Streptococcus pneumoniae is a common cause of bacterial meningitis and is most often susceptible to both Ceftriaxone and Benzylpenicillin, though emerging resistance to both Benzylpenicillin and Ceftriaxone is noted. Up until 2021 Fiona Stanley PathWest Microbiology tested both agents against S. pneumoniae using Biomerieux E-test® MIC strips. The manufacturer in April 2017 released a statement claiming their Ceftriaxone E-test® strips were producing minor and major errors for isolates testing Penicillin resistant (MIC ≥0.12 mg/L) and Ceftriaxone susceptible or intermediate (MIC ≥1mg/L), and that a second method of testing was recommended to confirm the result. Furthermore, EUCAST has raised concerns regarding the Biomerieux Benzylpenicillin E-test strips and recommended laboratories confirm MICs between 0.2-2mg/L with Broth Microdilution (BMD). This is a small in-house study comparing Ceftriaxone and Benzylpenicillin MIC’s for S. pneumoniae from Biomerieux E-test® strips to the Biomerieux Vitek ST-03 card for streptococcal antimicrobial susceptibility testing, a newly improved automated Broth Microdilution method.
21 (72%) had Penicillin MICs between 0.2 and 2mg/L. 11 out of the 21 had discrepant MICs to the Vitek (52%). One isolate (3%) had a category call discrepancy. 97% of isolates did not result in a category call discrepancy, so our findings were that there was statistical concordance between the Vitek and E-test strips, and thus the Benzylpenicillin E-test strips could be relied on, disagreeing with EUCAST’s recommendation.
27 tested Penicillin resistant/Ceftriaxone susceptible. 17 out of 27 (63%) had discrepant MICs between the methods when tested in parallel, and 14 of those (82%) had discrepancies that would have resulted in a category call change using CLSI guidelines. The very susceptible results by E-test were considered acceptable on the Vitek as the lowest Ceftriaxone MIC breakpoint on the Vitek ST-03 card is <0.12mg/L
The concerns raised by EUCAST regarding the Benzylpenicillin E-tests were not demonstrated in this trial. A expected discrepancy between Vitek and E-test Ceftriaxone MIC results was noted, with the Vitek producing higher MIC's than E-test.