Urinary tract infections (UTIs) are one of the most common infections for which antibiotics are prescribed. The Infectious Diseases Society of America (IDSA) issued guidelines for the treatment of uncomplicated acute bacterial cystitis and acute pyelonephritis in women.¹ The presence of bacteria in the urine of an asymptomatic patient is known as asymptomatic bacteriuria. The IDSA also has published guidelines on indications for the screening and treatment of asymptomatic bacteriuria in various patient populations.²

Epidemiology

Asymptomatic bacteriuria is common, with varying prevalence by age, sex, sexual activity, and the presence of genitourinary abnormalities(Table 1^3–8). In healthy women, the prevalence of bacteriuria increases with age, from about 1 percent in females five to 14 years of age to more than 20 percent in women at least 80 years of age living in the community.³ Escherichia coli is the most common organism isolated from patients with asymptomatic bacteriuria. Infecting organisms are diverse and include Enterobacteriaceae,Pseudomonas aeruginosa, Enterococcus species, and group B streptococcus. Organisms isolated in patients with asymptomatic bacteriuria will be influenced by patient variables: healthy persons will likely haveE. coli, whereas a nursing home resident with a catheter is more likely to have multi-drug–resistant polymicrobic flora(e.g.,P. aeruginosa). Enterococcus species and gram-negative bacilli are common in men.^9,10

Diagnosis

The presence of a significant quantity of bacteria in a urine specimen properly collected from a person without symptoms or signs of a UTI characterizes asymptomatic bacteriuria.¹¹ Quantitative criteria for identifying significant bacteriuria in an asymptomatic person are: (1) at least 100,000 colony-forming units (CFUs) per mL of urine in a voided midstream clean-catch specimen; and (2) at least 100 CFUs per mL of urine from a catheterized specimen^9,12,13 (Table 2). According to the IDSA guideline, the diagnosis of asymptomatic bacteriuria in women is appropriate only if the same species is present in quantities of at least 100,000 CFUs per mL of urine in at least two consecutive voided specimens.^2,3

The leukocyte esterase and nitrite tests often are used in primary care settings to evaluate urinary symptoms; however, they are not useful for diagnosing UTI in an asymptomatic patient. A urine dipstick leukocyte esterase test showing trace or more white blood cells has a sensitivity of 75 to 96 percent and specificity of 94 to 98 percent for detecting pyuria¹⁴; however, pyuria is not specific for UTI and may occur with other inflammatory disorders of the genitourinary tract (e.g., vaginitis). Urinalysis with microscopic examination for bacteria remains a useful test for the identification of bacteriuria.

Limitations of the dipstick nitrite test in diagnosing bacteriuria include: infection with non-nitrite–producing pathogens; delays between obtaining and testing the sample; and insufficient time since the last void for nitrites to appear at detectable levels. Combining the leukocyte esterase and nitrite tests results in higher specificity than using either test alone.

Premenopausal, Nonpregnant Women

Premenopausal, nonpregnant women with asymptomatic bacteriuria experience no adverse effects and usually will clear their bacteriuria spontaneously. However, these women are more likely to experience subsequent symptomatic UTI than women who do not have asymptomatic bacteriuria.¹⁵ One study randomized women with bacteriuria to receive one week of nitrofurantoin (Furadantin) or placebo; those receiving the antibiotic had a significantly lower prevalence of bacteriuria at six months, but not at one year.¹⁶ The patients treated with antibiotics were just as likely as those in the placebo arm to have a symptomatic UTI in the year after therapy. Although women with asymptomatic bacteriuria are more likely to have subsequent symptomatic UTIs, treatment of asymptomatic bacteriuria does not decrease the frequency of symptomatic UTI or prevent further episodes of bacteriuria. Asymptomatic bacteriuria has not been shown to be associated with detrimental long-term outcomes (e.g., hypertension, renal failure, genitourinary cancer, or decreased survival). For these reasons, the IDSA does not recommend screening for or treatment of asymptomatic bacteriuria in premenopausal nonpregnant women.²

Pregnant Women

Women with asymptomatic bacteriuria during pregnancy are more likely to deliver premature or low-birth-weight infants and have a 20- to 30-fold increased risk of developing pyelonephritis during pregnancy compared with women without bacteriuria.¹⁶ A Cochrane systematic review found that studies have consistently reported that treatment of asymptomatic bacteriuria in pregnancy decreases the risk of subsequent pyelonephritis from a range of 20 to 35 percent to a range of 1 to 4 percent.¹⁷ Antimicrobial treatment of asymptomatic bacteriuria also improves fetal outcomes, with decreases in the frequency of low-birth-weight infants and preterm delivery.^18,19 Early studies usually continued antimicrobial therapy for the duration of pregnancy; however, more recent studies reported similar benefits in patients treated for 14 days with nitrofurantoin or trimethoprim/sulfamethoxazole (TMP/SMX; Bactrim, Septra) compared with those treated with continuous antimicrobial therapy to the end of pregnancy.²⁰ The IDSA recommends a course of three to seven days of antimicrobial therapy for pregnant women with asymptomatic bacteriuria.² A Cochrane systematic review found insufficient evidence to determine whether a single dose regimen is as effective as treatments of longer duration.²¹

Because leukocyte esterase and nitrite tests have low sensitivity for identifying bacteriuria in women who are pregnant, these patients should be screened with urine cultures²²; however, the optimal frequency of urine culture screening has not been established. A single urine culture at the end of the first trimester generally is recommended based on clinical outcomes and cost-effectiveness.^23,24 Women with asymptomatic bacteriuria or symptomatic UTI during pregnancy should be treated(Table 3) and should undergo periodic screening for the duration of their pregnancy. The IDSA makes no recommendations for subsequent screening of pregnant women found to have no asymptomatic bacteriuria at the initial screen.²

Women with Diabetes

Studies of women with diabetes show no difference between initially asymptomatic bacteriuric and nonbacteriuric women in the incidence of UTI, mortality, or progression to diabetic complications at 18 months²⁵ or 14 years.²⁶ In a study of antibiotic therapy versus no therapy for women with diabetes and asymptomatic bacteriuria, antimicrobial therapy did not delay or decrease the frequency of symptomatic UTI or the rate of hospitalization for UTI or other causes at up to three years’ follow-up.²⁷ These studies support the IDSA guidelines² that screening for or treatment of asymptomatic bacteriuria in women with diabetes is not indicated.

Older Patients with Asymptomatic Bacteriuria

Studies of asymptomatic bacteriuria in pre- and postmenopausal women report similar outcomes regardless of age.^28,29 A study of ambulatory women in a long-term care facility who were assigned to receive antimicrobial therapy or placebo for bacteriuria showed a decrease in prevalence of asymptomatic bacteriuria at six months among those receiving antibiotics, but no significant difference in symptomatic episodes.³⁰ Adverse outcomes attributable to asymptomatic bacteriuria were not observed in a cohort of ambulatory male veterans older than 65 years at several years’ follow-up.¹⁰

Clinical trials of older residents in long-term care facilities have shown no benefits from screening for or antimicrobial treatment of asymptomatic bacteriuria.^31–33 Although antimicrobial treatment does not decrease symptomatic infection or improve survival, there is an increased incidence of adverse antimicrobial effects and reinfection with antibiotic-resistant organisms. Thus, the IDSA does not recommend screening for or treatment of asymptomatic bacteriuria in older patients.²

Patients with Spinal Cord Injuries

Patients with spinal cord injuries have a higher prevalence of asymptomatic bacteriuria and symptomatic UTI.^6,34 Patients with spinal cord injuries and with asymptomatic bacteriuria treated using antibiotics uniformly showed early recurrence of bacteriuria following therapy. When treated with seven to 14 days of antibiotics, 93 percent of patients were again bacteriuric by 30 days.³⁵ Posttreatment urine cultures showed increased antimicrobial resistance as well. A prospective, randomized trial in patients with asymptomatic bacteriuria and intermittent catheterization showed similar rates of UTI at follow-up, whether or not prophylactic antimicrobials were administered.³⁶ Although there are few trials addressing the treatment of asymptomatic bacteriuria in patients with spinal cord injuries, review articles and consensus guidelines support the IDSA recommendations² that asymptomatic bacteriuria should not be screened for or treated in patients with spinal cord injuries.

Patients with Indwelling Urethral Catheters

Patients with chronic indwelling Foley catheters are uniformly bacteriuric, but treatment is warranted only if the patient is symptomatic. Urine that is cloudy or foul-smelling often prompts a call from a long-term care facility to the physician, with an expectation that an evaluation, if not antibiotic therapy, will be ordered. However, in the asymptomatic patient, cloudy or foul smelling urine is not an indication for urinalysis, culture, or antimicrobial treatment. A study of residents in long-term care facilities with chronic indwelling catheters and bacteriuria who were treated with cephalexin (Keflex) or no therapy showed no differences in the incidence of fever or reinfection; however, patients who received antibiotic therapy had twice the incidence of subsequent microbial resistance to cephalexin.³⁷

When possible, the indwelling catheter should be removed, and the patient should receive clean intermittent catheterization to reduce the risk of symptomatic UTI. The replacement of a chronic indwelling Foley catheter is associated with a low risk for bacteremia, and antimicrobial treatment or prophylaxis is not indicated for this procedure.³⁸ A study in young women with short-term catheterization reported increased symptomatic infection over two weeks following catheter removal, when asymptomatic bacteriuria persisted 48 hours after the removal of the indwelling catheter.³⁹ Accordingly, the IDSA recommends that asymptomatic bacteriuria should not be screened for or treated in patients with an indwelling urethral catheter, but that treatment of women with persistent catheter-acquired bacteriuria at least 48 hours after catheter removal may be considered.²