Antimicrobial susceptibility patterns in Pseudomonas aeruginosa: data from a multicenter Intensive Care Unit Surveillance Study (ISS) in the United States.

The objectives of this study were to analyze susceptibility rates and patterns in Pseudomonas aeruginosa isolates from patients in intensive care units (ICU). A total of 2209 isolates in 1995/1996 and 2672 in 2001/2002 were tested at United States sites participating in the ICU Surveillance Study. In both periods, of the agents tested, amikacin was the most active and ciprofloxacin, the least. Resistance to common antipseudomonal agents tested increased from 1995/1996 to 2001/2002; the rise was least for amikacin (2%) and greatest for ciprofloxacin (16%). The proportion of isolates susceptible to all six antipseudomonal agents tested since 1996 decreased from 60.4% to 48.9% in 2001/2002. Examination of MIC distributions for the two periods showed that for some drugs, e.g. imipenem and ceftazidime, the populations of susceptible and resistant isolates remained distinct, although the resistant population increased. For other drugs, e.g. amikacin and piperacillin-tazobactam, the MIC distribution shifted upward over time. The categorical agreement between agents of the same or like classes for isolates tested in 2001/2002 was highest for ciprofloxacin and levofloxacin (93.2%, with 1.2% major errors) and lowest for the aminoglycosides (81.3%, with 10.2% major errors). We can conclude that resistance to antipseudomonal agents among ICU isolates of P. aeruginosa, especially fluoroquinolones, is increasing. The resistance rate for some antipseudomonal agents may not accurately reflect shifts in the MIC distribution curve.