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Am Fam Physician. 2003;67(12):2584-2587

The prevalence of trimethoprim-sulfameth-oxazole (TMP-SMX) resistance among Escherichia coli isolates from female patients with urinary tract infections (UTIs) is now approximately 10 to 20 percent in the United States. Consequently, many physicians are empirically prescribing fluoroquinolones or nitrofurantoin as first-line therapy for UTIs. Karlowsky and colleagues present data on antibiotic resistance rates for these alternative medications.

The authors used in vitro susceptibility data taken from The Surveillance Network Database—USA (Focus Technologies), a nationwide hospital laboratory database. E. coli isolates included in the data analysis were restricted to the first isolate of E. coli from a clean-catch midstream sample from outpatient women of all ages. National data for the calendar years 1998 to 2001 were included.

The study confirmed resistance rates of 17.5 percent for TMP-SMX, 1.9 to 2.5 percent for the fluoroquinolones (ciprofloxacin, levofloxacin, norfloxacin, and ofloxacin), and 0.8 percent for nitrofurantoin. Annual differences in resistance rates from 1998 to 2001 were less than 1.5 percent for all the antibiotics studied.

Among E. coli isolates that were known to be resistant to TMP-SMX, the resistance rate to ciprofloxacin increased to 9.5 percent, and nitrofurantoin resistance rose slightly to 1.9 percent. Of the small number of nitrofurantoin-resistant isolates, the incidence of concurrent resistance to ciprofloxacin was 29.8 percent.

The authors conclude that nationwide levels of E. coli resistance to TMP-SMX remain in the 10 to 20 percent range, while some resistance to nitrofurantoin and fluoroquinolones also remains. They found that isolates resistant to one antibiotic were more likely to be resistant to other antibiotics as well.

editor's note: Although in vitro resistance to TMP-SMX has reached substantial levels nationwide, in vivo urinary concentrations of the antibiotic are high. In addition, clinical cure rates for UTIs have not differed in studies comparing TMP-SMX–sensitive and TMP-SMX–resistant organisms. However, when there is a clinical preference for the use of alternative antibiotics, nitrofurantoin offers several advantages—less resistance, lower cost, and a narrower antimicrobial spectrum compared with the fluoroquinolones.—b.z.

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