Evaluation of penicillin and oxacillin susceptibility testing was conducted on two hundred
Staphylococcus lugdunensis
isolates. Disc diffusion with penicillin 1 IU (P1, EUCAST) and penicillin 10 IU (P10, CLSI) was compared with nitrocefin discs (Cefinase®) and automated broth microdilution (Vitek2®). Oxacillin susceptibility was extrapolated from cefoxitin 30μg disc diffusion (FOX) and compared with Vitek2®. Reference methods were
blaZ
and
mecA
PCR. Penicillin zone diameter and zone edge correlated with
blaZ
in all except two P10 susceptible isolates (VME; very major error) and one P1 resistant isolate (ME). One hundred and forty-eight isolates were
blaZ
-negative of which one hundred and forty-six and one hundred and forty-nine isolates were susceptible by P1 and P10 respectively. One hundred and twenty-seven isolates were penicillin susceptible by Vitek2®. Vitek2® overcalled resistance in twenty-one
blaZ
-negative, twenty P1 and twenty-two P10 susceptible isolates (Vitek2® ME rate, 14.2%). Two
mecA
-positive isolates were oxacillin resistant by FOX and Vitek2® (categorical agreement). However, eighteen FOX susceptible,
mecA
-negative isolates tested resistant by Vitek2®. In conclusion, Vitek2® over-estimated penicillin and oxacillin resistance compared with disc diffusion and PCR. Disc diffusion with zone edge interpretation was more accurate and specific than automated broth microdilution for
S. lugdunensis
in our study.
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