Asymptomatic bacteriuria (ABU) is common. The frequency varies among different populations, depending on factors such as age, sex, and underlying disorders (eg, diabetes mellitus or spinal cord injury).  One study in hospitalized patients identified obesity and iron deficiency anemia as independent risk factors for ABU. 
The frequency of ABU in different populations is as follows:
Preschool girls, <2%
Pregnant women, 2-9.5%
Women aged 65-80 years, 18-43%
Men aged 65-80 years, 1.5-15.3%
Women older than 80 years, 18-43%
Men older than 80 years, 5.4-21%
Patient characteristics also influence the microbiology of ABU. Escherichia coli is the most common organism and is the most likely to occur in healthy persons. A variety of organisms may be found, however, including Enterobacteriaceae, Pseudomonas aeruginosa, Enterococcus species, and group B Streptococcus. In men, Enterococcus species and gram-negative bacilli are common. Catheterized nursing home residents may have polymicrobial ABU. 
Laboratory criteria for the diagnosis of ABU in a midstream clean-catch urine specimen are as follows  :
For women, 2 consecutive specimens with isolation of at least 100,000 colony-forming units (cfu) per mL of the same bacterial species
For men, a single specimen with isolation of at least 100,000 cfu/mL of a single bacterial species
For the diagnosis of ABU in a catheterized urine specimen, the laboratory criterion is a single bacterial species isolated in a quantitative count of at least 100 cfu/mL.  This applies to both women and men.
In most patient populations, treatment of ABU is not clinically beneficial, and consequently, screening for ABU is not recommended.  The US Preventive Services Task Force advises against screening men and nonpregnant women for asymptomatic bacteriuria; there is adequate evidence to suggest that screening is ineffective in improving clinical outcomes. 
An important exception is pregnant women, for whom ABU carries significant risks and treatment provides important benefits. Antibiotic treatment may also be valuable for children aged 5-6 years and before invasive genitourinary procedures.  However, the consensus is that catheterization has no clinical significance and that antibiotic prescription is not indicated in any of the following:
Elderly ABU patients
Healthy school girls and young women
Patients who have indwelling catheters or undergo intermittent urinary catheterization
A study by Lin et al suggests the need for greater focus on optimizing the use of antibiotics in patients with enterococcal bacteriuria; overtreatment of ABU is common, especially in patients with pyuria. 
Asymptomatic bacteriuria (ABU) is uncommon in the pediatric population (see Table 1 below).
Table 1: Frequency of Asymptomatic Bacteriuria in Pediatric Patients (Open Table in a new window)
|Infants and toddlers (≤ 36 mo)||0.4-1.8||0.5-2.5|
|School-age children and adolescents||1.1-1.8||~ 0|
In premenopausal and nonpregnant women, the frequency of asymptomatic bacteriuria (ABU) is 0.8-5.2%. ABU in this population is associated with more frequent urinary tract infections (UTIs) and subsequent ABU but with no other long-term adverse outcome. Screening for ABU in this population is not recommended, and antibiotic treatment does not reduce the frequency of symptomatic UTI. 
The frequency of ABU in healthy young men is essentially zero. Thus, screening for ABU in this population is not recommended.
The frequency of ABU in older adults is as follows:
Age 50-65 years - 2.8-8.6% in women, 0.6-1.5% in men
Age 65-80 years - 5.8-16% in women, 1.5-15.3% in men
Age older than 80 years - 18-43% in women, 5.4-21% in men
Several factors appear to account for the increasing frequency of ABU with advancing age, including the following:
Obstructive uropathy (eg, urinary stones, prostatic hypertrophy, uterine prolapse, or cystocele)
Decreased bactericidal activity in prostatic secretions
Perineal soiling with fecal matter in women with dementia
Increased instrumentation of the urinary tract
Reduced Tamm-Horsfall protein secretion in urine
Increased uropathogens in the postmenopausal vagina and introitus
No morbidity or mortality from ABU has been demonstrated in older adults, though the data are limited; 76% of ABU episodes resolve spontaneously. Screening for ABU is not recommended. Antibiotic treatment does not reduce the frequency of symptomatic UTI or improve survival; instead, it leads to an increased incidence of adverse antibiotic effects and reinfection with antibiotic-resistant organisms. 
In older adults with chronic urinary incontinence, ABU can be difficult to differentiate from symptomatic UTI. In such cases, delaying antibiotic treatment for 1 week while offering supportive treatment such as increased fluid intake is an acceptable therapeutic option; up to 50% of women with UTI will have symptom relief or show spontaneous improvement in 1 week without antibiotics. 
Among institutionalized adults, the frequency of ABU is 25-53% in women and 19-37% in men. Risk factors include urinary or bowel incontinence and dementia. Screening for ABU in this population is not recommended, and antibiotic treatment does not improve survival or the frequency of symptomatic UTI.
In pregnant women, the frequency of asymptomatic bacteriuria (ABU) in the first trimester is 2-9.5%. Previous urinary tract infection (UTI) or lower socioeconomic status is associated with a higher frequency of ABU.
Older studies found that ABU in pregnancy is significant because 20-30% of untreated cases progress to acute pyelonephritis, usually at the end of the second trimester or early in the third trimester. Acute pyelonephritis in pregnancy is associated with premature labor. Studies of perinatal outcomes in pregnant women with untreated ASB have yielded mixed results, with a number of studies finding an increased risk of premature delivery, lower birth weight, or both, while other studies failed to find an association. 
In a 2015 prospective cohort study with an embedded randomized controlled trial in 4283 women with an uncomplicated singleton pregnancy, ABU was not associated with preterm birth; ABU showed a significant association with pyelonephritis, but the absolute risk of pyelonephritis in untreated ABU was low: pyelonephritis developed in 5 of 208 women (2.4%) with untreated or placebo-treated ABU, compared with 24 of 4035 (0.6%) women without ABU (adjusted odds ratio 3.9, 95% confidence index 1.4–11.4).
Nevertheless, because of the dangers posed by ABU in pregnancy, screening for ABU is a standard aspect of prenatal care. The US Preventive Services Task Force recommends screening for asymptomatic bacteriuria with urine culture at 12 to 16 weeks’ gestation or at the first prenatal visit (grade A recommendation). 
At least 1 urine culture should be performed at the end of the first trimester; 2 consecutive cultures are preferable because 1-2% of women with a negative initial urine culture develop ABU and experience acute pyelonephritis later in pregnancy. Urine dipstick and microscopic analysis are inadequate for identifying ABU in these patients.
A Cohcrane review concluded that antibiotic treatment in pregnant women with ABU can reduce the risk of pyelonephritis and may reduce risk of low birthweight and preterm birth. However, supporting evidence for those findings was of very low quality. 
Guidelines from the Infectious Diseases Society of America recommend 3–7 days of antibiotic therapy for treatment of ABU in pregnancy.  A Single-dose regimens have been studied, but may be less effective. 
One of the following agents may be used  :
Treatment of ABU in pregnancy reduces the frequency of acute pyelonephritis to 2-3%. After treatment of ABU, periodic (eg, monthly) follow-up urine cultures are recommended. (See Urinary Tract Infections in Pregnancy.)
Patients With Spinal Cord Injuries
In patients who have spinal cord injury with bladder impairment, the frequency of asymptomatic bacteriuria (ABU) is 70-100%. ABU in these patients is associated with the development of acute pyelonephritis, urosepsis, and renal failure. However, screening for ABU in this population is not recommended, and antibiotic treatment does not improve survival or the frequency of symptomatic urinary tract infections (UTIs).
Patients with spinal cord injury who receive antibiotics for ABU have uniformly showed early recurrence of bacteriuria after therapy.  Intermittent urinary catheterization and, in men, sphincterotomy with a condom catheter, producing a low-pressure bladder, significantly reduce morbidity and mortality from UTIs.
Patients With Diabetes Mellitus
Asymptomatic bacteriuria (ABU) is more common in patients of all ages with either type 1 or type 2 diabetes mellitus, compared with patients who do not have diabetes. The increased frequency is probably secondary to autonomic neuropathy of the bladder. Diabetic patients with ABU are more likely to have albuminuria and symptomatic UTIs, but their hemoglobin A1C levels are not significantly higher than those of diabetic patients without ABU. 
The frequency of ABU in patients with diabetes mellitus is 7.9-17.7% in females and 1.5-2.2% in males. There is no indication of adverse outcomes in women.
Screening is not recommended, and treatment with antibiotics is not beneficial. A randomized, controlled trial found that treatment of asymptomatic bacteriuria in women with diabetes does not appear to reduce complications. These investigators concluded that diabetes itself should not be an indication for screening for or treatment of ABU. 
Renal Transplant Recipients
In renal transplant recipients, asymptomatic bacteriuria (ABU) is principally a concern in the initial months after transplantation: the frequency of ABU is 41% in the first month, 21% in the second month, and 0.01% after 3 months. The risks of ABU in these patients include acute pyelonephritis, sepsis, and graft loss.  In 11% of patients, persistent ABU develops and leads to urologic complications.
Screening for ABU is indicated in the immediate postoperative period and for up to 6 months after transplantation. Current practice is to initiate prophylactic antibiotics in the perioperative period and continue them long-term, and to shorten the period of indwelling catheter use. These measures have reduced morbidity to the point that there is no association between ABU and graft loss. Organ donors should be screened and treated in advance for ABU.
Short-term bladder catheterization is associated with a 2-7% frequency of asymptomatic bacteriuria (ABU) for each day that the catheter is in place. The frequency is higher in women than in men. Symptomatic urinary tract infection (UTI) occurs in 26% of women by 14 days after catheter removal.
Screening for ABU is not indicated unless the patient has other risk factors for UTI, however. Antibiotic treatment is possibly beneficial in women with persistent ABU 48 hours after catheter removal. In general, the most effective strategy for reducing the incidence of catheter-related ABU is to reduce catheter use. 
ABU is a universal finding in patients with indwelling catheters that have been in place for longer than 30 days. These patients are at risk for acute pyelonephritis, urosepsis, catheter obstruction, renal stones, vesicoureteral reflux, renal failure, and (eventually) bladder cancer.
Unfortunately, treatment of ABU in these patients does not decrease the incidence of fever and usually leads to the development of resistant bacterial strains. In asymptomatic patients with indwelling urethral catheters, cloudy or foul-smelling urine is not an indication for urinalysis, culture, or antimicrobial treatment.