Asymptomatic Bacteriuria

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)

Asymptomatic bacteriuria (ABU) represents a prevalent clinical condition that frequently results in unwarranted antimicrobial therapy. ^[7]

Women

Epidemiologic data indicate that the prevalence of ABU escalates with advancing age, particularly affecting women more than men. Notably, in elderly women, the prevalence exceeds 15%, and this figure surges to as high as 50% among residents of long-term care facilities. Despite its prevalence, ABU typically does not progress to clinically significant urinary tract infections, and empiric antibiotic treatment has not demonstrated a benefit in patient outcomes.

In response to these findings, the Infectious Disease Society of America (IDSA) revised its guidelines in 2019 for the management of ABU, underscoring the necessity to weigh the potential risks and benefits of treatment. ^[7] The guidelines advocate for targeted screening and treatment in specific populations where intervention has proven beneficial. This includes pregnant women, who should be screened for ABU during the first trimester and treated if results are positive, and individuals undergoing endoscopic urologic procedures, who also should be screened and treated to prevent complications.

Conversely, the guidelines recommend against routine treatment of ABU in other populations, including individuals with diabetes, neutropenia, spinal cord injuries, or those with indwelling urinary catheters, as intervention does not enhance clinical outcomes and may contribute to adverse effects. ^{[7, 8]} These adverse effects include the development of antimicrobial resistance, the risk for Clostridioides difficile infection, and increased healthcare costs.

In premenopausal and nonpregnant women, the frequency of ABU ranges from 1-5%, and in postmenopausal women aged 50-70 years, it is 2.8-8.6%. Asymptomatic bacteriuria is linked with more frequent UTIs but no other long-term adverse outcomes. Screening and antibiotic treatment for ABU in these groups are not recommended as they do not reduce the frequency of symptomatic UTIs. ^{[1, 4]}

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

Older adults

Community-dwelling individuals aged 70 years or older have ABU rates of 10.8-16% in women and 3.6-19% in men. In long-term care facilities, the rates are 25-50% in women and 15-50% in men. ^[1]

No significant morbidity or mortality has been shown from ABU in older adults, with 76% of episodes resolving spontaneously. Screening and antibiotic treatment are not recommended as they do not reduce symptomatic UTI frequency or improve survival but increase adverse effects and reinfection rates with antibiotic-resistant organisms. ^[1]

In older adults with chronic urinary incontinence, differentiating ABU from symptomatic UTI can be challenging. Delaying antibiotics for a week while increasing fluid intake is an acceptable approach, as up to 50% of women show improvement or symptom relief spontaneously within this period without antibiotics. ^[9]

Factors contributing to increased ABU in older adults include obstructive uropathy, decreased bactericidal activity in prostatic secretions, perineal soiling, neuromuscular diseases, increased urinary tract instrumentation, urinary catheters, reduced Tamm-Horsfall protein secretion, and increased uropathogens in postmenopausal women.

For older patients with functional and/or cognitive impairments presenting with bacteriuria and delirium but without local genitourinary symptoms or systemic signs of infection, the IDSA guidelines recommend assessing other causes and careful observation instead of antimicrobial treatment. However, if the patient with bacteriuria shows systemic signs like fever potentially indicating severe infection (sepsis), broad-spectrum antimicrobial therapy should be initiated. ^[1]

Among institutionalized adults, common risk factors for ABU include urinary or bowel incontinence and dementia. Screening and antibiotic treatment do not improve survival or reduce symptomatic UTIs in this group. ^[1] ​

Antibiotic stewardship

To address these concerns, numerous antibiotic stewardship programs across the United States have implemented protocols aimed at reducing unnecessary treatment of ABU. ^[7] These programs emphasize the critical importance of judicious screening and therapeutic intervention, reserving treatment for cases where there is a clear evidence-based potential benefit, thereby optimizing patient care and resource utilization.

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.

Pregnant women are more susceptible to urinary tract infections (UTIs) owing to physiologic changes during pregnancy. ^[10] UTIs affect about 2-15% of all pregnant women, with ABU impacting 2-7%. There is no strong evidence linking untreated ABU to acute pyelonephritis, but treating ABU can lower the risks for low birth weight and preterm birth, supporting the practice of screening for ABU in the first trimester using a single urine culture. For treatment, short courses of β-lactams, nitrofurantoin, or fosfomycin are recommended. The evidence on cystitis during pregnancy is scarce. Acute pyelonephritis is linked to higher maternal complications and, in some cases, to preterm delivery and low birth weight. Preferred treatments for pyelonephritis include amoxicillin with an aminoglycoside, third-generation cephalosporins, or carbapenems. Limited data on recurrent UTIs during pregnancy make it challenging to recommend prophylactic measures.

Studies from the late 1900s and early 2000s indicate ABU in pregnancy is significant because 20-30% of untreated cases may progress to acute pyelonephritis, typically in the late second or early third trimester. Acute pyelonephritis during pregnancy is associated with premature labor. Research on perinatal outcomes for pregnant women with untreated ABU shows mixed results; some studies report increased risks for premature delivery and lower birth weight, whereas others find no such associations. ^[11]

A 2015 study involving 4283 women with uncomplicated singleton pregnancies found no link between ABU and preterm birth. However, there was a significant association between ABU and pyelonephritis, although the absolute risk remained low: 2.4% in untreated or placebo-treated women with ABU versus 0.6% in women without ABU. ^[12]

Screening for ABU is a standard part of prenatal care due to its potential risks. The US Preventive Services Task Force recommends urine culture screening at 12 to 16 weeks’ gestation or at the first prenatal visit. ^[4]  Similarly, the Canadian Task Force on Preventive Health Care suggests screening once during the first trimester. ^[13]  It is advised to perform at least one urine culture at the end of the first trimester, with two consecutive cultures being preferable to detect any cases that develop ABU and subsequent acute pyelonephritis later in pregnancy.

A retrospective cohort analysis by Langermans and colleagues found no statistical difference in the presence of ABU or its impact on obstetric outcomes when comparing first and second trimester screenings. ^[14]

A review concluded that antibiotic treatment for pregnant women with ABU could reduce the risk for pyelonephritis and might lower the risk for low birth weight and preterm birth, although the evidence supporting these findings is of very low quality. ^[15]

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]  Single-dose regimens have been studied, but may be less effective. ^[16]  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 individuals. ^[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]  Asymptomatic bacteriuria 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 ABU. 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 ABU subsequently were treated with antibiotics. ^[17]

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 notably more prevalent in patients of all ages with either type 1 or type 2 diabetes mellitus compared to those without diabetes, likely due to autonomic neuropathy affecting the bladder. Diabetic patients with ABU tend to experience albuminuria and symptomatic urinary tract infections (UTIs) more frequently, although their hemoglobin A1C levels do not significantly differ from diabetic patients without ABU. ^[18]

A prospective case-control study involving 437 adults, with a gender distribution of 61% female and 39% male and an average age of 70.5 years, reported an overall ABU prevalence of 20.1%. Females exhibited a higher prevalence at 27%, compared to 9.4% among males. The study highlighted that higher glycated hemoglobin levels and a history of UTI in the previous year significantly increased the likelihood of ABU, whereas higher vitamin B12 levels were associated with a reduced likelihood. ^[19]

In patients with diabetes mellitus, the frequency of ABU ranges from 10.8-16% in females and 0.7-11% in males, with no adverse outcomes noted in women. ^[1]  The overall occurrence of ABU in individuals with type 2 diabetes mellitus (T2DM) stands at 23.7%, with age, female gender, duration of T2DM, HbA1C levels, hypertension, hyperlipidemia, neuropathy, and proteinuria identified as related risk factors. ^[20]

Despite these findings, screening for ABU is not recommended, and antibiotic treatment has not shown benefits. A randomized controlled trial specifically concluded that treating ABU in women with diabetes does not reduce complications, suggesting that diabetes alone should not be a criterion for ABU screening or treatment. ^{[1, 21]}

 

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. ^[22]  In 11% of patients, persistent ABU develops and leads to urologic complications.

Screening for ABU usually is performed in the immediate postoperative period and for up to 6 months after transplantation. There is no consensus on the diagnosis and management of ABU 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 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. ^[23]

In a study assessing the treatment of ABU post renal transplantation, 80 patients were divided into a treatment group and a control group. ^[24] Surprisingly, the treatment group experienced higher rates of UTI and pyelonephritis compared to the control group. The survival analysis supported these findings, showing increased risks for both conditions in the treated group. The most common bacterium found was Escherichia coli, many of which were resistant to common antibiotics. These results suggest that treating ABU early after a transplant may not be beneficial and actually could increase the risk of complications.

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]

Clinical trials increasingly are showing that screening for and treating ABU does not provide benefits for most kidney transplant (KTx) recipients. ^[25] However, there is a lack of sufficient data to either support or advise against a "screen-and-treat strategy" for ABU during the initial 1–2 months following a transplant or when there is an indwelling urinary catheter present. Despite its common occurrence, ABU post-KTx remains a relatively under-researched issue.

The majority of studies on the management and screening of ABU are observational and often biased regarding interventions. ^[26] Current evidence generally discourages the screening and treatment of ABU, as antibiotic administration has not been shown to significantly reduce the rates of secondary symptomatic urinary tract infections (UTIs) or improve graft function. Additionally, there is no notable effect on mortality or other clinical outcomes. Furthermore, frequent antibiotic use may significantly increase the risk of recurrence and the emergence of antibiotic-resistant bacterial strains.

A systematic review and meta-analysis found no significant differences were observed between study groups concerning patient and graft outcomes, including graft function, graft loss, hospitalization due to UTI, all-cause mortality, or acute rejection. ^[27] The findings suggest that screening and treating kidney transplant patients for ABU does not reduce the incidence of future symptomatic UTIs, nor does it impact antimicrobial resistance or graft outcomes. Further randomized controlled trials (RCTs) are needed to determine if early treatment of ASB within the first two months post-transplantation is beneficial.

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 catheter-associated ABU 48 hours after catheter removal significantly (p = 0.00021) reduced the rates of catheter-associated urinary tract infections from 8.01 to 2.67 per 1,000 catheter days. ^[28] About 75% of patients with catheter-associated ABU post-catheter removal developed symptomatic UTIs upon follow-up. Conclusions: Detecting catheter-associated ABU 48 hours post-catheter removal enhances the surveillance process and allows for early identification of potential urinary tract infections.

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]

Asymptomatic bacteriuria 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.