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Urine Culture

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Also known as: Urine Culture and Sensitivity; Urine C and S
Formal name: Culture, Urine

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, two fist-sized organs 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 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 media) 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 help to identify 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. Typically, 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; 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 to be significant in number or type, smear it on a slide, dry it, and stain it with dyes. This test is called a gram stain. The laboratorian examines the microorganisms under the microscope. Different types of bacteria will exhibit characteristic colors and shapes. For instance, the bacterium Escherichia coli, which causes the majority of urinary tract infections, will appear as pink (gram-negative) rods under the microscope. Lactobacillus, which is a common vaginal contaminant in women's urine samples, will appear as thin purple (gram-positive) rods. Some of the bacteria, such as Lactobacillus, are easy for an experienced laboratorian to identify, are nonpathogenic, 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.

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 are one or more pathogens present, further testing is performed. Biochemical tests are used to identify which bacteria are present and susceptibility testing is done to identify antimicrobial agents that are likely to inhibit the growth of the bacteria. The results of these laboratory tests allow the physician to select the best antibiotic treatment to resolve 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. 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.

For catheterized specimens, a urine sample is taken by inserting a thin flexible tube or catheter through the urethra into the bladder. This is performed by a trained health care provider. The urine is collected in a sterile container at the other end of the tube.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

Generally none; however, you may be instructed not to urinate for at least an hour before the test and/or to drink a glass of water 15-20 minutes before sample collection. This will help to ensure that you can produce enough urine for the sample. Follow the instructions provided for collecting a clean catch urine sample.