Eighty-eight Staphylococcus aureus clinical isolates meeting criteria for 
borderline oxacillin resistance (intermediate susceptibility or resistance to 
oxacillin but susceptibility to amoxicillin/clavulanic acid upon disk diffusion 
testing) were studied to determine optimal test techniques and conditions for 
differentiating borderline oxacillin-resistant Staphylococcus aureus (BORSA) 
from methicillin-resistant Staphylococcus aureus (MRSA). Further testing 
revealed three distinct resistance patterns: 61 strains (69%) consistently met 
BORSA criteria and had average beta-lactamase levels five- to six-fold higher 
than oxacillin-susceptible controls; 11 strains (13%) were markedly 
heteroresistant MRSA with delayed appearance of resistant colonies leading to 
spurious susceptibility to amoxicillin/clavulanic acid; 16 strains (18%) 
appeared to be oxacillin-susceptible on repetitive testing. Under conditions 
used to elicit intrinsic methicillin resistance in Staphylococcus aureus, a 
large percentage of BORSA appeared resistant to amoxicillin/clavulanic acid. 
This clearly shows that BORSA may be misidentified as MRSA while heteroresistant 
MRSA may appear to be BORSA. It is concluded that amoxicillin/clavulanic acid 
zone sizes should be measured after a full 24 hours of incubation, that 
susceptibility testing of Staphylococcus aureus under certain environmental 
conditions should be interpreted with caution, and that MIC testing is the most 
reliable technique for differentiating these two resistance patterns in 
Staphylococcus aureus.