Asymptomatic Bacteriuria

Etiology

The etiology of asymptomatic bacteriuria has not been conclusively determined. Asymptomatic bacteriuria is more common among women than among men probably because of the shorter female urethra, which gives bacteria from the urethral meatus and the perineum a shorter distance to the bladder. In fact, most women have transient bacteriuria after sexual intercourse, but few of these women will develop symptomatic infections because the body's normal defense mechanisms prevent symptomatic infection in most cases.  In the elderly, it is thought that incomplete bladder emptying contributes to the increased incidence of asymptomatic bacteriuria.[4]

Epidemiology

Asymptomatic bacteriuria is a common clinical finding. While less than 0.5% of infants and toddlers have asymptomatic bacteriuria, the incidence increases with age. The incidence is 5% or less among healthy premenopausal women, up to 15% or greater in women and men age 65-80 years, and it continues to climb after age 80 to as high as 40% to 50% of long-term care residents. At any age, the incidence of asymptomatic bacteriuria is higher among females than among males. Escherichia coli is the most common bacteria identified in asymptomatic bacteriuria.[5]

History and Physical

Patients with asymptomatic bacteriuria are, by definition, asymptomatic. They have no symptoms that can be attributed to bacteria in the urine. This is completely different from the evaluation and management of symptomatic bacteriuria or urinary tract infections.  Several factors are thought to increase the likelihood of asymptomatic bacteriuria.[6]  These include:

• obstructive uropathy (stones, prostatic hypertrophy, cystocele, etc.),
• fecal soiling of the perineum (especially in women),
• indwelling urinary catheters, and
• frequent instrumentation of the urinary tract.

Evaluation

Diagnosis of asymptomatic bacteriuria is made by urine culture. Either a properly collected clean-catch specimen or a catheterized specimen is acceptable. The Infectious Diseases Society of America (IDSA) has established criteria for diagnosing asymptomatic bacteriuria.

Midstream clean catch urine specimen:

• For women, two consecutive specimens with isolation of the same bacteria species with at least 100,000 colony-forming units (CFUs) per ml of urine.
• For men, a single specimen with isolation of one bacteria species with at least 100,000 CFUs per ml of urine.

Catheterized specimen:

• For women or men, a single specimen with isolation of one bacteria species with at least 100 CFUs per ml of urine.

Urine dipstick for leukocyte esterase will reliably identify pyuria, but it is not specific for asymptomatic bacteriuria. (Pyuria may result from other inflammatory disorders of the genitourinary tract.) Urinary dipstick for nitrites is also of limited usefulness because of infection with non-nitrite-producing organisms, the delay between collection and testing of the specimen, and insufficient time since the last voiding for nitrites to be produced at detectable levels. The combination of the dipstick for leukocyte esterase and nitrites is more specific for asymptomatic bacteriuria than either test alone. Urinalysis with microscopic exam for bacteria is a useful, but non-quantitative, way to identify bacteriuria.  

Pregnant women should be screened for asymptomatic bacteriuria with a urine culture. The optimal timing and frequency of screening urine cultures in pregnancy have not been established, but obtaining a screening urine culture at the end of the first trimester of pregnancy is recommended.  

Other than in pregnancy, there are few indications to screen for asymptomatic bacteriuria. Two possible clinical scenarios in which a screening urine culture might be appropriate include patients undergoing urological procedures in which mucosal bleeding is expected, such as resection of the prostate, and patients who are in the first three months following renal transplantation.[7]

Treatment / Management

Most patients with asymptomatic bacteriuria will not develop symptomatic urinary tract infections and will have no adverse consequences from asymptomatic bacteriuria.  Specifically, children, patients with diabetes, older patients, patients with spinal cord injuries, and patients with indwelling urinary catheters do not benefit from treatment with antibiotics for asymptomatic bacteriuria. Treatment in these patients does not decrease the incidence of symptomatic urinary tract infections or improve survival. However, it does increase the likelihood of adverse effects from antibiotics and the development of antibiotic-resistant bacteria.

In contrast, treatment of pregnant women with asymptomatic bacteriuria has been shown to be beneficial. Antimicrobial treatment of asymptomatic bacteriuria in pregnancy decreases the risk of pyelonephritis, low-birthweight infants, and preterm delivery.  In addition, patients undergoing urologic procedures in which mucosal bleeding is expected, such as with resection of the prostate, and patients who are in the first three months following renal transplantation probably should be treated. There is evidence that treatment of asymptomatic bacteriuria in these patients decreases the risk of symptomatic urinary tract infection.    

Treatment should be guided by the results of urine culture and sensitivity. Amoxicillin, amoxicillin/clavulanate, cefuroxime, cephalexin, and nitrofurantoin are considered safe for use in pregnancy. [7][8][9] Treatment duration should be for 3 to 7 days for pregnant women, and at least one follow-up urine culture should be performed. (B3)

Differential Diagnosis

• Acute pyelonephritis

• Bladder cancer

• Chlamydia (chlamydial genitourinary infections)

• Cystitis (nonbacterial)

• Herpes simplex

• Interstitial cystitis

• Pelvic inflammatory disease

• Renal and perirenal abscess

• Urethritis

• Vaginitis