Determination of trimethoprim-sulfamethoxazole resistance in Streptococcuspneumoniae by using the E test with Mueller-Hinton agar supplemented with sheep or horse blood may be unreliable

An international, multicenter study compared trimethoprim-sulfamethoxazole MICs for 743 Streptococcus pneumoniae isolates (107 to 244 isolates per cou ntry) by E test, using Mueller-Hinton agar supplemented with 5% defibrinate d horse blood or 5% defibrinated sheep blood, with MICs determined by the N ational Committee for Clinical Laboratory Standards broth microdilution ref erence method. Agreement within 1 log(2) dilution and minor error rates wer e 69.3 and 15.5%, respectively, on sheep blood-supplemented agar and 76.9 a nd 13.6%, respectively, with horse blood as the supplement. Significant int erlaboratory variability was observed. E test may not be a reliable method for determining the resistance of pneumococci to trimethoprim-sulfamethoxaz ole.