Asymptomatic Bacteriuria — Definition, Treatment and Prognosis

Asymptomatic bacteriuria (ASB) is significant bacteriuria in the absence of signs or symptoms referable to the urinary tract.

Microbiological criteria: in women, ≥10⁵ CFU/mL of the same organism in two consecutive clean-catch voided specimens; in men, ≥10⁵ CFU/mL in one clean-catch specimen; in either sex, ≥10² CFU/mL from a single catheterised specimen.

Pyuria is not part of the definition and does not convert ASB into infection — most bacteriuric older adults have pyuria.

ASB must be distinguished from UTI, which requires localising genitourinary symptoms or systemic features of infection in addition to bacteriuria.

Bacteriuria in older persons is common and usually asymptomatic, and its prevalence increases with advancing age.

Approximate prevalence: community-dwelling women over 70 — 10–20%; older men — up to 10%; long-term care facility women — 25–50%; men — 15–40%; patients with a long-term indwelling catheter — essentially 100%.

Predisposing factors are the same as for UTI: vaginal and urethral atrophy, urinary puddling from bed rest, bladder catheterisation, obstructive uropathy and diabetes.

Prevalence of ASB roughly parallels the prevalence of urinary incontinence, so incontinent geriatric patients commonly have significant bacteriuria.

Common organisms are E. coli, Klebsiella, Proteus, Enterococcus and, in catheterised patients, polymicrobial and frequently changing flora.

In the absence of obstructive uropathy, there is no evidence to support routine antimicrobial therapy for ASB in older persons.

Do not screen for or treat ASB in: healthy non-pregnant women, older men and women in the community, residents of long-term care facilities, patients with diabetes, patients with spinal cord injury, and patients with indwelling catheters.

Do not treat ASB in an older patient with delirium or a fall but no fever, no haemodynamic instability and no localising urinary symptoms — assess for other causes and observe carefully.

Treat ASB only in: pregnancy, and before urological procedures likely to cause mucosal bleeding (e.g. TURP, ureteroscopy with stone manipulation), where treatment is given shortly before the procedure.

Renal transplant recipients should not be routinely treated for ASB beyond the first month after transplantation.

One exception in the incontinence pathway: in an incontinent patient with new or worsening incontinence, bacteriuria may be treated once as part of the initial assessment; however, eradicating bacteriuria does not affect chronic stable incontinence in nursing home residents.

In patients with an indwelling catheter, do not perform surveillance cultures to guide management, because all chronically catheterised patients are bacteriuric, often polymicrobially, and the organisms change frequently.

The prognosis of ASB is benign. Studies suggesting increased mortality did not adjust for age and comorbidity; when adjusted, the excess mortality is explained by the fatal underlying diseases associated with bacteriuria, not by the bacteriuria itself.

ASB does not cause progressive renal damage, hypertension or chronic kidney disease in the non-obstructed urinary tract.

Treatment does not improve survival, does not reduce symptomatic infection and does not improve continence; earlier studies in hospitalised and institutionalised patients showed treatment was ineffective because of the very high rate of recurrence.

Harms of treating ASB are real: antimicrobial adverse effects, drug interactions, Clostridioides difficile infection, selection of multidrug-resistant organisms, and cost. There is also evidence that stable bacteriuria may be protective against symptomatic reinfection, so eradication can be counterproductive.