Asymptomatic bacteriuria in adults

INTRODUCTION — The term asymptomatic bacteriuria refers to isolation of bacteria (≥100,000 [10⁵] colony-forming units [CFU]/mL in a voided clean-catch specimen) from an individual without symptoms of urinary tract infection (UTI). Asymptomatic bacteriuria is common, especially in older patients and those admitted to the hospital or in long-term care. However, 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 routinely screening or treating asymptomatic bacteriuria in most individuals 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 "Urinary tract infection in kidney transplant recipients", section on 'Monitoring for asymptomatic bacteriuria'.)

DEFINITION

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

It is important to distinguish between nonspecific symptoms and symptoms specifically referable to UTI. Clinicians often attribute nonspecific symptoms (eg, delirium, behavioral changes, failure to thrive, falls) to UTI, particularly in older patients, who have a high prevalence of asymptomatic bacteriuria. In the absence of specific UTI symptoms, however, they should have a high threshold to diagnose UTI. Misdiagnosing asymptomatic bacteriuria as a UTI can result in antibiotic overuse as well as failure to diagnose and treat the true (potentially dangerous) cause of the symptoms. 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 may present without typical urinary symptoms. In such cases, fever, malaise, increased spasticity, and autonomic dysreflexia may be the only manifestations of UTI [1,2]. When such patients present with these 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'.)

Threshold for bacteriuria versus contamination — Growth of bacteria on urine culture can represent either true bladder bacteriuria or contamination of the specimen. Quantitative thresholds have been proposed to distinguish between the two. However, in a patient without symptoms specifically referrable to UTI, the distinction is not essential as asymptomatic bacteriuria should not be treated in most patients (and contamination of the specimen should never be treated). (See 'Rationale for not screening/treating' below.)

●Voided clean-catch specimens – In an asymptomatic patient, the threshold for bacteriuria from a clean-catch voided urine specimen is isolation of a single organism in quantitative counts ≥100,000 (10⁵) colony-forming units (CFU)/mL [1]. Lower levels of growth are more likely to represent urethral, vaginal, or fecal contamination.

Early studies had indicated that most asymptomatic patients with bacterial counts ≥100,000 (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 [3].

Guidelines from the Infectious Diseases Society of America (IDSA) indicate that in females, two consecutive urine cultures should be performed to confirm the persistence of bacteriuria before making the diagnosis of asymptomatic bacteriuria; two voided specimens with ≥100,000 (10⁵) CFU/mL are needed to predict bladder bacteriuria with the same degree of accuracy as a single straight catheterization specimen [3-7]. However, in clinical practice, it is rarely necessary to establish the diagnosis since, as above, specific monitoring or management is rarely warranted. Thus, we recommend not routinely repeating urine cultures to confirm asymptomatic bacteriuria.

●Catheterized specimens – The threshold for asymptomatic bacteriuria from a catheterized urine specimen is isolation of a single organism in quantitative counts ≥100,000 (10⁵) CFU/mL [8].

When specimens are obtained from an existing indwelling catheter, lower levels of bacterial growth are more likely to reflect bacteria colonizing the catheter (and contaminating the culture). In contrast, specimens collected through straight catheterization or a newly placed indwelling catheter are less likely to have contamination from the catheter or urethra, and in such cases, lower levels of bacterial growth (eg, ≥100 [10²] to <100,000 [10⁵] CFU/mL) probably represent true bladder bacteriuria. However, as above, making this distinction in an asymptomatic individual is typically not necessary, and maintaining a high threshold for bacteriuria among patients who undergo catheterization is appropriate to reduce potential triggers of unnecessary antibiotic therapy.

The threshold for 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'.)

Irrelevance of pyuria — Pyuria (≥10 leukocytes/microL of uncentrifuged urine) does not indicate that bacteriuria represents a UTI and should not prompt antibiotic use in patients without specific symptoms. Asymptomatic bacteriuria is frequently accompanied by pyuria. As an example, the prevalence of pyuria in patients with diabetes mellitus and asymptomatic bacteriuria is almost 80 percent [9].

Pyuria is also not a surrogate marker for bacteriuria and frequently occurs in its absence [10-12]. This was illustrated in a study of urine samples from asymptomatic older females; 60 percent of samples with pyuria had no bacteriuria [11]. Similarly, in another study of healthy premenopausal females who underwent daily collection of voided urine, the predictive value of pyuria for asymptomatic bacteriuria with Escherichia coli was 4 to 8 percent, depending on the pyuria threshold [13].

EPIDEMIOLOGY

Females — The prevalence of asymptomatic bacteriuria among healthy females increases with age, from about 1 percent among school-aged individuals to >20 percent among those over 80 years of age residing in the community [10,14,15].

Among young adults and adolescents, asymptomatic bacteriuria correlates with sexual activity; as an example, prevalence is greater among premenopausal married females than nuns of the same age (4.6 versus 0.7 percent, respectively) [16]. Pregnant and nonpregnant females have a similar prevalence (2 to 7 percent) [15]. In young healthy females, asymptomatic bacteriuria is transient; it rarely lasts longer than a few weeks. (See 'Benign natural history' below.)

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 [9]. (See 'Patients with diabetes mellitus' below.)

Prevalence of asymptomatic bacteriuria increases with increasing morbidity burden and decreasing self-rated health status [17]. It is particularly high (in some studies, as high as 50 percent) among patients residing in skilled nursing facilities or admitted to the hospital [18-20].

Males — Asymptomatic bacteriuria is rare among healthy young males [21]. Among males older than 75 years residing in the community, prevalence is 6 to 15 percent [15]. Males with diabetes mellitus do not appear to have a higher prevalence of bacteriuria than those without [22]. Among older hospitalized male adults and those in long-term care settings, prevalence may approach 40 percent [23].

Patients with indwelling bladder catheters — All patients with an indwelling catheter in place long enough ultimately develop bacteriuria, which has been estimated to increase at a rate of approximately 3 to 10 percent per day of catheterization [24,25]. 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 [26].

RATIONALE FOR NOT SCREENING/TREATING — We suggest not screening for nor treating asymptomatic bacteriuria. There are a few exceptions for whom screening and treatment may be warranted; these include pregnant persons, patients undergoing urologic intervention, and recent renal transplant recipients. (See 'Potential indications to screen/treat' below.)

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 [1,27]. 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, when adjusted for baseline morbidity, is not associated with a clinically significant risk of major complications, such as bacteremia, or long-term adverse effects, such as chronic kidney disease or mortality [14,17,28-30]. 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 [29].

The benign outcome of asymptomatic bacteriuria, at least in healthy young females, may be because it is often transient. Many episodes of bacteriuria in females are not detected on repeat specimens [10,13]. 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 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 [13]. Even individuals who appear to have repeated growth of the same organism on urine culture may actually have transient colonization with different isolates. Routine clinical microbiological tests do not distinguish between the innumerable strains of uropathogen species, such as E. coli, which can have strikingly diverse genetic and phenotypic characteristics. Several studies using a variety of techniques have found that, in 20 to 40 percent of patients with asymptomatic bacteriuria and repeated growth of E. coli on two consecutive urine specimens, the second isolate was a different strain [31-33].

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 [1,34]. 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 (risk ratio [RR] 3.77, 95% CI 1.40-10.15) [35]. Other studies have found that treatment of asymptomatic bacteriuria is associated with emergence of antibiotic resistance in subsequently isolated uropathogens [36,37]. 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 [37].

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 [38]. 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 [39]. (See 'Pathogenesis' below.)

Lack of treatment benefit

Nonpregnant adults — There is no role for routine screening for or treating asymptomatic bacteriuria in the general, nonpregnant population [1,27]. Although asymptomatic bacteriuria is associated with subsequent UTI in multiple studies [10,40-42], treatment does not appear to reduce the frequency of symptomatic infection or prevent other adverse outcomes [14,35,43-47]. Uncommon exceptions are discussed elsewhere. (See 'Potential indications to screen/treat' below.)

Several trials have failed to find a clear benefit of treating asymptomatic bacteriuria [35,45,46]. 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 (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 [35]. 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 [43].

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 [48]. 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 that in males [49,50]. Nevertheless, asymptomatic bacteriuria is not associated with an increased risk of adverse outcomes in such patients when adjusted for baseline morbidity [17,49,51-54]. Furthermore, antimicrobial treatment has not been shown to be of benefit in such patients [36,55]. (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 [36]. 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 [55]. In addition, bacteriuria tends to recur after therapy in older adults, with emergence of antibiotic resistance [36,56]. (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. 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 [9,38,57]. In one study, increased risk occurred mainly in females using insulin and those with a duration of disease longer than 10 years [58]. In particular, patients with advanced or severe disease as determined by end-organ damage may have a higher incidence of asymptomatic bacteriuria [9]. The association of higher hemoglobin A1c levels with higher levels of bacteriuria has been variable across studies [9,58-61].

Asymptomatic bacteriuria is associated with subsequent UTI in patients with diabetes mellitus [62-64]. 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) [59,63]. 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 [64].

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 [65,66]. 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 [65]. 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) [66,67].

Patients undergoing nonurologic surgery

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. This recommendation is consistent with those of the 2018 International Consensus on Orthopedic Infections [68] and the IDSA [1]. Routine urine cultures prior to orthopedic surgery had previously been common practice.

Large observational studies do not demonstrate a clear association between perioperative bacteriuria and subsequent prosthetic joint infection [69-73]. Additionally, discordance between the pathogens isolated from urine cultures and from subsequent prosthetic joint infections argues that preoperative bacteriuria does not directly cause postoperative joint infection [74]. As examples:

●In a retrospective, multicenter study that included over 20,000 joint replacement procedures, the incidence of prosthetic joint infections was low overall and not statistically different among those with a positive preoperative urine culture compared with those with a negative culture (0.51 versus 0.71 percent) [73]. No patients with a culture-positive joint infection had the same organism grow on preoperative urine culture.

●In a prospective, multicenter cohort study that included nearly 2500 patients undergoing total hip or knee arthroplasty, screening identified asymptomatic bacteriuria in 12 percent [69-72]. Although patients with preoperative asymptomatic bacteriuria were more likely to have a prosthetic joint infection in the subsequent year than those without (4.3 versus 1.4 percent infection rate), treatment of the bacteriuria, which was at the discretion of the clinician, was not associated with a decreased risk of infection, and the organisms isolated from the urine were not the same as those from the surgical site infection. The observed association between asymptomatic bacteriuria and joint infection was likely related to confounding factors.

Additionally, preoperative urine cultures have not been associated with reduced rates of arthroplasty-associated infections, and discontinuing the practice does not lead to an increase in them [72,75]. After a tertiary referral center implemented an intervention to refuse preoperative urine cultures from the orthopedics clinic, the rate of urine cultures plummeted (from 87 to 1 per 100 elective arthroplasty procedures during the baseline and intervention periods, respectively) and there were only three prosthetic joint infections out of nearly 2000 arthroplasties performed during the intervention period [72]. All three infections were caused by Staphylococcus aureus (which is not a typical urinary pathogen).

Other nonurologic procedures — There is no role for screening for or treatment of asymptomatic bacteriuria among patients undergoing other nonurologic surgery [1]. Preoperative asymptomatic bacteriuria has not been independently associated with an increased risk of surgical site infection (SSI), and treatment of bacteriuria has not been associated with a decreased SSI risk [76,77].

As an example, in a retrospective study that included nearly 18,000 patients who had urine cultures performed prior to undergoing cardiovascular, orthopedic, or vascular surgery at a Veterans Affairs center in the United States, 617 (3.5 percent) had asymptomatic bacteriuria [77]. After adjustment for age, sex, and comorbidities, the rate of SSI was similar among those with and without asymptomatic bacteriuria (2.4 versus 1.6 percent). Asymptomatic bacteriuria was also not associated with an increased risk of postoperative UTI. Furthermore, receipt of an antibiotic with activity against the isolated bacteria was not associated with a reduced risk of SSI or postoperative UTI.

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 (see 'Patients with indwelling bladder catheters' above); nevertheless, several trials have failed to demonstrate any benefit to treatment of asymptomatic bacteriuria in catheterized patients [78,79]. 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 [79]. 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 [78]. Studies of programs aimed at reducing treatment of asymptomatic bacteriuria in chronically catheterized patients have not identified any increase in UTI-related adverse effects [80].

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 bacteriuria' 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 [81,82]. Many patients with spinal cord injury have indwelling catheters. Among them, 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' above.)

POTENTIAL 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 (UTI) 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 — Issues related to screening for and treating asymptomatic bacteriuria following renal transplant are discussed in detail elsewhere. (See "Urinary tract infection in kidney transplant recipients", section on 'Monitoring for asymptomatic bacteriuria' and "Urinary tract infection in kidney transplant recipients", section on 'Early ureteral stent removal'.)

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

Pathogen factors — Although we do not understand at the individual patient level why a particular strain may cause asymptomatic bacteriuria instead of UTI, some (but not all [86,87]) strains that cause asymptomatic bacteriuria may have subtle adaptations that promote a tradeoff between less efficient pathogenesis and relatively rapid growth. For example, some strains isolated from asymptomatic patients display loss of fimbrial adhesins or O-antigen specific side chains. Both have been implicated in the pathogenesis of symptomatic infection, and it is possible that reduced expression in asymptomatic bacteriuria strains allows for relatively rapid growth, resulting in efficient bladder colonization [88-90]. Other strains demonstrate diminished capacity for red blood cell hemagglutination and hemolysis compared with strains implicated in symptomatic UTIs [91,92]. Altogether, some strains implicated in asymptomatic bacteriuria may be less virulent, having either evolved from uropathogenic E. coli through virulence attenuation or originated from commensal, non-uropathogenic intestinal E. coli [91-98].

Because of the ability of asymptomatic bacteriuria strains to outcompete cystitis and pyelonephritis strains in some studies and preliminary data that inappropriate treatment of asymptomatic bacteriuria may predispose to cystitis [38], some investigators have suggested that colonization with certain asymptomatic bacteriuria strains of E. coli may actually be protective against infection with more invasive uropathogens [99]. (See "Bacterial adherence and other virulence factors for urinary tract infection" and 'Adverse effects of antibiotics' above.)

Host factors — The absence of symptoms in patients with asymptomatic bacteriuria could also reflect differences in the host response [100,101]. A study of children with asymptomatic bacteriuria demonstrated lower levels of neutrophil toll-like receptor 4 (TLR4) expression compared with age-matched controls [100]. In mice, TLR4 controls the mucosal response to E. coli, and inactivation of TLR4 can lead to a carrier state that resembles asymptomatic bacteriuria [101]. Hyporesponsive TLR2 signaling has also been linked to asymptomatic bacteriuria [102]. (See "Toll-like receptors: Roles in disease and therapy".)

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 5^th to 6^th 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 10^th to 12^th 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

●Definition – 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 'Definition' above.)

•Lack of symptoms – “Asymptomatic” indicates no symptoms specifically referable to a UTI (eg, dysuria, urinary frequency or urgency, suprapubic pain, flank pain, or costovertebral angle tenderness). 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. Misdiagnosing UTI in this situation can result in antibiotic overuse and failure to diagnose and treat the true cause of the patient’s symptoms. (See 'Definition of asymptomatic bacteriuria' above.)

•Bacteriuria count over a threshold – The threshold distinguishing asymptomatic bacteriuria from contamination in a clean-catch voided or catheterized urine specimen is isolation of a single organism in quantitative counts ≥100,000 (10⁵) colony-forming units (CFU)/mL. (See 'Threshold for bacteriuria versus contamination' above.)

•Irrelevance of pyuria – Pyuria is not a surrogate marker for bacteriuria. In the setting of asymptomatic bacteriuria, pyuria is not an indication of a UTI that warrants therapy. (See 'Irrelevance of pyuria' above.)

●No role for screening/treating in general population – 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.)

•Benign natural history – Asymptomatic bacteriuria is common, particularly in females, and is not associated with long-term adverse effects. 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 'Benign natural history' above.)

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

●Potential indications for screening – Pregnancy, urologic procedures that are expected to result in mucosal bleeding, and recent renal transplantation are situations in which it may be reasonable to screen for and treat asymptomatic bacteriuria. These potential indications are discussed in detail elsewhere. (See "Urinary tract infections and asymptomatic bacteriuria in pregnancy", section on 'Asymptomatic bacteriuria' and "Surgical treatment of benign prostatic hyperplasia (BPH)", section on 'Antibiotic prophylaxis' and "Urinary tract infection in kidney transplant recipients", section on 'Monitoring for asymptomatic bacteriuria'.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Thomas M Hooton, MD, who contributed to earlier versions of this topic review.