Betsy Foxman, PhDbfoxman@umich.edu,     Department of Epidemiology, University of Michigan School of Public Health, 1415 Washington Heights, Ann Arbor, MI 48109-2029, USA

Urinary tract infection (UTI) is one of the most common bacterial infections, accounting for 0.9% of all ambulatory visits in the United States. This review defines the major UTI syndromes, their occurrence and recurrence, bacteriology, risk factors, and disease burden.

Keywords

Asymptomatic bacteriuria

Cystitis

Pyelonephritis

Catheter-associated urinary tract infection

Key points

Disease description

Urinary tract infection (UTI), an infection anywhere in the urinary tract (urethra, bladder, ureters, or kidneys) is very common. In 2007 in the United States there were 10.5 million ambulatory visits for UTI, accounting for 0.9% of all ambulatory visits.1 Almost one-fifth (21.3%) of these visits were to hospital emergency departments. UTI is among the most common primary diagnoses for United States women visiting emergency departments.2 Prevalence of UTI is high among inpatients also: in a 2004 survey of symptomatic UTI among 49 Swiss hospitals, UTI was detected in 3.7% of those who had been catheterized for at least 24 hours during their hospital stay, and in 0.9% of those who had not been catheterized.3

This review defines the major UTI syndromes (Table 1), their occurrence and recurrence, bacteriology, risk factors, and disease burden. Although the bacteria that cause UTI are increasingly resistant to antibiotic therapies (reviewed elsewhere in this issue), and modern molecular techniques have made it possible to better characterize the genetic lineages of uropathogens, UTI occurrence and risk factors, with few exceptions, have remained relatively constant.

UTI syndromes and their occurrence: asymptomatic bacteriuria, cystitis, pyelonephritis, and catheter-associated UTI

The urinary tract has a portal to the outside, making it particularly susceptible to invasion by microbes. Bacteria normally inhabit the tissues around the urethral opening, and frequently colonize the urine. Among men, culture of a random initial void will find 1% to 5% colonized with Escherichia coli; urethral colonization is higher among men whose female sex partner has a UTI.4 Among women, urinary colonization rates are higher; the vaginal cavity and rectal opening are close to the urethral opening, and women have more moist periurethral areas where bacteria grow. On entering the urethra, the bacteria are more likely to ascend to the female bladder than the male bladder because of the shorter urethral length. The overall prevalence of asymptomatic bacteriuria (ASB) in women is 3.5%,5 but is much higher following sexual intercourse.6 Among both men and women, the prevalence of asymptomatic bacteriuria increases with age.5,7 Laboratory findings for ASB are the same as for other UTI syndromes (ie, a positive urine culture and urinalysis), but there are no signs or symptoms referable to the urinary tract (Table 1).

With the exception of during pregnancy, ASB is not a treatable condition. Treatment of ASB in the otherwise healthy individual often results in a symptomatic UTI,8 and increases selection for antibiotic-resistant bacteria. The Infectious Disease Society of America recommends against screening for and treating ASB among catheterized patients.9 However, physiologic changes during pregnancy make the pregnant woman with ASB more susceptible to pyelonephritis.10 The prevalence of ASB in pregnancy ranges from 2% to 20%; and one-fifth to two-fifths of these may develop pyelonephritis if untreated.11 Pyelonephritis can be life threatening to both the mother and infant.11 Screening and treating ASB during pregnancy may reduce this risk by 77%.12

The high frequency of ASB does complicate the diagnosis of UTI, as urinary symptoms considered enigmatic of UTI, namely frequency, urgency and dysuria, are not solely caused by UTI. Vaginitis, chlamydia, and gonorrhea also cause urinary symptoms. Therefore, the chance of ASB and urinary symptoms occurring together by chance alone is not insignificant, especially as these conditions are also associated with sexual activity. By contrast, up to half of women at high risk of UTI (sexually active women aged 18–29 years) with frequency, urgency, and dysuria will have a negative urine culture when the limit of detection is 1000 cfu/mL urine.13,14

In cystitis, urinary symptoms are confined to the bladder, although upper tract involvement occurs. Among premenopausal women, frequency, urgency, and dysuria are the most common symptoms. Among postmenopausal women, the elderly, and children, the patient may present with malaise, nocturia, incontinence, or a complaint of foul-smelling urine (see Table 1). Cystitis is very common. For example, among veteran users of the Veterans’ Administration health care, the annual incidence was 4.3% among women and 1.7% among men.15 This figure is similar to population-based estimates from the Calgary Health Region of Canada, where the annual incidence of community-onset UTI identified using laboratory surveillance was 3% for females and 0.5% for males.16 Estimates based on self-reported history of physician diagnosis during the past year are higher: approximately 12.6% per year for women and 3.0% per year for men.17 Estimated lifetime risk of UTI for women based on self-reported history of physician diagnosis is 60.4%.18

Urinary symptoms may or may not be present in pyelonephritis; the patient may present with fever and chills, back pain, nausea, and vomiting (see Table 1). The incidence of pyelonephritis is an order of magnitude lower than cystitis (59.0/10,000 for females and 12.6/10,000 for males), but the patterns by age and sex are very similar.19 Risks are higher for females than for males, but these differences decrease with age (Fig. 1). In the United States, the incidence of hospitalization for acute pyelonephritis is 11.7 in 10,000 for women and 2.4 in 10,000 for men.20 A population-based study in California estimated the incidence of hospitalization for pyelonephritis among children to be 31 in 100,000 in 2005. The incidence varied substantially by age, with children younger than 1 year having the highest rates.21

Every day that a urinary catheter is in place increases the risk of bacteriuria by 3% to 10%.22 However, unless there are symptoms referable to the urinary tract or there are generalized symptoms such as fever, chills, or malaise with no identifiable cause (see Table 1), catheter-associated bacteriuria is not a treatable condition.9 Catheters are the major risk factor for hospital-acquired UTI, which account for almost one-third of all hospital-acquired infections. The estimated rates of catheter-associated UTI vary by service: in an analysis of 15 hospitals in the Duke Infection Control Outreach Network, the rates were 1.83 per 1000 catheter days for patients in intensive care, compared with 1.55 per 1000 catheter days for other patients.23 The risk of UTI with catheterization varies only slightly by catheter type. A multicenter, randomized controlled trial in the United Kingdom compared antimicrobial-impregnated, antiseptic-coated (silver alloy), and standard polytetrafluoroethylene-coated catheters in 7102...