The time interval was defined as the time between the last
day of antibiotic exposure and the occurrence of subsequent
nosocomial infection. As described previously [7], we
characterized exposure as occurring within either 10, 20,
40, or 60 days before the diagnosis of nosocomial infection.
If patients had not received antibiotics within 60 days, they were considered as not previously exposed. The incidence of
antibiotic-resistant infection in this subgroup was regarded as
the baseline level during the study period.
Antimicrobial susceptibility testing was performed using
the disk diffusion method according to the standards of
Clinical and Laboratory Standards Institute. Polymicrobial
infection was defined as isolation of more than 1 microorganism
from the same sample.