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Urine Culture

Test Overview

A urine culture is a test to find and identify germs (usually bacteria) that may be causing a urinary tract infection (UTI). Urine in the bladder normally is sterile—it does not contain any bacteria or other organisms (such as fungi). But bacteria can enter the urethra and cause an infection.

A urine sample is kept under conditions that allow bacteria and other organisms to grow. If few organisms grow, the test is negative. If organisms grow in numbers large enough to indicate an infection, the culture is positive. The type of organisms causing the infection are identified with a microscope or by chemical tests.

Urinary tract infections are more common in women and girls than in men. This may be partly because the female urethra is shorter and closer to the anus, which allows bacteria from the intestines to come into contact more easily with the urethra. (See a picture of the female urinary tract.) Men also have an antibacterial substance in their prostate gland that reduces their risk.

If the urine culture is positive, other tests may be done to help choose which antibiotic will do the best job treating the infection. This is called sensitivity testing.

Why It Is Done

A urine culture may be done to:

  • Find the cause of a urinary tract infection (UTI).
  • Make decisions about the best treatment for a UTI. This is called sensitivity testing.
  • Find out whether treatment for a UTI worked.

How To Prepare

You do not need to do anything before you have this test. If you are taking or have recently taken antibiotics, tell your doctor.

You will need to collect a urine sample. Avoid urinating just before having this test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

You will be asked to collect a clean-catch midstream urine sample for testing. The first urine of the day is preferred because bacterial levels will be higher.

Clean-catch midstream urine collection

This method helps protect the urine sample from germs that are normally found on the penis or vagina.

  • Wash your hands before collecting the urine.
  • If the collection container has a lid, remove it carefully and set it down with the inner surface up.
  • Clean the area around your penis or vagina.
    • A man should retract the foreskin, if present, and clean the head of his penis thoroughly with medicated towelettes or swabs.
    • A woman should spread open the folds of skin around her vagina with one hand, then use her other hand to clean the area around her vagina and urethra thoroughly with medicated towelettes or swabs. She should wipe the area from front to back to avoid spreading bacteria to the vagina that is normally found around the anus.
  • Begin urinating into the toilet or urinal. A woman should continue holding apart the folds of skin around the vagina while she is urinating.
  • After the urine has flowed for several seconds, place the collection container in the stream and collect about 60 mL (2 fl oz) of this "midstream" urine without stopping the flow.
  • Do not touch the rim of the container to your genital area.
  • Do not get toilet paper, hair, feces, or menstrual blood in the urine sample.
  • Finish urinating into the toilet or urinal.
  • Carefully replace the lid on the container. Wash your hands. Return the urine sample to the lab. If you are collecting the urine at home and cannot get it to the lab within an hour, refrigerate the sample. It can be refrigerated for up to 24 hours. Follow the instructions from your lab.

How It Feels

Collecting a urine sample does not cause pain.

Risks

There is no chance for problems while collecting a urine sample.

Results

A urine culture is a test to detect and identify organisms (usually bacteria) that may be causing a urinary tract infection (UTI). Urine culture results are usually ready in 1 to 3 days. Some organisms take longer to grow in the culture; for this reason, results may not be available for several days.

Urine culture
Normal:

No bacteria or other organisms (such as fungi) grow in the culture. The culture result is negative.
Abnormal:

Organisms (usually bacteria) grow in the culture. The culture result is positive.

A count of 100,000 or more bacteria per milliliter (mL) of urine may be caused by an infection. A count ranging from 100 to 100,000 could be either caused by infection or by contamination of the sample (you may need a repeat urine culture). If the count is 100 or less, infection is unlikely; however, a count of 100 or less may also be seen if you are already taking antibiotics.

If test results are positive, sensitivity testing may be done to help make decisions about treatment.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Taking antibiotics or just finished taking them.
  • Taking water pills (diuretics) or drinking a large amount of liquid. This may dilute your urine and reduce the number of bacteria in the sample.
  • Taking a lot of vitamin C.

What To Think About

  • A urine culture done in the early stage of a urinary tract infection (UTI) may be less accurate than one that is done after the infection becomes established.
  • A urine culture may be done when an abnormal result from a urinalysis (such as an increased number of white blood cells) shows signs of an infection. For more information, see the topic Urine Test.
  • A urine culture is not always done for a healthy woman with symptoms of a urinary tract infection and a positive urine test result.
  • A urine culture may be repeated after the UTI has been treated to make sure the infection is cured.
  • A health professional may collect a urine sample by placing a urinary catheter into the bladder. This method is sometimes used to collect urine from a person in the hospital who is very ill or unable to provide a clean-catch sample. Using a catheter to collect a urine sample reduces the chance of getting bacteria from the skin or genital area in the urine sample, but catheter use sometimes causes a UTI.
  • People who have a urinary catheter in place for a long time are at high risk of developing a UTI.
  • Collecting a urine sample from a small child or baby may be done by using a special plastic bag with tape around its opening (a U bag). The bag is attached around the child's genitals until he or she urinates (usually within an hour). Then the bag is carefully removed. To collect a urine sample from a very sick baby, a doctor may insert a needle through the baby's abdomen directly into the bladder (suprapubic tap).
  • To diagnose tuberculosis that has spread to the urinary tract, a special test will be done using all of the first morning urine on three separate days.
  • Sensitivity testing helps your doctor choose the best medicine to treat specific types of bacteria or fungus that may be causing a UTI.
  • Some types of bacteria or fungi may take several weeks to grow in the culture.

References

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

Credits

By Healthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer W. David Colby IV, MSc, MD, FRCPC - Infectious Disease
Last Revised May 28, 2010

Last Revised: May 28, 2010

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & W. David Colby IV, MSc, MD, FRCPC - Infectious Disease

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