Asymptomatic Bacteriuria

Asymptomatic bacteriuria is defined as the presence of one or more species of bacteria growing in the urine at specified quantitative counts (≥10⁵ colony-forming units [CFU]/mL or ≥10⁸ CFU/L), irrespective of the presence of pyuria, in the absence of signs or symptoms attributable to urinary tract infection (UTI). ^[1] 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. ^[2]

The frequency of ABU in different adult populations is as follows ^[1] :

• Premenopausal women, 1-5%
• Pregnant women, 1.5-9.5%
• Women aged 50-70 years, 2.8-8.6%
• Women aged 70 years or older in the community, 10.8 -16%
• Men aged 70 years or older in the community, 3.6-19%
• Women aged 70 years or older  in a long-term care facility,  25-50%
• Men aged 70 years or older  in a long-term care facility, 15-50%
• Women with diabetes, 10.8-16%
• Men with diabetes, 0.7-11%
• Individuals with spinal cord injury and intermittent cathetar use, 23-69%
• Individuals with spinal cord injury and sphincterotomy/condom catheter, 57%
• Individuals with long-term catheter use, 100%

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. ^[1]

Laboratory criteria for the diagnosis of ABU in a midstream clean-catch urine specimen in patients without indwelling catheters are as follows ^[1] :

• 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 of both men and women, organisms present in lower quantitative counts likely represent contamination of the urine specimen from organisms present in the biofilm along the device rather than true bacteriuria and, in these patients, ≥10⁵ CFU/mL remains the most appropriate diagnostic criteria for bladder bacteriuria. Lower quantitative counts (≥10² to < 10⁵ CFU/mL) isolated from urine specimens collected by “in and out” catheterization or following insertion of a new indwelling catheter suggest true bacteriuria, but the clinical significance of these lower quantitative counts in people without symptoms has not been evaluated. ^[1]

In most patient populations, treatment of ABU is not clinically beneficial, and consequently, screening for ABU is not recommended. ^[3] 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. ^[4]

Antibiotic treatment may also be valuable for children aged 5-6 years and before invasive genitourinary procedures. ^[5] 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
• Diabetic 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. ^[6]

Asymptomatic bacteriuria (ABU) is uncommon in the pediatric population (see Table 1 below) with a normal urinary tract and does not appear to be associated with important harms. The Infectious Disease Society of America (IDSA) clinical practice guidelines recommend against screening for or treating ABU in infants and children. ^[1]

Table 1: Frequency of Asymptomatic Bacteriuria in Pediatric Patients (Open Table in a new window)

In premenopausal and nonpregnant women, the frequency of asymptomatic bacteriuria (ABU) is 1-5% and in postmenopausal women (aged 50-70 years) it is 2.8-8.6%. ABU in these population is associated with more frequent urinary tract infections (UTIs) but with no other long-term adverse outcome. Screening for ABU in these population is not recommended, and antibiotic treatment does not reduce the frequency of symptomatic UTI. ^{[1, 4]}

The frequency of ABU in healthy young men is essentially zero. Thus, screening for ABU in this population is not recommended. ^[4]

The frequency of ABU in older adults is as follows ^[1] :

• Age 70 years or older in the community - 10.8 -16% in women, 3.6-19% in men
• Age 70 years or older in a long-term care facility - 25-50% in women, 15-50% 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
• Neuromuscular disease
• Increased instrumentation of the urinary tract
• Urinary catheters
• 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. ^[1]

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. ^[7]

Among institutionalized adults 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. ^[1]

In older patients with functional and/or cognitive impairment and bacteriuria and delirium (acute mental status change, confusion) and without local genitourinary symptoms or other systemic signs of infection (eg, fever or hemodynamic instability), the IDSA guidelines recommend assessment for other causes and careful observation rather than antimicrobial treatment. However, for the bacteriuric patient with fever and other systemic signs potentially consistent with a severe infection (sepsis) and without a localizing source, broad-spectrum antimicrobial therapy directed against urinary and nonurinary sources should be initiated. ^[1]

In pregnant women, the frequency of asymptomatic bacteriuria (ABU) in the first trimester is 1.5-9.5%. 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. ^[8]

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). ^[9]

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 B recommendation). ^[4]  The Canadian Task Force on Preventive Health Care issued a weak recommendation in favor of screening pregnant women once during the first trimester with urine culture for AUB. ^[10]

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.

Langermans and colleagues conducted a retrospective cohort analysis to determine the optimal timing for asymptomatic bacteriuria screening during pregnancy.  After comparing the outcomes of 655 patients screened during the first trimester to 1350 patients screened during the second trimester, the researchers reported no statistical difference for the presence of asymptomatic bacteriuria or for the clinical impact on obstetrical outcomes. ^[11]

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. ^[12]

Guidelines from the Infectious Diseases Society of America recommend  4–7 days of antibiotic therapy for treatment of ABU in pregnancy rather than shorter duration. ^[1]  A Single-dose regimens have been studied, but may be less effective. ^[13]

The optimal duration of therapy is antimicrobial-specific. Nitrofurantoin and beta-lactam antimicrobials (usually ampicillin or cephalexin) are preferred because of their safety in pregnant women. ^[1] :

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.)

In patients who have spinal cord injury (SCI) with bladder impairment, the frequency of asymptomatic bacteriuria (ABU) is 23-69% with intermittent catheter use and 57% with sphincterotomy and a condom catheter. ^[1] 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 by the IDSA, and antibiotic treatment does not improve survival or the frequency of symptomatic urinary tract infections (UTIs). ^[1]

The Veterans Health Administration (VHA) guidelines outlining care for persons with SCI, however, recommend a yearly urinalysis and urine culture as part of an annual physical checkup, regardless of whether signs or symptoms of infection are present. Obtaining these tests in asymptomatic patients is essentially a screening for AUB. Although the VHA guideline does not explicitly recommend treatment of AUB review of 2 years of annual examination visits uncovered that 35% of cases of AUB were subsequently treated with antibiotics. ^[14]

Patients with spinal cord injury who receive antibiotics for ABU have uniformly showed early recurrence of bacteriuria after therapy. ^[1] Intermittent urinary catheterization and, in men, sphincterotomy with a condom catheter, producing a low-pressure bladder, significantly reduce morbidity and mortality from UTIs.

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. ^[15]

The frequency of ABU in patients with diabetes mellitus is 10.8-16% in females and 0.7-11% in males. ^[1] There is no indication of adverse outcomes in women.

Screening is not recommended, and treatment with antibiotics is not beneficial. ^[1] 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. ^[16]

In renal transplant recipients, asymptomatic bacteriuria (ABU) is principally a concern in the initial months after transplantation: the frequency of ABU is 23-24% in the first month, 10-17% in 2-12 months, and 2-9% after 12 months. ^[1] The risks of ABU in these patients include acute pyelonephritis, sepsis, and graft loss. ^[17] In 11% of patients, persistent ABU develops and leads to urologic complications.

Screening for ABU is usually performed in the immediate postoperative period and for up to 6 months after transplantation. There is no consensus on the diagnosis and management of asymptomatic bacteriuria and 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.

However, antibiotic use also has harmful effects. Antimicrobial use is a key driver for antimicrobial resistance selection which has become an important issue in the field of transplantation, where antimicrobial resistance is a rapidly evolving. An increase in antimicrobial resistance rates in renal transplantations with bacteriuria has been reported. In addition, antimicrobial use is associated with direct adverse effects, including fluoroquinolone-induced tendinopathy, and Clostridium difficile–associated diarrhea. ^[18]

In 2019, the IDSA released revised guidelines that recommend against screening for or treating ABU in renal transplant recipients more than 1 month after transplantation. The guidelines found insufficient evidence to recommend for or against screening or treatment of ABU within the first month following renal transplantation. ^[1]

 

Short-term bladder catheterization is associated with a 3-5% frequency of asymptomatic bacteriuria (ABU) for each day that the catheter is in place. ^[1] 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. ^[1]

ABU is a universal finding in patients with indwelling catheters that have been in place for longer than 30 days. ^[1] 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.