Urine Culture

Urine is one of the body’s waste products. It is produced in the kidneys and collected in the bladder until a person urinates. Normally, the urine does not contain significant numbers of any microorganism. However, if bacteria or yeast are introduced into the urinary tract, they can multiply and cause a urinary tract infection, called a UTI.

Most UTIs are caused by Escherichia coli (E. coli), one of the most common human bacteria. Other frequently identified bacteria are Proteus, Klebsiella, and Staphylococcus saprophyticus.

If bacteria are detected in the urine, they will then be tested for their antibiotic susceptibility so that an effective treatment can be administered.

How is the sample collected for testing?

A “mid-stream” urine sample is necessary for a culture so bacteria present around the urethra and on the hands are not introduced. The procedure for collecting a clean “mid-stream” includes the following steps:  

• Hands should be washed just before beginning the collection.
• A soap should be used to clean the penis in males, and females should wash the external genitalia, holding the labia apart.
• Repeat this procedure three times.
• Do not collect the initial stream of urine since it may be contaminated with skin and urethral bacteria.
• Midway through the urination process, collect a sample of urine in a sterile screw-top container (hence the name “mid-stream” urine).
• Tightly cap the container and wash your hands thoroughly.
• The sample should be taken to the laboratory as quickly as possible to prevent the further growth of organisms.

Uncontaminated specimens can also be obtained from catheterised patients following the same hygienic procedures for the end of the catheter. Urine can also be collected directly from the bladder using a needle and syringe in a process known as a suprapubic aspiration, but this is usually reserved for infants and young children where it has been difficult to obtain a reliable mid-stream urine sample.

A sample of the urine is then streaked across the surface of one or more agar plates and placed in an incubator at body temperature for 24 hours. If there is no growth on the agar plates at the end of that time, the culture is considered negative for significant number of microorganisms that could cause an infection. If bacteria or yeast are present, the total number of organisms is counted (colony count), and the organisms are identified by additional biochemical testing. Further tests determine which antibiotics are likely to be effective in treating the infection.

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.