Junichi Yoshida, Tetsuya Kikuchi, Nobuo Matsubara, Ikuko Asano, Nobumichi Ogami

It is not known whether or not ward-specific antimicrobial use density (AUD) affects the ratio of methicillin-resistant Staphylococcus aureus (MRSA) in culture-positive S. aureus. A 60-month study was attempted to ascertain the association between inpatient MRSA ratio and ward-specific AUDs as well as the former and latter study intervals, specimen types, and ward specialty. During the study, the professionals in infection control regulated the use of broad-spectrum antimicrobials and those for MRSA. By both month and ward, the ratio of inpatients positive for MRSA to those positive for S. aureus was calculated. Factors associated with MRSA ratio included AUDs averaged for the sampling month and its previous month, outpatient MRSA ratio by age, ward specialty, specimen type, and half intervals to represent historical changes. Of a total of 4,245 strains of S. aureus isolated during the 5-year study, 2,232 strains (52.6%) were MRSA. By year, outpatient MRSA ratio at age ≥15 decreased in later years, as did inpatient MRSA ratio. Multivariate analysis for inpatient MRSA ratio revealed a positive risk in AUDs for meropenem (odds ratio [OR] 1.761; 95% confidence interval [CI] 1.761-2.637, P = 0.01), imipenem-cilastatin (OR 1.583; 95% CI 1.087-2.306, P = 0.02), ampicillin-sulbactam (OR 1.623; 95% CI 1.114-2.365, P = 0.01), and minocycline (OR 1.680; CI 1.135-2.487, P = 0.01), respiratory care ward (OR 2.292; 95% CI 1.085-4.841, P = 0.03), and outpatient MRSA ratio (OR 1.536; 95% CI 1.070-2.206, P = 0.02). Use of broad-spectrum antimicrobials, such as meropenem, imipenem-cilastatin, and ampicillin-sulbactam may increase inpatient MRSA ratio. Ward factor should be included in MRSA surveillance because of the possible effect on AUD and considering patients' backgrounds.