INTERPRETIVE CRITERIA AND QUALITY-CONTROL PARAMETERS FOR TESTING OF SUSCEPTIBILITIES OF HAEMOPHILUS-INFLUENZAE AND STREPTOCOCCUS-PNEUMONIAETO TRIMETHOPRIM AND TRIMETHOPRIM-SULFAMETHOXAZOLE

Two hundred twenty-eight strains of Haemophilus influenzae and 234 str ains of Streptococcus pneumoniae were tested by broth microdilution an d disk diffusion methods for susceptibility to trimethoprim (TMP) and TMP-sulfamethoxazole (SMX) to evaluate proposed criteria. Data are pre sented to support the proposed TMP MIC breakpoints of less than or equ al to 2.0 mu g/ml for susceptibility and greater than or equal to 4.0 mu g/ml for resistance for both species and TMP-SMX MIC breakpoints of less than or equal to 2.0-38 mu g/ml for susceptibility and greater t han or equal to 4.0-76 mu g/ml for resistance. Corresponding zone diam eter breakpoints for H. influenzae for both drugs are proposed: less t han or equal to 10 mm = resistant; greater than or equal to 16 mm susc eptible. A 10-laboratory study documented reproducibility of such test s with standard control strains. The following control limits are prop osed for tests of H. influenzae ATCC 49247 against TMP: MIC, 0.12 to 0 .5 mu g/ml; zone diameter, 27 to 33 mm. The current limits for TMP-SMX were confirmed, For tests of S. pneumoniae ATCC 49619, MICs of TMP we re 1.0 to 4.0 mu g/ml and the current TMP-SMX MIC range was confirmed. , Disk susceptibility tests of either drug against pneumococci were no t reproducible, and consequently neither quality control limits nor in terpretive criteria could be established. Endpoint interpretation and lot-to-lot variability in Mueller-Hinton agars were significant factor s leading to interlaboratory variability.