Susceptibility of Staphylococcus aureus to vancomycin: analysis of minimum inhibitory concentrations in two tertiary care hospitals in eastern South Dakota.

Background: Vancomycin has been considered the accepted standard of therapy for methicillin-resistant Staphylococcus aureus (MRSA) and is commonly used as an alternative for methicillin-suceptible Staphylococcus aureus (MSSA) in cases of allergy or intolerance to other agents. MRSA strains exhibiting elevated vancomycin MICs (minimum inhibitory concentration) within the susceptible range have been associated with increased treatment failure and mortality. The aim of our study was to evaluate for evidence of elevated vancomycin MICs among S. aureus isolates associated with pneumonia or skin/soft tissue infection (SSTI) in our adult inpatient population.

Methods: S. aureus vancomycin MIC data was retrospectively reviewed for the period of July 1, 2008, to December 31, 2008. Adult inpatients were included in our analysis if treatment was initiated with an anti-staphylococcal agent for at least 48 hours for either pneumonia or SSTI. Patient demographics and susceptibilities to non-vancomycin antistaphylococcal agents were collected.

Results: A total of 272 patients met inclusion criteria. Only three patients were infected with S. aureus isolates found to have elevated vancomycin MICs within the susceptible range (> 1 mcg/mL to < or = 2 mcg/mL). This included two patients with pneumonia (one MRSA, one MSSA) and one patient with SSTI caused by MRSA. All S. aureus isolates were susceptible to linezolid and trimethoprim-sulfamethoxazole (TMP-SMX), with tetracycline susceptibility exceeding 90 percent in all subsets.

Conclusions: Elevated vancomycin MICs were uncommon in our sample among adult inpatients with pneumonia or SSTI caused by S. aureus. Vancomycin appears to remain a viable option for the treatment of these infections in our patient population.