Urine culture

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Also known as: Urine M/C/S
Formal name: Urine culture

At a Glance

Why Get Tested?

When to Get Tested?

If you experience symptoms of a UTI, such as pain during urination

Sample Required?

A mid-stream "clean" urine sample

The Test Sample

What is being tested?

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. Because urine itself can serve as a culture medium, any bacteria present, including contaminating microorganisms, will multiply rapidly if the urine sample is allowed to stand at room temperature, particularly in the warmer months of the year. This could potentially lead to false-positive or false-negative results. For this reason, urine samples must be processed in the laboratory within 30 minutes of collection. If this is not possible, the specimen must be refrigerated (at about 4°C) immediately: it can then be kept for up to 4-6 hours without any significant changes.

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.

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 from front to back, holding the labia apart.
  • Do not collect the initial stream of urine since it may be contaminated with skin and urethral bacteria.
  • Midway through the urination process, collect 20-30 millilitres of urine in a sterile screw-top container (hence the name ‘mid-stream’ urine).
  • Tightly cap the container, wash your hands thoroughly and label the container.
  • 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.

How is the sample tested in the laboratory?

A sample of the urine is initially assessed under microscope in a counting chamber or a cell conting instrument and visible cells are counted. The presence of large numbers of white blood cells (‘pus cells’) is strongly indicative of a UTI. Appreciable numbers of squamous epithelial cells (originating from skin) indicate a poorly collected specimen. A small amount of urine (usually 10 microlitres) is then cultured and inoculated plates are 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 growing, the total number of organisms is counted (colony count), and the organisms are identified by additional biochemical testing. The number of bacteria present in the sample helps to distinguish between true presence of bacteria in the urine and contamination. The concentration of viable bacteria in a urine sample is usually expressed in colony forming units per litre (CFU/L). If greater than 108 CFU/L of a recognised uropathogen (for example Escherichia coli or Proteus vulgaris) are present in a specimen, this is considered to be reliable evidence of the presence of bacteria in the urinary tract. However, this rule has been validated for uncomplicated urinary tract infections in adult women and clinical reasoning should be employed when interpreting urine culture results from men and symptomatic children.

Further tests determine which antibiotics are likely to be effective in treating the infection.

The Test

Common Questions

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.