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Many minor and superficial skin and wound infections are diagnosed by the doctor based on a clinical evaluation and their experience. In addition to general symptoms, many skin infections have characteristic signs, such as the appearance of a plantar wart, and typical locations on the body, such as athlete's foot between the toes. A clinical evaluation cannot, however, definitively tell the doctor which microorganism is causing a wound infection or the treatment to which this microorganism is likely to be susceptible. For that, laboratory testing is sought but does not always yield a result. When a result is given it will usually guide therapy.

Laboratory tests
Laboratory testing is primarily used to diagnose bacterial wound infections, to identify the microorganism responsible, and to determine its likely susceptibility to specific antimicrobial agents. Sometimes testing is also performed to detect and identify fungal infections. Sample collection may involve swabbing the surface of a wound, aspiration of fluid or pus with a needle and syringe, and/or the collection of a tissue biopsy. If anaerobic microorganisms are suspected, then special collection and transport measures must be used to keep the sample from being exposed to oxygen. For fungal evaluation, scrapings of the skin may be collected.

Testing may include:

  • Gram stain - used along with the wound culture. Special staining allows bacteria to be evaluated under the microscope. They may be distinguished by their shape - cocci (spheres) or bacilli (rods) - and separated by colour into gram positive or gram-negative microorganisms. The results of this test should be available the same day the specimen is received in the laboratory and can give the doctor preliminary information about the quality of the specimen and potential organisms that may be causing the infection.
  • Bacterial wound culture - this is the primary test used to determine the cause of a bacterial infection. The sample is streaked onto or into nutrient media and incubated at body temperature to grow and identify any bacteria present in the sample. Part of this test includes the identification of MRSA when it is present. Results of bacterial wound cultures are usually available within 48 hours from the time the specimen is received in the laboratory. Results of special cultures for slow growing organisms, such as fungi or mycobacteria, may require several weeks.
  • Antimicrobial susceptibility - a follow-up test to the wound culture. When a pathogen is identified and isolated using the wound culture, this test is used to determine the bacteria's likely susceptibility to certain drug treatments. This information helps guide the doctor in selecting appropriate antibiotics for treatment. These results are typically available within 24 hours after identification of the microorganism that is causing the infection.

Other microbiological tests that may be ordered include:

  • KOH prep - a rapid test performed to microscopically detect fungal elements (cellular structures) in a sample.
  • Fungal culture - ordered when a fungal infection is suspected. Many fungi are slow-growing and may take several weeks to identify.
  • AFB culture and smear - ordered when a mycobacterial infection is suspected.
  • Blood culture - ordered when septicaemia is suspected.
  • Urine culture - ordered when a urinary tract infection is suspected.
  • DNA or RNA testing to detect genetic material of a specific organism.

Non-laboratory tests
Imaging scans such as ultrasounds or x-rays may be ordered to evaluate the extent of tissue damage and to look for areas of fluid/pus in selected cases.

Last Review Date: August 14, 2017