A urine test is one of the most common methods used to detect a bacterial infection in the body. Urine contains waste products filtered from the blood by the kidneys. When bacteria cause an infection in the urinary tract or elsewhere in the body, they can often be detected in a urine sample.
What is a urinary tract infection?
A urinary tract infection (UTI) is an infection caused by bacteria getting into the urinary tract, which includes the urethra, bladder, ureters, and kidneys. UTIs are the most common type of bacterial infection and can affect any part of the urinary tract.
Some common symptoms of a UTI include:
- A burning feeling when urinating
- Frequent urination
- Urgent need to urinate
- Cloudy, dark, bloody, or strange smelling urine
- Pelvic pain or pressure
- Low fever
UTIs can be acute or chronic. Acute UTIs come on suddenly but can be cured with antibiotics. Chronic UTIs recur or last for more than 2-4 weeks despite treatment.
What causes a UTI?
UTIs are typically caused when bacteria enter the urinary tract through the urethra and begin to multiply. The most common cause of UTIs is the bacterium Escherichia coli (E. coli), which normally lives in the intestines. Improper bathroom hygiene, sexual intercourse, using a diaphragm, genetics, and medical conditions that block urine flow increase UTI risk.
How is a UTI diagnosed?
Doctors use a simple urine test called a urinalysis to check for signs of infection. This involves collecting a urine sample and sending it to a lab for evaluation under a microscope. The lab checks for:
- White blood cells – Higher than normal levels indicate inflammation/infection
- Red blood cells – May signal damage to the urinary tract
- Bacteria – Confirm presence of infection
- Leukocyte esterase – An enzyme released by white blood cells fighting bacteria
- Nitrites – Produced when certain bacteria break down urine
If the urinalysis indicates a UTI, the sample may be cultured to identify the specific bacteria causing infection. Doctors also consider symptoms when diagnosing.
Can other bacterial infections be detected?
Although UTIs are the most common type of bacterial infection identified from urine tests, other bacterial infections in the body can also produce positive results.
Kidney infections
Bacteria that cause a kidney infection (pyelonephritis) often appear in urine. Symptoms include fever, chills, nausea, vomiting, flank pain, and pus/blood in the urine.
Bladder infection
A bladder infection (cystitis) also manifests in urinalysis results. Symptoms include pelvic pressure/discomfort and increased urgency and frequency of urination.
Prostatitis
A bacterial infection of the prostate may show up in a urine culture. Some signs arepelvic pain, difficult and painful urination, and painful ejaculation.
STDs
Sexually transmitted diseases like chlamydia and gonorrhea can be detected through a urine test. These often have no symptoms but can cause issues like pelvic inflammatory disease if untreated.
What other tests help diagnose bacterial infections?
While a simple urine test can identify many bacterial infections, other tests help give a complete diagnosis:
- Blood tests – May show elevated white blood cell count from infection
- Imaging tests – CT scan or ultrasound check for any anatomical abnormalities or kidney stones
- Cystoscopy – Use of a camera-equipped instrument to view the urethra and bladder
- Urine culture – Growing bacteria from a urine sample to identify the specific strain
What bacteria commonly cause infections detected by urine tests?
The most common bacteria identified by urine testing include:
Escherichia coli (E. coli)
This bacterium normally lives in the intestines but is the cause of 80-85% of UTIs. Certain strains are more likely to cause infection than others.
Klebsiella pneumoniae
Associated with up to 10% of UTIs. More likely to affect hospital patients, diabetics, and those with weak immune systems.
Proteus mirabilis
Accounts for less than 5% of UTIs but is known for forming kidney stones. Often enters the urinary tract when using a urinary catheter.
Staphylococcus saprophyticus
Causes about 5-15% of UTIs, mainly in younger women. Less likely to be drug-resistant than other UTI bacteria.
Enterococcus faecalis
A normal gut bacterium that can sometimes infect the urinary tract, especially in hospital settings. Often resistant to antibiotics.
How are bacterial infections treated based on urine test results?
Once a bacterial infection is confirmed through a urine test, treatment focuses on eliminating the bacteria with antibiotics. Choice of antibiotic depends on:
- Type of bacteria identified through urine culture
- Severity and location of infection
- Patient drug allergies and medical history
- Local antibiotic resistance trends
For uncomplicated UTIs, the antibiotic treatment is usually a 3-day course for women and 7-10 days for men. More serious kidney infections require 2-3 weeks of treatment. It’s important to take the full course even if symptoms resolve to prevent recurrence.
Commonly prescribed UTI antibiotics
| Antibiotic | Notes |
|---|---|
| Trimethoprim/sulfamethoxazole | Often the first choice for uncomplicated UTIs |
| Ciprofloxacin | Used for more resistant infections |
| Nitrofurantoin | Cannot treat kidney infections |
| Fosfomycin | Single dose treatment option |
Can a urine test miss a bacterial infection?
Although urinalysis and urine culture are the standard methods for diagnosing UTIs and other bacterial infections, it’s possible for the results to be negative even when an infection exists. This occurs when:
- The urine sample was collected incorrectly and did not contain enough bacteria
- Antibiotics were already taken and reduced the bacterial levels
- The infection is caused by an uncommon organism the culture didn’t test for
- The bacteria are located in an area the urine sample could not access, like the prostate
- There are not enough bacteria early or late in the infection
For this reason, doctors also closely evaluate symptoms when interpreting urine test results. A negative urine culture with high suspicion of infection may warrant a repeat test or further evaluation.
Conclusion
In summary, a standard urine test called a urinalysis is an effective way to screen for bacterial infections. It can detect signs of inflammation and the presence of bacteria that may be causing infection in the urinary tract or other parts of the body. The most common type of infection identified is a UTI, often caused by E. coli bacteria. However, urine testing may occasionally miss an infection that is present but not shedding enough bacteria. By considering urine test results together with a patient’s symptoms, doctors can accurately diagnose and treat most bacterial infections.