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Asymptomatic bacteriuria in adults

Asymptomatic bacteriuria in adults
Authors:
Thomas Fekete, MD
Thomas M Hooton, MD
Section Editor:
Stephen B Calderwood, MD
Deputy Editor:
Allyson Bloom, MD
Literature review current through: Apr 2022. | This topic last updated: Jun 29, 2021.

INTRODUCTION — The term asymptomatic bacteriuria refers to isolation of bacteria in an appropriately collected urine specimen from an individual without symptoms of urinary tract infection (UTI). Asymptomatic bacteriuria is common, but most patients with asymptomatic bacteriuria have no adverse consequences and derive no benefit from antibiotic therapy. With few exceptions, nonpregnant patients should not be screened or treated for asymptomatic bacteriuria.

The rationale for not screening or treating asymptomatic bacteriuria in the general population is discussed in this topic.

Asymptomatic bacteriuria in the few patient populations that may warrant screening and treatment is discussed in detail elsewhere. These populations include:

Pregnant persons. (See "Urinary tract infections and asymptomatic bacteriuria in pregnancy".)

Patients undergoing urologic procedures in which mucosal bleeding is anticipated. (See "Prostate biopsy", section on 'Preparation' and "Placement and management of indwelling ureteral stents", section on 'Preparation' and "Surgical treatment of benign prostatic hyperplasia (BPH)", section on 'Antibiotic prophylaxis'.)

Patients who recently received a renal transplant. (See "Kidney transplantation in adults: Urinary tract infection in kidney transplant recipients", section on 'Screening'.)

CLINICAL DEFINITIONS — Asymptomatic bacteriuria is defined as isolation of a specified quantitative count of bacteria in an appropriately collected urine specimen from an individual without symptoms of urinary tract infection (UTI).

Bacteriuria threshold — Quantitative thresholds have been established to distinguish bladder bacteriuria from urethral contamination. The definitions of bacteriuria are slightly different for voided clean-catch specimens and catheterized specimens.

Voided clean-catch specimens — The threshold for asymptomatic bacteriuria from a clean-catch voided urine specimen is isolation of a single organism in quantitative counts ≥105 colony-forming units (CFU)/mL [1]. For females, a second specimen should be obtained (preferably within two weeks) to confirm growth of the same organism over the same quantitative threshold. For males, a single urine specimen meeting the criteria is sufficient for making the diagnosis.

Rationale for the count threshold – The high quantitative threshold is intended to increase the likelihood that bacteriuria reflects bladder bacteriuria rather than urethral, vaginal, or fecal contamination. Early studies had indicated that most asymptomatic patients with bacterial counts ≥105 CFU/mL from voided urine had corresponding bacteriuria on a specimen obtained through straight catheterization, but only a minority of those with lower counts did [2]. Similarly, patients with bacterial counts ≥105 CFU/mL from a voided specimen are more likely to have a repeat culture with the same organism growth compared with patients with lower counts.

Rationale for repeat specimen collection for females – For females, a confirmatory specimen is recommended to improve specificity for persistent bacteriuria, as bacteriuria can represent contamination and can also be transient. Two consecutive voided specimens are needed to predict bladder bacteriuria with the same degree of accuracy as a single urine specimen obtained through a catheter. In studies of voided and catheterized urine specimens from asymptomatic females, a bacterial count of ≥105 CFU/mL from a catheterized specimen was confirmed by a repeat catheterized specimen in >95 percent of cases, whereas a count ≥105 CFU/mL from a voided urine specimen was confirmed in a second voided specimen in only 80 percent of cases [2-6]. Two consecutive positive cultures from a voided specimen predicted a third positive culture with 95 percent confidence. Studies in healthy young females suggest that most episodes of bacteriuria in females are transient and not detected on repeat specimens [7,8]. As an example, in a study of 104 healthy premenopausal females with recurrent UTI who prospectively collected daily voided urine for culture and urinalysis, 45 experienced at least one episode of asymptomatic bacteriuria ≥105 CFU/mL with a uropathogen over a 90-day follow-up period; however, bacteriuria was seldom detected for more than two days in a row (eg, in only 9 of 43 episodes of E. coli bacteriuria), and only one participant had bacteriuria for more than five days in a row [8].

Rationale for a single specimen for males – There are fewer data on microbiologic criteria for diagnosis of asymptomatic bacteriuria in males, but the diagnostic threshold is less stringent in them in part because external contamination during voiding is an unlikely cause of significant bacteriuria. The most rigorous report informing the threshold is a study of 59 asymptomatic males with the incidental finding of ≥105 CFU/mL of an Enterobacteriaceae; this finding was reproducible with repeat culture one week later in 98 percent of cases [9]. (See "Acute simple cystitis in men".)

The definition of a positive urine culture in the setting of symptoms of cystitis or complicated UTI is distinct and discussed in detail elsewhere. (See "Sampling and evaluation of voided urine in the diagnosis of urinary tract infection in adults", section on 'Definition of a positive culture'.)

Catheterized specimens — The threshold for asymptomatic bacteriuria from a catheterized urine specimen is isolation of a single organism in quantitative counts ≥105 CFU/mL [10]. The sample does not have to be repeated for confirmation.

Specimens collected through straight catheterization (or a newly placed catheter) are less likely to have urethral contamination than voided specimens, but the significance of positive cultures in asymptomatic catheterized adults is unclear. Patients with long-term indwelling catheters often have low-level bacteriuria, often with multiple organisms, which often reflects colonization of the catheter. In such patients, the threshold for significant bacteriuria in asymptomatic patients is higher than in those with symptoms, to increase specificity and reduce overuse of antimicrobials [10]. There have been no comparisons of culture yields from urethral catheterized specimens and suprapubic aspiration specimens. (See "Catheter-associated urinary tract infection in adults", section on 'Definitions'.)

Definition of asymptomatic — Asymptomatic bacteriuria refers to patients who have no symptoms specifically referable to a UTI (eg, dysuria, urinary frequency or urgency, suprapubic pain in patients with simple cystitis and fevers with cystitis symptoms, flank pain, or costovertebral angle tenderness in patients with acute complicated UTI). The clinical presentation and diagnosis of UTI is discussed in detail elsewhere. (See "Acute simple cystitis in women", section on 'Diagnostic approach' and "Acute simple cystitis in men", section on 'Diagnostic approach' and "Acute complicated urinary tract infection (including pyelonephritis) in adults", section on 'Diagnostic approach'.)

This distinction is important, since many people, particularly older patients, are diagnosed with UTI when they present with nonspecific symptoms (such as delirium, behavioral changes, failure to thrive, falls, etc) and bacteriuria, when in fact there is some alternate cause of their nonspecific symptoms, and they actually have asymptomatic bacteriuria. In the absence of fever, local or systemic symptoms, or signs of infection, clinicians should have a high threshold before using such nonspecific symptoms to diagnose a UTI. The pitfalls of diagnosing UTI in older adults are discussed in detail elsewhere. (See "Approach to infection in the older adult", section on 'Urinary tract infection'.)

By contrast, in some patients with severe neurogenic bladder, such as those with spinal cord injury, UTI often presents without typical urinary symptoms. In such cases, fevers, malaise, increased spasticity, and autonomic dysreflexia may be the only manifestations of UTI [1,11]. When such patients present with such symptoms in the absence of alternate potential causes, it is reasonable to attribute them to UTI when bacteriuria is present. (See "Chronic complications of spinal cord injury and disease", section on 'Urinary tract infection'.)

Irrelevance of pyuria — Although most patients with symptomatic UTI have pyuria (≥10 leukocytes/microL of uncentrifuged urine), it is not a surrogate marker for bacteriuria or UTI and frequently occurs in their absence [7,12,13]. This was illustrated in a study of urine samples from asymptomatic older females; 60 percent of samples with pyuria had no bacteriuria [12]. Similarly, in another study of healthy premenopausal females who underwent daily collection of voided urine, the predictive value of pyuria for asymptomatic bacteriuria with E. coli was 4 to 8 percent, depending on the pyuria threshold [8].

Conversely, asymptomatic bacteriuria is frequently accompanied by pyuria, which does not indicate a symptomatic UTI that warrants therapy. As an example, the prevalence of pyuria in patients with diabetes mellitus and asymptomatic bacteriuria is almost 80 percent [14].

EPIDEMIOLOGY

Females — The prevalence of asymptomatic bacteriuria among healthy females increases with advancing age, from about 1 percent among schoolgirls to >20 percent among those over 80 years of age residing in the community [7,15,16]. It correlates with sexual activity; as an example, prevalence is greater among premenopausal married women than nuns of the same age (4.6 versus 0.7 percent, respectively) [17]. Pregnant and nonpregnant females have a similar prevalence (2 to 7 percent) [16]. In young healthy women, asymptomatic bacteriuria is transient; it rarely lasts longer than a few weeks.

Prevalence among females with diabetes mellitus is 8 to 14 percent and is usually correlated with duration and presence of long-term complications of diabetes, rather than with metabolic parameters of diabetes control [14].

Males — Asymptomatic bacteriuria is rare among healthy young males [18]. Among males older than 75 years residing in the community, prevalence is 6 to 15 percent [16]. Males with diabetes mellitus do not appear to have a higher prevalence of bacteriuria than those without [19].

PATHOPHYSIOLOGY — The absence of symptoms in patients with asymptomatic bacteriuria could reflect characteristics specific to the pathogen, the host, or both.

Pathogen factors — The microbiology of asymptomatic bacteriuria is similar to that of cystitis and pyelonephritis, although some strains capable of producing asymptomatic bacteriuria may have subtle adaptations that facilitate pathogenesis. For example, attachment of bacteria via fimbrial adhesins is thought to be important for the establishment and persistence of symptomatic infection. Some bacterial strains with reduced capability for fimbriae expression appear to have the capacity for relatively rapid growth that thus allows them to cause asymptomatic bacteriuria [20].

Alternatively, strains implicated in asymptomatic bacteriuria may be less virulent and, therefore, may not necessarily be true pathogens [21-25]. For example, E. coli strains recovered from spinal cord injury patients with asymptomatic bacteriuria demonstrate diminished capacity for red blood cell hemagglutination and hemolysis compared with strains implicated in symptomatic urinary tract infections [23,24]. Even if they persist, such strains are unlikely to progress to serious infection. Based on this notion, some investigators have suggested that colonization with "uroprotective" strains of E. coli may be protective against infection with more invasive uropathogens [26]. (See "Bacterial adherence and other virulence factors for urinary tract infection" and 'Adverse effects of antibiotics' below.)

Host factors — The absence of symptoms in patients with asymptomatic bacteriuria could also reflect differences in the host response [27,28]. A study of children with asymptomatic bacteriuria demonstrated lower levels of neutrophil toll-like receptor 4 (TLR4) expression compared with age-matched controls [27]. In mice, TLR4 controls the mucosal response to E. coli, and inactivation of TLR4 can lead to a carrier state that resembles asymptomatic bacteriuria [28]. (See "Toll-like receptors: Roles in disease and therapy".)

Bladder microbiome — While routine cultures of urine collected via suprapubic aspiration or straight catheterization are usually sterile in healthy, asymptomatic persons, more sensitive detection methods have identified a normal bladder-associated microbiome [29]. As with other mucosal surfaces, the flora of the bladder consists of various species of bacteria and varies by the individual. In females, there is some overlap between the vaginal and bladder flora. Study of the bladder microbiome is in its infancy, and its relevance to human health remains to be determined.

RATIONALE FOR NOT TREATING — In general, we suggest not screening for nor treating asymptomatic bacteriuria. There are a few exceptions in whom screening and treatment are warranted; these include pregnant persons, patients undergoing urologic intervention, and recent renal transplant recipients.

The presence of pyuria on a urinalysis does not alter our suggestion to avoid screening for or treating asymptomatic bacteriuria in most individuals; in someone who is asymptomatic, pyuria is not indicative of a urinary tract infection (UTI). (See 'Irrelevance of pyuria' above.)

Our recommendations are consistent with those from the Infectious Diseases Society of America (IDSA) and the US Preventive Services Task Force [1,30]. The IDSA guidelines also cover asymptomatic bacteriuria in children, which is discussed elsewhere. (See "Urinary tract infections in infants and children older than one month: Clinical features and diagnosis", section on 'Differential diagnosis'.)

Benign natural history — Although asymptomatic bacteriuria has been associated with subsequent UTI in some patient populations (as discussed below), asymptomatic bacteriuria is not associated with long-term adverse effects, such as chronic kidney disease or mortality [15,31,32]. As an example, in a study of over 600 females with diabetes mellitus, asymptomatic bacteriuria was not independently associated with declines in renal function or hypertension after a six-year follow-up [32].

The benign outcome of asymptomatic bacteriuria, at least in healthy young females, may be because of its transient nature [7,8]. (See 'Voided clean-catch specimens' above.)

Lack of treatment benefit

Nonpregnant adults — There is no role for routine screening for or treating asymptomatic bacteriuria in the general, nonpregnant population [1,30]. Although asymptomatic bacteriuria is associated with subsequent UTI in some studies [7], treatment does not appear to reduce the frequency of symptomatic infection or prevent other adverse outcomes [15,33-38]. Uncommon exceptions are discussed elsewhere. (See 'Select indications to screen/treat' below.)

Several trials have failed to find a clear benefit of treating asymptomatic bacteriuria [33,36,37]. In a meta-analysis of nine trials that included females and males from outpatient, geriatric, and nursing home settings, treatment of asymptomatic bacteriuria had no statistically significant effect on the incidence of symptomatic UTI (risk ratio [RR] 1.11, 95% CI 0.51-2.43), complications (RR 0.78, 95% CI 0.35-1.74), or death (RR 0.99, 95% CI 0.70-1.41) compared with no treatment or placebo [33]. Although antibiotics initially sterilize the urine in almost all patients, bacteriuria recurs in approximately one-half of treated patients and can spontaneously resolve in untreated patients, such that the prevalence is similar between treated and untreated individuals at one year [34].

Even many individuals with immunocompromising conditions do not appear to be at greater risk of adverse outcomes from untreated asymptomatic bacteriuria. In a study of 260 females with rheumatologic disease, of whom 94 percent were taking an immunosuppressive agent, asymptomatic bacteriuria was identified in 9 percent [39]. After a median of 12 months of follow-up, the rates of symptomatic UTI among those with and without asymptomatic bacteriuria at baseline were not statistically different (17 versus 12 percent), and no woman with asymptomatic bacteriuria developed sepsis or pyelonephritis requiring hospitalization. Data are more limited for other immunocompromised populations, but there is no clear evidence to support screening or treatment of asymptomatic bacteriuria in such patients.

Older patients — There is no role for screening for or treatment of asymptomatic bacteriuria among older adults, either in the community or in health care facilities [1].

Asymptomatic bacteriuria is common in such patients, with reported rates of 6 to 16 percent in females in the community, 25 to 54 percent of females in skilled nursing facilities, and rates at about half of those among males [40,41]. Nevertheless, asymptomatic bacteriuria is not associated with an increased risk of adverse outcomes in such patients [40,42-46]. Furthermore, antimicrobial treatment has not been shown to be of benefit in such patients [47,48]. (See 'Nonpregnant adults' above.)

As an example, in a randomized trial of 50 older females residing in a skilled nursing facility who had asymptomatic bacteriuria, treatment of all episodes of bacteriuria did not decrease urogenital morbidity or mortality over the course of a year compared with treatment for only symptomatic UTI [48]. In another randomized trial of 191 nursing home residents with incontinence and bacteriuria, eradicating bacteriuria had no short-term effects on the severity of chronic urinary incontinence [47]. In addition, bacteriuria tended to recur after therapy, with emergence of antibiotic resistance [48]. (See "Medical care in skilled nursing facilities (SNFs) in the United States", section on 'Asymptomatic bacteriuria'.)

Patients with diabetes mellitus — There is no role for screening for asymptomatic bacteriuria in patients with diabetes mellitus. However, screening for microalbuminuria is routine in such patients, and the presence of pyuria on urinalysis may result in a reflexed urine culture that leads to the incidental discovery of asymptomatic bacteriuria. If it is identified, we suggest not treating asymptomatic bacteriuria in such patients. Although diabetes mellitus is associated with asymptomatic bacteriuria in females, which in turn is associated with subsequent UTI, including severe UTI, available evidence suggests that treating asymptomatic bacteriuria in patients with diabetes mellitus does not improve outcomes.

Overall, diabetes mellitus is associated with an approximate three- to fourfold increase in the risk of asymptomatic bacteriuria in females, from 6 percent to 18 to 26 percent [14,49,50]. In one study, increased risk occurred mainly in females using insulin and those with a duration of disease longer than 10 years [51]. In particular, patients with advanced or severe disease as determined by end-organ damage or elevated levels of glycosylated hemoglobin reportedly have a higher incidence of asymptomatic bacteriuria [14].

As in the general population, asymptomatic bacteriuria is associated with subsequent UTI in patients with diabetes mellitus [52-54]. In a cohort of females with type 2 diabetes mellitus, those with asymptomatic bacteriuria at baseline had a higher risk of UTI over the subsequent 18 months (34 versus 19 percent) [53,55]. Overall, however, most symptomatic UTIs in this cohort occurred among females who had negative baseline urine cultures. Similarly, in a prospective study of 496 adults with diabetes mellitus, asymptomatic bacteriuria was associated with an approximate fourfold increased risk of hospitalization for acute complicated UTI associated with sepsis [54].

Nevertheless, multiple studies have found that antibiotic therapy of asymptomatic bacteriuria is associated with no reduction in symptomatic infection and a high rate of recurrent bacteriuria once antibiotics are discontinued [56,57]. The best data come from a trial in which 105 females with diabetes mellitus and asymptomatic bacteriuria were randomly assigned to 14 days of antibiotics or placebo [56]. At four weeks after the end of therapy, a greater proportion of patients in the antibiotic group cleared the bacteriuria (80 versus 22 percent with placebo). However, after this initial follow-up period, the group assignment was revealed, and patients were followed for a mean of 27 months; bacteriuria was assessed at three-month intervals, and patients who originally received antibiotics were treated during subsequent episodes. Over this long-term follow-up, patients in the antibiotic group had nearly five times the number of antibiotic days compared with the placebo group, but there were no differences between the groups in UTI incidence (including pyelonephritis), hospitalization for UTI, or timing of UTI onset.

Other studies have suggested that the rate of recurrence of asymptomatic bacteriuria in patients with diabetes mellitus is approximately 50 to 80 percent despite effective therapy (over follow-up ranging from one to several years) [57,58].

Patients undergoing nonurologic surgery — There is no role for screening for or treatment of asymptomatic bacteriuria among patients undergoing nonurologic surgery, including joint arthroplasty [1]. (See 'Joint arthroplasty' below.)

In a retrospective study of 489 males who had urine cultures performed prior to undergoing orthopedic, cardiothoracic, or vascular procedures, bacteriuria was uncommon (11 percent of patients) [59]. Preoperative bacteriuria was not associated with an increased risk of surgical site infection. Furthermore, the incidence of subsequent UTI was not decreased with antibiotic therapy for bacteriuria (3 of 43 untreated versus 2 of 11 treated patients).

Joint arthroplasty — We suggest not routinely performing urinalysis or culture in patients without urinary symptoms prior to or following joint arthroplasty. If a patient is found to have perioperative bacteriuria in the confirmed absence of urinary symptoms, we recommend not treating with antibiotics. Although there are no data from large trials to inform this issue [60], there is no evidence of a clear association between joint infections and bacteriuria, and the typical pathogen profiles of these infections are very different.

Most observational studies do not demonstrate a clear association between perioperative bacteriuria and subsequent development of prosthetic joint infection. In a prospective, multicenter study of nearly 2500 patients undergoing total hip or knee arthroplasty, screening identified asymptomatic bacteriuria (≥105 colony-forming units (CFU)/mL in the absence of signs or symptoms of UTI) in 12 percent [61-64]. In the year following surgery, there were 43 prosthetic joint infections (1.7 percent). Although patients with preoperative asymptomatic bacteriuria were more likely to have a prosthetic joint infection than those without (4.3 versus 1.4 percent infection rate, odds ratio 3.23, 95% CI 1.67-6.27), treatment of the bacteriuria, which was at the discretion of the clinician, was not associated with a decreased risk of infection. Moreover, the organisms isolated from the urine were not the same as those from the surgical site infection in any patient with asymptomatic bacteriuria. These results complement those of other large observational studies that did not demonstrate an association between bacteriuria or symptomatic UTI and prosthetic joint infection and also noted that organisms isolated from the urine are not the same as those causing surgical site infections [61,64]. Other small studies evaluating patients with asymptomatic preoperative bacteriuria who proceeded to surgery (with preoperative or postoperative antibiotic therapy) failed to find any instances of subsequent joint infections attributable to the urinary pathogens [65,66].

Some studies have suggested an association between postoperative UTI and prosthetic joint infections, but these are mainly limited to small case reports [67-69]. In one large retrospective study, post-arthroplasty UTI was independently associated with a subsequent surgical site infection [70]. However, the criteria for UTI diagnosis used in the study were not specified, and there was no evaluation of a microbiologic link between the uropathogens and those causing the joint infections.

Some experts have advocated screening and treatment for asymptomatic bacteriuria in patients undergoing joint arthroplasty because of the biological plausibility of subsequent surgical site infection and the relative ease of treatment [71]. We believe that further data are warranted before such a recommendation can be made. The very low infection risk of total joint arthroplasty, the disparity of flora between bacteriuria and surgical site infection, and the delays as well as the small but real risks related to screening and treating bacteriuria do not support universal screening. (See 'Adverse effects of antibiotics' below.)

In contrast, evaluation and treatment are indicated in pre- and postoperative patients who have symptomatic UTI. (See "Acute simple cystitis in men", section on 'Treatment' and "Acute complicated urinary tract infection (including pyelonephritis) in adults", section on 'Management' and "Acute simple cystitis in women".)

Patients with indwelling bladder catheters — There is no role for screening for or treatment of asymptomatic bacteriuria among patients with indwelling bladder catheters [1]. Bacteriuria is extremely common among patients with indwelling catheters, and treatment does not improve patient outcomes.

All patients with an indwelling catheter in place long enough ultimately develop bacteriuria, which has been estimated to occur at a rate of approximately 3 to 10 percent per day of catheterization [72,73]. This bacteriuria is mostly asymptomatic. As an example, in a study of nearly 1500 newly catheterized patients, 235 developed bacteriuria over the course of one to two weeks, but more than 90 percent of cases were asymptomatic [74]. Among patients with a chronic indwelling catheter, bacteriuria is associated with a higher risk of complicated UTI than in the absence of a catheter.

Nevertheless, several trials have failed to demonstrate any benefit to treatment of asymptomatic bacteriuria in catheterized patients [75,76]. As an example, in a trial of patients in the intensive care unit (ICU) with an indwelling catheter in place for >48 hours and asymptomatic bacteriuria, a short course of antibiotics and replacement of the catheter did not reduce the rate of UTI with sepsis, duration of ICU stay, or incidence of subsequent positive urine cultures [76]. Similarly, in a trial of patients with long-term indwelling catheters, a course of cephalexin given whenever bacteriuria was identified did not reduce the rate of febrile episodes, incidence of bacteriuria, or catheter malfunctioning [75]. Studies of programs aimed at reducing treatment of asymptomatic bacteriuria in chronically catheterized patients have not identified any increase in UTI-related adverse effects [77].

Patients with spinal cord injury — Determining whether bacteriuria reflects asymptomatic bacteriuria or UTI in patients with spinal cord injury or severe neurogenic bladder can be difficult because they may have atypical manifestations of UTI (see 'Definition of asymptomatic' above). In the absence of potential symptoms of UTI, there is no role for screening or treating bacteriuria in such patients [1].

Although patients with spinal cord injury have a high prevalence of asymptomatic bacteriuria, bacteriuria tends to recur early after therapy or prophylaxis [78,79]. Many patients with spinal cord injury have indwelling catheters, among whom treatment for asymptomatic bacteriuria does not confer a clinical benefit; data in such patients without indwelling catheters are more limited. (See 'Patients with indwelling bladder catheters' above.)

Nevertheless, avoiding unnecessary antibiotic therapy is a particular priority in these patients, as they have a high rate of symptomatic UTI, and excessive antibiotic exposure increases the risk of drug-resistant uropathogens, which complicate UTI treatment. (See 'Adverse effects of antibiotics' below.)

Adverse effects of antibiotics — Beyond the potential direct adverse effects of antibiotics (eg, toxicity or intolerance, risk of Clostridioides difficile colitis), overuse of antibiotics is well known to drive antibiotic resistance at both the individual and the community or institutional level (see "Antimicrobial stewardship in hospital settings", section on 'Adverse effects of antimicrobial use'). Eliminating treatment of asymptomatic bacteriuria has been identified as an important target of efforts to reduce unnecessary antibiotic administration [1,80]. There is also some evidence that treating asymptomatic bacteriuria could increase the risk of subsequent UTI.

Evidence from a number of studies supports the potential risks of antibiotic use in asymptomatic bacteriuria. In a meta-analysis of nine trials evaluating treatment for asymptomatic bacteriuria, antibiotics increased the incidence of any adverse event (RR 3.77, 95% CI 1.40 to 10.15) [33]. Other studies have found that treatment of asymptomatic bacteriuria is associated with emergence of antibiotic resistance in subsequently isolated uropathogens [48,81]. As an example, in one study, treatment of asymptomatic bacteriuria was associated with higher rates of resistance to amoxicillin-clavulanate (25 versus 4 percent), trimethoprim-sulfamethoxazole (34 versus 12 percent), and ciprofloxacin (44 versus 19 percent) among subsequent E. coli urinary isolates [81].

Furthermore, in a trial of healthy, premenopausal, nonpregnant females with asymptomatic bacteriuria, a greater proportion of the participants randomly assigned to receive antibiotic treatment for the bacteriuria experienced a subsequent symptomatic UTI over the following year compared with the females who did not receive antibiotics [50]. Confidence in the results from this trial is limited by the lack of blinding and a placebo control, and the unexpected pathogen profile (with E. coli accounting for only a third of cases). Nevertheless, if these findings are confirmed by additional studies, they support the concept that asymptomatic bacterial colonization can protect against superinfection with more virulent strains and should not be treated [82]. (See 'Pathophysiology' above.)

SELECT INDICATIONS TO SCREEN/TREAT

Pregnancy — Asymptomatic bacteriuria during pregnancy has been associated with adverse pregnancy outcomes. Screening for asymptomatic bacteriuria is warranted for pregnant persons [1,83]. This is discussed in detail separately. (See "Urinary tract infections and asymptomatic bacteriuria in pregnancy", section on 'Asymptomatic bacteriuria'.)

Patients undergoing urologic intervention — Screening for and treatment of asymptomatic bacteriuria are warranted for patients undergoing urologic procedures in which mucosal bleeding is anticipated [1,83]. Untreated bacteriuria is associated with infectious complications following urologic interventions, with a higher risk associated with procedures that disturb mucosal integrity (such as transurethral prostate interventions, percutaneous stone surgery). In trials of patients with asymptomatic bacteriuria undergoing transurethral resection of the prostate, antibiotic treatment reduced the risk of postoperative complicated urinary tract infection and bacteremia [84,85]. Antibiotic prophylaxis prior to urologic procedures is discussed elsewhere. (See "Prostate biopsy", section on 'Preparation' and "Placement and management of indwelling ureteral stents", section on 'Preparation' and "Surgical treatment of benign prostatic hyperplasia (BPH)", section on 'Antibiotic prophylaxis'.)

Renal transplant recipients — Some experts screen for and treat asymptomatic bacteriuria in the first few months following transplantation. This is discussed in detail elsewhere. (See "Kidney transplantation in adults: Urinary tract infection in kidney transplant recipients", section on 'Screening' and "Kidney transplantation in adults: Urinary tract infection in kidney transplant recipients", section on 'Asymptomatic bacteriuria'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Urinary tract infections in adults" and "Society guideline links: Asymptomatic bacteriuria in adults".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

Basics topics (see "Patient education: Asymptomatic bacteriuria (The Basics)")

SUMMARY AND RECOMMENDATIONS

Asymptomatic bacteriuria is defined as isolation of a specified quantitative count of bacteria in an appropriately collected urine specimen from an individual without symptoms or signs of urinary tract infection (UTI). (See 'Clinical definitions' above.)

The threshold for asymptomatic bacteriuria from a clean-catch voided or catheterized urine specimen is isolation of a single organism in quantitative counts ≥105 colony-forming units (CFU)/mL. For females providing a voided specimen, a second specimen should be obtained (preferably within two weeks) to confirm growth of the same organism over the same quantitative threshold. A single specimen is sufficient for a voided urine from males or for urine obtained through catheterization. (See 'Bacteriuria threshold' above.)

Asymptomatic bacteriuria refers to patients who have no symptoms specifically referable to a UTI (eg, dysuria, urinary frequency or urgency, suprapubic pain in patients with simple cystitis and fevers with cystitis symptoms, flank pain, or costovertebral angle tenderness in patients with acute complicated UTI). In the absence of fever or systemic signs of infection, clinicians should have a high threshold before using nonspecific symptoms (such as delirium, behavioral changes, failure to thrive) to diagnose a UTI. (See 'Definition of asymptomatic' above.)

Pyuria is not a surrogate marker for bacteriuria. Additionally, in the setting of bacteriuria, pyuria is not an indication of a symptomatic UTI that warrants therapy. (See 'Irrelevance of pyuria' above.)

Asymptomatic bacteriuria is common, particularly in females. Although noncatheterized patients with asymptomatic bacteriuria have a higher risk for subsequent symptomatic UTI, bacteriuria itself is not associated with long-term adverse effects. Furthermore, for most patients, there is no evidence that antibiotic treatment of asymptomatic bacteriuria reduces the frequency of symptomatic infection or infection-related adverse effects. (See 'Rationale for not treating' above.)

In addition to the direct adverse effects of antibiotics, antibiotic treatment of asymptomatic bacteriuria is associated with emergent drug resistance in uropathogens, making treatment of subsequent symptomatic UTIs more complicated. There is also some evidence that treating asymptomatic bacteriuria could increase the risk of subsequent UTI. (See 'Adverse effects of antibiotics' above.)

There is no role for routine screening the general, nonpregnant population for asymptomatic bacteriuria. For most nonpregnant patients who are found to have asymptomatic bacteriuria, we recommend not treating with antibiotics (Grade 1B). This also applies to older patients, patients with diabetes mellitus, patients with an indwelling bladder catheter, and patients undergoing nonurologic surgery. (See 'Lack of treatment benefit' above.)

There are a few exceptions to this approach. Pregnancy, urologic procedures that are expected to result in mucosal bleeding, and recent renal transplantation are indications to screen for and treat asymptomatic bacteriuria. These indications are discussed in detail elsewhere. (See "Urinary tract infections and asymptomatic bacteriuria in pregnancy", section on 'Asymptomatic bacteriuria' and "Kidney transplantation in adults: Urinary tract infection in kidney transplant recipients", section on 'Screening' and "Surgical treatment of benign prostatic hyperplasia (BPH)", section on 'Antibiotic prophylaxis'.)

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