Common Laboratory Tests
Most UTIs are detected by performing a urinalysis and then confirmed with a urine culture. If there are pathogenic bacteria present, then susceptibility testing is done to make sure that the antibiotic that the doctor chooses will effectively treat the microorganism causing the infection.
- Urinalysis. A clean catch (midstream) sample collection is important to minimize sample contamination. White blood cells (WBC, leukocytes), red blood cells (RBC), nitrites, and bacteria in the urinalysis may indicate a UTI.
- Urine Culture. Urine is streaked on a thin layer of nutrient gel (agar plate), then incubated for 24-48 hours. Any bacteria that grow on the agar are counted and identified. Usually, if a person has a UTI, there will be a high colony count of one type of bacterium that will be present. If there are three or more types of bacteria present (or, in a woman, if there are vaginal bacteria such as lactobacillus or diphtheroids), then it is considered to be a contaminated sample and no further work is done on the culture. A repeat sample may need to be submitted if the person's symptoms persist. If there is a predominant growth of one type of bacterium present in high number, then susceptibility testing is done (sometimes the doctor may refer to it as "sensitivity" testing). The ability of different antibiotics to inhibit the growth of the bacteria in a test tube or on an agar plate helps predict which antibiotics are likely to be effective treatments.
Follow-up Laboratory Tests
If a doctor suspects that a UTI may have spread into the bloodstream, the doctor may order a blood culture. If a doctor suspects that a person's symptoms may be due to a sexually transmitted disease, such as chlamydia or gonorrhea, then the doctor may test for one or more STDs.
- Blood Culture. Two samples of blood are taken to look for any bacteria in the blood, which is normally sterile. The blood samples are incubated in a broth medium at body temperature and examined at regular intervals for bacterial growth. When a person is septic from a urinary tract infection, the same microorganism is found in both the blood and the urine cultures. Susceptibility testing will provide information on which antibiotic is likely to eradicate the pathogen from both the blood and the urine.
Imaging scans and special X-rays may be used to look for anatomical problems and/or signs of an underlying disease or condition. If a person has recurrent or chronic UTIs, one or more of the following procedures may be ordered. Each gives the doctor different information.
- Kidney and bladder ultrasound – uses sound waves to determine the structures of the bladder and the kidney; they can be seen as light and shadowed areas.
- Voiding cystourethrogram (VCUG) – an x-ray test that examines the urethra and bladder while the bladder fills and empties
- Nuclear scans – several types may be used to examine the function and shape of the bladder and kidneys. For each scan type, a radioactive dye is injected into a vein. The dye is carried to the bladder and kidney, allowing the visualization of any structural abnormalities.
- Cystoscopy – a flexible tube about the diameter of a straw is threaded up the urethra and into the bladder. It allows a doctor to look at the surface of the inside of urethra and bladder. It can help identify blockages and abnormalities. If a stone is present, other instruments can be inserted up through the cystoscope that may allow a stone to be removed or broken into smaller pieces with a laser. The crushing of these stones is called lithotripsy. Urine and tissue sample can also be taken using the cystoscope.
- Intravenous pyelogram (IVP) – used to look at the whole urinary tract. An opaque dye is injected into a vein, then travels to the kidney and bladder. A series of x-rays are taken, which may reveal obstructions or structural abnormalities.