Asymptomatic bacteriuria is the presence of bacteria in the properly collected urine of a patient that has no signs or symptoms of a urinary tract infection. Asymptomatic bacteriuria is very common in clinical practice. While few infants and toddlers have asymptomatic bacteriuria, the incidence increases with age. The incidence is up to 15% or greater in women and men age 65 to 80 years and as high as 40% to 50% after age 80. Most patients with asymptomatic bacteriuria will never develop symptomatic urinary tract infections and will have no adverse consequences from asymptomatic bacteriuria. In an age when decreasing unnecessary antibiotic use is emphasized, the important clinical question is this: Which patients with asymptomatic bacteriuria benefit from treatment? The answer is that most patients with asymptomatic bacteriuria do not benefit from treatment. There are, however, a few exceptions. There is sufficient evidence that a pregnant woman with asymptomatic bacteriuria should be treated. Also, patients undergoing urologic procedures in which mucosal bleeding is expected and patients who are in the first three months following renal transplantation should probably be treated for asymptomatic bacteriuria.
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.
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.
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. These include:
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:
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.
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. Treatment duration should be for 3 to 7 days for pregnant women, and at least one follow-up urine culture should be performed.
Cost-effective use of health care resources necessitates careful scrutiny to avoid unnecessary tests and treatments. "Antibiotic stewardship," or the appropriate use of antibiotics in a way that improves patient outcomes, reduces microbial resistance, and decreases the spread of infections caused by multi-drug-resistant organisms, is essential. As such, a rational and evidence-based approach to the evaluation and treatment of asymptomatic bacteriuria is important. It is not appropriate to routinely order a urinalysis and urine culture and sensitivity test on every patient admitted to the hospital unless there is a clinical reason to suspect a symptomatic or an occult urinary tract infection. Additionally, when the clean-catch urine culture of a 70-year-old female (who is admitted for a diagnosis unrelated to her urinary tract and has no urinary symptoms) increases to >100,000 CFU/ml of Escherichia coli, which it often will, it is appropriate to resist the temptation to treat the culture results. Treatment is unnecessary, and may even be harmful.