Many minor and superficial skin and wound infections are diagnosed by the doctor based on a physical examination, signs and symptoms, and her 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 what treatment is likely to be effective. For that, laboratory testing is required.
Laboratory testing is primarily used to diagnose bacterial wound infections, to identify the microorganism responsible, and to determine its likely susceptibility to specific antimicrobial drugs. Sometimes testing is also performed to detect and identify fungal infections. Sample collection may involve swabbing the surface of a wound to collect cells or pus, aspiration of fluid or pus with a needle and syringe, and/or the collection of a tissue biopsy. For fungal evaluation, scrapings of the skin may be collected.
Testing may include:
- Bacterial culture – This is the primary test used to diagnose a bacterial infection. Part of this evaluation involves the identification of methicillin-resistant Staphylcoccus aureus (MRSA) when it is present. Results of bacterial wound cultures are usually available within 24-48 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.
- Gram stain – This test is usually performed in conjunction with the wound culture. It is a special staining procedure that allows bacteria to be evaluated under the microscope. The results of this test are usually available the same day the sample is received in the laboratory and can give the doctor preliminary information about the microorganisms that may be causing the infection.
- Antimicrobial susceptibility – A follow-up test to the wound culture. When a pathogen is identified 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 about 24 hours after isolation of the microorganism that is causing the infection.
Other tests that may be ordered include:
- KOH prep – A rapid test performed to detect fungi in a sample. The sample is treated with a special solution, placed on a slide, and examined under a microscope.
- 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. Most AFB are slow-growing and may take several weeks to identify.
- Blood culture – Ordered when infection from a wound may have spread and septicemia is suspected.
- Molecular testing to detect genetic material of a specific organism.
- Basic metabolic panel (BMP) or Comprehensive metabolic panel (CMP) – May be ordered to detect underlying conditions that can affect wound healing, including a glucose test to detect diabetes
In some cases, 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.