Antibiotic choice is increasingly important in view of the increasing antibiotic resistance among typical UTI organisms. The choice also depends on patient characteristics (age, pregnancy status, prior history of UTI), complicated versus uncomplicated UTI, knowledge of the most likely pathogen (including local susceptibility patterns), severity of the condition and the hospital formulary.

Historically, Trimethoprim-sulfamethoxazole (TMP-SMX) was the first choice antimicrobial. However, due to adverse effects of the sulfamethoxazole, Trimethoprim is now used alone. E Coli resistance is increasing to all antibiotics, and Trimethoprim is no exception. Figures in some areas report an increase in E.coli resistance to Trimethoprim from between 0-5% pre-1990, to around 30% now [8]. Risk factors for Trimethoprim resistance include diabetes, recent hospitalisation, and current use of any antibiotic. E.coli resistance to quinolones is also increasing in the UK from 0.8% in 1990 to 3.7% in 1999 [9].

Different patients also have varying susceptibilities reflecting the need to take into account individual risk factors when choosing empiric treatment. Women older than 50 have been found to have increased rates of infection resistant to quinolones [10].