone size limits of 21 to 28 mm are recommended for tests with Escherichia coli ATCC 25922, and limits of 19 to 26 mm are recommended for tests with Staphylococcus aureus ATCC 25923. For screening Mueller-Hinton agar, tests with Streptococcus faecalis ATCC 33186 or ATCC 29212 are recommended: zones should be fairly clear and greater than or equal to 22 mm (trimethoprim disks) or greater than or equal to 24 mm (trimethoprim-sulfamethoxazole disks).