Oxacillin screening test to determine penicillin susceptibility in sites other than cerebrospinal fluid (CSF)
According to the current Clinical and Laboratory Standards Institute (CLSI) guidelines, an isolate of S. pneumoniae is considered susceptible to penicillin if the zone of inhibition around the 1 μg oxacillin disk is ≥20 mm. Thus, the isolate is oxacillin-susceptible, and penicillin can be administered. A follow-up minimum inhibitory concentration (MIC) should be performed only if the zone of inhibition measures ≤19 mm, as the degree of resistance cannot be determined. The MIC susceptibility test results obtained for penicillin also predict ampicillin, cefotaxime, imipenem, and other beta-lactam antibiotics, precluding the necessity for performing specific tests against these drugs.
Note: CLSI states that oxacillin is the surrogate (screening) test for penicillin. A susceptible zone predicts susceptibility to penicillin, ampicillin, and other antibiotics. Note that pneumococcal infections are not treated with oxacillin.
S. pneumoniae isolates from CSF
The oxacillin screening test is not recommended for isolates of S. pneumoniae from CSF; the MIC test should be performed. In some laboratories, direct MIC testing is done for all clinically significant S. pneumoniae isolates regardless of source, providing a definitive result one day sooner.
