Asymptomatic Bacteriuria (Bacteria in Urine, No Symptoms)

What is asymptomatic bacteriuria?

Asymptomatic bacteriuria (ABU) is the abnormally high number of bacteria in the urine without any symptoms associated with an urinary tract infection. Normally urine is sterile, meaning that there are no microbes in it, until it reaches the distal urethra. Here bacteria attached to the urethral lining pass out with the urine. Overall this small number of bacteria is not significant and does not usually indicate a potential health risk. In asymptomatic bacteriuria, the number of bacteria in the urine exceeds 100,000 organisms (cfu ~ colony-forming units) per milliliter (mL).

Bacteria are the most common infectious agents of a urinary tract infection. The term urinary tract infection denotes a symptomatic state usually characterized by burning upon urination, frequent urination, strong smelling urine, urethral discharge and discoloration of the urine. Other symptoms such as a fever, lower abdominal and pelvic pain, and/or flank pain are also present to varying degrees but are non-specific for a UTI. With asymptomatic bacteriuria, these symptoms are absent despite the large number of bacteria in the urine. However, about 30% of such cases may become a symptomatic infection (urinary tract infection) usually within one year.

Most cases of asymptomatic bacteriuria are detected upon routine urinary examination or for other related disorders. The dilemma for most doctors is whether to treat asymptomatic bacteriuria or not. The use of antibiotics can complicate the condition and lead to UTIs with bacterial species that are more difficult to treat. In certain high risk groups, however, treatment is warranted as there is a major risk of the bacteria traveling up the urinary tract and causing a kidney infection even though there were no symptoms lower down the urinary tract.

Causes of Asymptomatic Bacteriuria

Asymptomatic bacteria is caused by largely the same bacteria that causes urinary tract infections. In most instances, Escherichia coli (E.coli) is isolated, most of which are from the rectum.  A number of other microorganisms may also be isolated including Enterobacteriaceae, Pseudomonas aeruginosa, Enterococcus species, and group B Streptococcus.

The reasons for the presence of large number of bacteria  in the urine does not differ significantly from that or a urinary tract infection, although a tainted sample needs to also be excluded as a possible cause. There appears to be certain groups at risk which may not be indicative of the etiology. Instead it is in these groups that screening is more likely to occur since there is a greater risk of ABU developing into a symptomatic infection at a later stage. These groups at risk include :

  • Pregnant women.
  • Person over 65 years of age.
  • Patients with anatomical abnormalities of the urinary tract.
  • Patients with spinal cord injuries.
  • Immobilized older patients particularly those in nursing homes or hospital.
  • Diabetics.
  • Renal (kidney) transplantation patients.
  • Catheterized patients.
  • Patients with infected kidney stones.
  • HIV/AIDS patients.

Asymptomatic bacteriuria is uncommon in infants and children but is more likely in the event of urinary problems such as vesicoureteral reflux.

Diagnosis of Asymptomatic Bacteriuria

The diagnosis of asymptomatic bacteriuria depends on the manner in which the sample is collected. For men and women who are capable of normal voiding, the midstream clean-catch urine sample is preferred. For women, the testing of two consecutive samples should reveal the presence of 100,000 or more CFU per mL. For men, a single specimen with more than 100,000 cfu/mL is sufficient for a diagnosis. This applies to only a single species of bacteria.

In catheterized patients, the options are obviously limited and only the catheterized urine sample is available. Since urine is being drained directly from the bladder, it should be sterile although all catheterized patients are bacteriuric. Therefore the presence of at least 100 cfu/mL or more of a single bacterial species is needed for a diagnosis in men or women.

Treatment of Asymptomatic Bacteriuria

Not all patients with asymptomatic bacteriuria should be treated in contrast to symptomatic infections (UTIs) where treatment is always indicated. Of the various ABU risk groups who are more likely to develop symptomatic infection, only certain individuals should be treated. This includes pregnant women, children with vesicoureteral reflux, renal transplant patients, those with infected kidney stones and patients with upcoming urologic procedures.

Treatment should not be initiated in catheterized patients as this elimination of the bacteria in ABU increases the risk of other more pathogenic bacteria entering the urinary tract. Catheterized patients who are treated for ABU are also more likely to develop yeast infections. However, should a symptomatic infection arise, treatment should be commenced immediately.

The antibiotics that may be used includes :

  • trimethoprim/sulfamethoxazole
  • nitrofurantoin
  • ciprofloxacin
  • amoxicillin-clavulanate.


  1. Asymptomatic Bacteriuria in Adults. American Family Physician
  2. Asymptomatic Bacteriuria. Emedicine Medscape

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