[The change and significance of vancomycin minimal inhibitory concentration against methicillin-resistant Staphylococcus aureus isolates from inpatients with lower respiratory tract infection].

Objective To investigate the change and significance of vancomycin minimal inhibitory concentration (MIC) against methicillin-resistant Staphylococcus aureus (MRSA) isolates. Methods We analyzed the data of inpatients with lower respiratory tract infection, with positive cultures of MRSA from airway samples, at respiratory ward or respiratory intensive care unit (RICU) between 2000 and 2011. The MIC of vancomycin was determined by the agar dilution method. Results There were 295 patients [210 males, 85 females, mean age (73±12) years (range, 18-98)] with a positive culture of MRSA from airway samples.The arithmetic mean of vancomycin MIC against MRSA isolates fluctuated from 0.99 to 1.60 mg/L. The number of defined daily doses (DDDs) of vancomycin from the whole hospital had an influence on the mean of vancomycin MIC in the next year (r=0.64, P=0.04). But the vancomycin DDDs from respiratory department were not related with the mean of vancomycin MIC in the next year (r=0.33, P=0.32). The patients were divided into 2 groups according the MIC of vancomycin against MRSA isolates: MIC=2 mg/L group (n=43) and MIC<2 mg/L group (n=252). There was no difference in mortality [14 cases (32.6%) and 73 cases (29.0%)](χ2=0.23, P=0.63), the clinical success rates [26 cases(60.5%) and 156 cases(61.9%)](χ2=0.03, P=0.85) and bacterial success rates [21 cases(48.8%) and 106 cases(42.1%)](χ2=1.20, P=0.27) between the 2 groups. But the average hospitalization days were significantly prolonged (Z=3.09, P=0.00)in the MIC=2 mg/L group [40(27, 93) days]as compared to that in the MIC<2 mg/L group [30(20, 52) days]. The average treatment time [10(1, 19) days and 3(0, 12)days, Z=-2.79, P<0.01] was also longer in the MIC=2 mg/L group. In a multiple stepwise regression analysis, male gender (OR=3.58)and acute physiology and chronic health evaluation Ⅱ (APACHE Ⅱ) scores(OR=1.06) were independently associated with vancomycin MIC=2 mg/L. Conclusions In inpatients with MRSA lower respiratory tract infection at respiratory ward or RICU between 2000 and 2011, the vancomycin MIC fluctuated, which was related with vancomycin DDDs from the whole hospital. The average hospitalization days and treatment time were significantly prolonged in patients with MRSA isolates with a higher vancomycin MIC(=2 mg/L ). APACHE Ⅱ score was an independent risk factor for vancomycin MIC being 2 mg/L. Key words: Staphylococcal infections; Methicillin resistance; Vancomycin; Microbial sensitivity tests