Asymptomatic bacteriuria (ASB) is a laboratory diagnosis, not a disease. It is defined as the presence of a significant quantity of bacteria in a properly collected urine sample from a person who has no signs or symptoms of a urinary tract infection. “Asymptomatic” means without symptoms, and “bacteriuria” simply means bacteria in the urine.

To understand why this is not considered an infection in most people, it is crucial to differentiate between colonization and infection.

• An infection, like a typical UTI (cystitis), occurs when harmful, virulent bacteria invade the bladder lining, triggering a strong inflammatory response from the body’s immune system. This battle between the bacteria and the immune system is what causes the classic UTI symptoms of pain, burning, and urgency.
• Colonization, which is what happens in ASB, is a much more peaceful state. Bacteria are present in the bladder, but they are not invading the bladder wall or causing inflammation. The person’s immune system recognizes the bacteria but tolerates their presence, and the bacteria coexist without causing any trouble.

A helpful analogy is to think of the urinary tract as a quiet residential street.

• A UTI is like having rowdy vandals on the street who are actively breaking windows and causing a commotion. This requires an emergency response.
• Asymptomatic bacteriuria is like having a group of quiet tourists who are simply present on the street, coexisting peacefully with the residents. They are not causing any damage. Trying to forcibly remove these harmless tourists with powerful antibiotics can often cause more disruption than leaving them alone.

I’ve often found that explaining ASB as “harmless bacterial colonization” helps patients understand why we don’t always treat it, even if lab results show bacteria.

The cause is bacteria colonizing the bladder. The pathway is the same as for a UTI: bacteria from the perineal area (the skin around the genitals and anus) ascend the urethra and enter the bladder.

The most common bacterium found in ASB is Escherichia coli (E. coli), the same organism responsible for the majority of symptomatic UTIs. Other bacteria, like Klebsiella and Enterococcus, can also be involved.

The key difference lies not always in the type of bacteria, but in the interaction between the bacteria and the host’s immune system. In many cases of ASB, the strains of E. coli are less virulent, they lack the specific “weapons” (like pili that help them stick to the bladder wall) that more aggressive strains use to cause an infection. Furthermore, the host’s immune system, for various reasons, does not mount a strong inflammatory response to the presence of these bacteria, leading to a state of peaceful colonization rather than a full-blown infection.

I’ve seen ASB in patients with no symptoms at all, just routine screening before surgery or during prenatal care. It’s a reminder that lab results don’t always tell the whole story.

You develop ASB when bacteria successfully establish a colony in your bladder without triggering an infection. The risk factors for developing ASB are conditions that make it easier for bacteria to enter and thrive in the bladder.

The prevalence of ASB is very low in young, healthy individuals but increases significantly in certain populations. Key risk factors include:

• Being Female: Due to the shorter urethra, it is much easier for bacteria to ascend into the bladder.
• Older Age: ASB is extremely common in the elderly, affecting up to 20% of women over 65 and up to 50% of men and women in long-term care facilities.
• Diabetes Mellitus: People with diabetes are more likely to have ASB, possibly due to changes in their immune system and urine composition.
• Indwelling Urinary Catheters: A catheter provides a direct highway for bacteria to enter the bladder.
• Spinal Cord Injuries or other neurological conditions that affect the ability to empty the bladder completely.

In elderly patients with chronic catheters, ASB is almost expected. What matters more is recognizing when it turns into something that needs treatment, like a real UTI.

By definition, people with asymptomatic bacteriuria do not experience any of the typical UTI symptoms.

This means a person with ASB does not have:

• Pain or burning during urination (dysuria).
• An urgent or frequent need to urinate.
• Pelvic pain.

This is the most critical point for patients and doctors. In some populations, particularly the elderly, other signs are often mistakenly attributed to a UTI when they are actually caused by something else. For example, cloudy or foul-smelling urine in an older adult, without other symptoms, is not an indication of a UTI and is often just a sign of ASB or dehydration. Similarly, a change in mental status, like confusion, in an elderly person has many potential causes. ASB should not be presumed to be the cause without ruling out other possibilities.

Many patients get concerned when they hear there are “bacteria in their urine.” I often explain that without symptoms, it’s like background noise, not a warning siren.

ASB is diagnosed through a urine culture from a properly collected midstream clean-catch urine sample. If it grows a significant number of a single type of bacteria (typically more than 100,000 colony-forming units/mL) in a person with no symptoms, the diagnosis of ASB is confirmed.

The “Don’t Look For It” Principle The most important modern concept in managing ASB is that in most cases, we should not even be looking for it. Major medical bodies, like the Infectious Diseases Society of America (IDSA), have published strong guidelines recommending against routine screening for and treatment of ASB in most populations (IDSA, 2019).

Screening for ASB is NOT recommended for the following groups:

• Most healthy, non-pregnant women.
• Older adults living in the community or long-term care facilities, even those with catheters.
• People with well-controlled diabetes.
• People with spinal cord injuries.

The reason is simple: finding the bacteria in these individuals often leads to unnecessary and harmful courses of antibiotics.

I always clarify for patients that a positive urine culture doesn’t equal infection. We look at the whole picture, including symptoms, risk factors, and why the test was ordered.

For the vast majority of people, the correct treatment for asymptomatic bacteriuria is no treatment. This can be a difficult concept to accept, but it is based on strong scientific evidence that highlights the dangers of over-treating a harmless condition.

Why Not to Treat ASB

• Preventing Antibiotic Resistance: This is the number one reason. Unnecessary antibiotic use is a major driver of creating “superbugs” that are resistant to treatment.
• Avoiding Side Effects: Antibiotics are powerful drugs that can have significant side effects, including allergic reactions, yeast infections, and C. difficile colitis.
• Disrupting Protective Colonization: Sometimes, the harmless bacteria causing ASB can actually prevent more virulent, dangerous bacteria from taking hold. Treating the harmless bacteria can open the door for a real, symptomatic UTI.

Unless there’s a pregnancy or surgery coming up, I usually avoid treating ASB. Treating bacteria that aren’t causing harm only risks causing bigger problems later.

The Critical Exceptions: Who MUST Be Treated

While the “do not treat” rule applies to most, there are two specific situations where screening and treating ASB is absolutely essential to prevent serious complications.

The two major groups who must always be treated for asymptomatic bacteriuria are:

1. Pregnant Women: Treatment is mandatory. Untreated ASB in pregnancy significantly increases the risk of the infection ascending to the kidneys (pyelonephritis), which in turn increases the risk of preterm labor and low birth weight for the baby.
2. Patients Undergoing Urologic Surgery: Any person who is about to undergo a urologic procedure that is expected to cause bleeding of the urinary tract lining (like a TURP for the prostate) must be screened and treated beforehand to prevent the bacteria from entering the bloodstream during the procedure and causing sepsis.

Asymptomatic bacteriuria is a common clinical finding that represents a peaceful colonization of the bladder, not an active infection. It is crucial for both patients and healthcare providers to understand that, for most people, this condition is benign and should not be treated. The impulse to prescribe antibiotics for any positive urine culture in a person without UTI symptoms must be resisted. Unnecessary treatment does not provide any benefit and contributes directly to the dangerous global crisis of antibiotic resistance. The exceptions to this rule, pregnant women and patients about to undergo certain urologic surgeries, are clear and critically important. In infectious conditions like ASB, the absence of symptoms is often more important than the lab result. I always remind my patients, sometimes, doing nothing is actually the best form of care.

Infectious Diseases Society of America (IDSA). (2019). Clinical Practice Guideline for the Management of Asymptomatic Bacteriuria. Retrieved from https://www.idsociety.org/practice-guideline/asymptomatic-bacteriuria/

Nicolle, L. E. (2014). Asymptomatic bacteriuria. Current Opinion in Infectious Diseases, 27(1), 90–96. https://doi.org/10.1097/QCO.0000000000000028

American College of Obstetricians and Gynecologists (ACOG). (2020). Urinary Tract Infections (UTIs). Retrieved from https://www.acog.org/womens-health/faqs/urinary-tract-infections

Centers for Disease Control and Prevention (CDC). (2022). Urinary Tract Infection. Retrieved from https://www.cdc.gov/antibiotic-use/uti.html