Risk factors for oxacillin/methicillin resistance in coagulase-negative staphylococci

The clinical variables associated with isolation of oxacillin- and methicillin-resistant, coagulase-negative staphylococci (CNS) from blood cultures of hospitalized patients were studied. One hundred CNS strains (49 oxacillin-susceptible; 51 oxacillin-resistant) isolated consecutively from one of two or more sets of blood cultures were collected. Only two variables were independently associated with recovery of oxacillin/methicillin-resistant strains by a multivariate analysis: length of hospital stay >10 days (OR 5·2, 95% CI = 1·7–15·7), and administration of antimicrobial agents in the previous 14 days (OR 4·5, 95% CI = 1·7–11·7). Analysis of the antibiotics administered indicated that only β-lactams were associated with a statistically significant risk of resistance to oxacillin/methicillin (OR of β-lactams vs no antibiotics = 6·94, 95% CI = 1·9–25·3; OR of non-β-lactams vs no antibiotics = 2·64, 95% CI = 0·8–8·3). Length of hospital stay (especially >10 days) and prior administration of antimicrobial agents (mainly β-lactams) independently predicted the presence of oxacillin/methicillin-resistant CNS in blood cultures.