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Urinary tract infections (UTIs) result in 3.6 million office visits each year and greater than 100,000 hospital admissions in the United States annually. They may be confined to the lower urinary tract resulting in cystitis or involve the upper urinary tract and cause pyelonephritis. This is an important distinction as pyelonephritis may result in renal parenchymal damage, urosepsis, and death. Enteric or coliform bacteria are responsible for most UTIs with Escherichia coli being the most commonly identified organism. UTIs are further divided into (1) uncomplicated UTIs, where there is no structural or anatomic abnormalities of the urinary tract, and (2) complicated UTIs, where they may be either structural or anatomic abnormalities of the urinary tract or functional predispositions to infection. In addition, microorganisms in patients with complicated UTIs are frequently multidrug resistant.

Because of certain unique characteristics of affected individuals, UTIs can also be classified according to the population affected, eg, in young women, in young men, during pregnancy, in diabetic patients, etc.

Ascending infection from the urethra is the most common route of infection. The periurethral epithelium is normally colonized by enteric bacterial flora that invade the bladder. This probably accounts for the markedly greater frequency of infections in females, in whom the urethra is short and is in close proximity to the vulvar and perianal areas, thereby increasing the likelihood of contamination. While the majority of UTIs are due to E coli, only a few serogroups of E coli (O1, O2, O4, O6, O7, O8, O75, O150, and O18ab) can actually cause infections. An important virulence factor of these uropathogens is the presence of adherence factors, such as Type I fimbriae, or P fimbriae which allow binding of the organism to uroepithelial cells. Bacteria rapidly multiply in the bladder and may travel up the ureters to the renal pelvis and parenchyma, thereby causing pyelonephritis.

Hematogenous spread to the renal parenchyma and/or bladder is less common, but should be considered in infections due to Staphylococcus aureus.

Multiple risk factors are described for UTIs (summarized in Table 38–1), but the most important ones for acute cystitis in young women are recent or frequent sexual activity and a history of previous episodes of cystitis.

Table 38–1. Risk Factors for Urinary Tract Infections.

The majority of UTIs are caused ...

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