Related tests: Urinalysis, Blood cultures, Susceptibility testing
The Test Sample
What is being tested?
The urine culture test detects and identifies bacteria and yeast in the urine. Urine is produced by the kidneys, located on either side of the spine at the base of the ribcage. The kidneys filter waste out of the blood and produce urine, a yellow fluid, to carry the wastes out of the body. Urine travels through tubes called ureters from the kidneys to the bladder, where it is stored temporarily, and then through the urethra as it is voided. Urine is generally sterile, but sometimes bacteria (or, more rarely, yeast) can move from the skin outside the urethra and migrate back up the urinary tract to cause a urinary tract infection (UTI).
With a urine culture, a small sample of urine is placed on one or more agar plates (a thin layer of a nutrient gel) and incubated at body temperature. Any microorganisms that are present in the urine sample grow over the next 24 to 48 hours as small circular colonies. The size, shape, and color of these colonies give clues as to which bacteria are present, and the number of colonies indicates the quantity of bacteria originally present in the urine sample. A laboratorian observes the colonies on the agar plate, counting the total number and determining how many types have grown. Ideally, if a good clean sample was collected for the test (see below), then the only bacteria present should be due to an infection. Usually, this will be a single type of bacteria that will be present in relatively large numbers. Sometimes, more than one type of bacteria will be present. This may be due to an infection that involves more than one pathogen (disease-causing microorganism); however, it is more likely to be due to contamination from the skin picked up during the urine collection.
The laboratorian will take a colony from each type of bacteria present that appears significant in number or type, smear it on a slide, dry it, and stain it with dyes (called a gram stain). The laboratorian then examines the microorganisms under the microscope. Different types of bacteria will have different colors and shapes that can differentiate different types of bacteria present. For instance, the bacterium Eschericia coli, which causes the majority of urinary tract infections, will appear as gram-negative rods (pink rods) under the microscope and Lactobacillus, which is a common vaginal contaminant in women’s urine samples, will appear as gram-positive bacilli (thin purple rods). Some of these bacteria, such as Lactobacillus, are easy for an experienced laboratorian to identify and are nonpathogenic (do not cause an infection) and do not require any further investigation. Others, such as gram-negative rods, represent groups of similar bacteria and will require additional testing to determine exactly which type of bacteria is present.
Based upon the information obtained, the laboratorian gives the doctor an initial report of the quantities and types of bacteria (or yeast) present in the urine. Examples would be: “no growth in 24 hours” (nothing grew on the agar), “less than 10,000 organisms/ml” (a small amount of bacteria was present) or “greater than 100,000 organisms per ml of gram negative rods, ID and susceptibility to follow” (the patient probably has an infection caused by a gram negative bacteria that needs to be further identified).
If there is no or little growth on the agar after 24 to 48 hours of incubation, the urine culture is considered negative for pathogens and the culture is complete. If there is one or more pathogen present, further testing is performed. Biochemical tests are used to identify which bacteria are present and susceptibility testing evaluates the ability of the bacteria to grow in the presence of a specific selection of antimicrobial drugs to help determine which drug treatments are likely to be effective in resolving the infection.
How is the sample collected for testing?
Urine for a culture can be collected at any time. Because of the potential to contaminate urine with bacteria and cells from the surrounding skin during collection (particularly in women), it is important to first clean the genitalia. Women should spread the labia of the vagina and clean from front to back; men should wipe the tip of the penis. As you start to urinate, let some urine fall into the toilet, then collect one to two ounces of urine in the sterile container provided, then void the rest into the toilet. This type of collection is called a mid-stream clean catch urine.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
When is it ordered?
A urine culture may be ordered when symptoms indicate the possibility of a urinary tract infection, such as pain and burning when urinating and frequent urge to urinate. Antibiotic therapy may be prescribed without requiring a urine culture for symptomatic young women, who have an uncomplicated lower urinary tract infection. If there is suspicion of a complicated infection, or symptoms do not respond to initial therapy, then a culture of the urine is recommended. Pregnant women without any symptoms may be screened for bacteria in their urine, which could affect the health and development of the fetus.
What does the test result mean?
The presence of a single type of bacteria growing at high colony counts (greater than 10,000 colony forming units (CFU)/ml) is considered a positive urine culture. A culture that is reported as no growth in 24 or 48 hours or less than 10,000 CFU/ml usually indicates that there is no infection. If the symptoms persist, however, the culture may be repeated to look for the presence of bacteria at lower colony counts (less than 10,000 CFU/ml) or other microorganisms that may cause these symptoms. The presence of white blood cells and low numbers of microorganisms in a symptomatic patient is a condition known as acute urethral syndrome.
If a culture shows growth of several different types of bacteria, then it is likely that the growth is due to contamination. This is especially true if the organisms present include Lactobacillus and common nonpathogenic vaginal bacteria in women. If the symptoms persist, the doctor may request a repeat culture on a sample that is more carefully collected.
The presence of a significant amount of a single type of bacteria usually indicates an infection. Susceptibility testing is performed to guide antimicrobial treatment. Any bacterial infection may be serious and can spread to other areas of the body if not treated. Since pain is often the first indicator of an infection, prompt treatment, usually with antibiotics, will help to alleviate the pain.
Is there anything else I should know?
Females get UTIs more often than males. Even school-age females may have frequent UTIs. For males with a culture-proven UTI, the doctor may order further tests to rule out the presence of a kidney stone or structural abnormality that could cause the infection.
If you have recurrent urinary tract infections, culture and susceptibility testing may be performed with each episode. For patients who have frequent UTIs, their bacteria may become resistant to antibiotics over time, making careful selection of antibiotic (and the full course of treatment) essential. Those with kidney disease and/or with diseases that affect the kidneys, such as diabetes and those with compromised immune systems, may be more prone to recurring UTIs.
Digestive Diseases Library