Development of resistance during antimicrobial therapy: a review of antibiotic classes and patient characteristics in 173 studies.

The incidence of emergent resistance and clinical factors affecting its development were evaluated by retrospective review of 173 studies encompassing over 14,000 patients. Eight antibiotic classes and 225 individual treatment regimens were evaluated. Emergent resistance occurred among 4.0% of all organisms and 5.6% of all infections treated. It appeared to be significantly more frequent with penicillin and aminoglycoside monotherapy, with significantly lower rates associated with imipenem-cilastatin, aztreonam, and combination therapy. Clinical failure also appeared to be significantly more likely to occur after emergence of resistance among organisms treated with fluoroquinolones or aminoglycosides. Infections associated with higher resistance rates were cystic fibrosis, osteomyelitis, and lower respiratory tract infections. Resistance was most common in patients in intensive care units or receiving mechanical ventilation. It was also significantly frequent among studies performed in university or teaching hospitals. Organisms associated with high resistance rates were Pseudomonas aeruginosa, Serratia, Enterobacter, and Acinetobacter sp. Factors such as infection type, underlying diseases, type of institution, and specific pathogens warrant consideration when examining emergent resistance.