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Rash (cont.)

Exams and Tests

There are many useful laboratory and special examinations which are helpful in the diagnosis of rash:

  • bacterial culture to check for bacteria on the skin or in a wound
  • microscopic examination of a scraping of skin with potassium hydroxide to look for fungus
  • blood tests such as antinuclear antibody (ANA), complete blood count (CBC), liver function tests (LFT), and thyroid function tests
  • nasal culture using a cotton tip swab to check for Staphylococcus and other bacteria
  • Gram stain (special staining of a sample prior to examination under a microscope) to identify bacteria types
  • Tzanck prep to look for herpes virus under the microscope
  • skin biopsy (small skin sample or scraping sent for microscopic examination)
  • patch test to determine contact allergies

Sampling skin material and viewing under direct microscopy is a fast and simple way to help diagnose a rash. When a superficial fungal or yeast infection is suspected, viewing a superficial skin scraping with a potassium hydroxide prep can reveal fungal hyphae or budding cells.

Likewise, suspected bacterial infection can be evaluated by a Gram stain or nasal swab culture. Viral lesions  typically caused by herpes simplex can be viewed under the microscope with a Tzanck smear which will show giant, multinucleate cells.

Blood tests can be helpful as well (for example, sudden onset of severe psoriasis may be associated with an HIV infection). Anti-streptolysin O (ASO) levels can be helpful in detecting a sudden onset of guttate psoriasis associated with a prior streptococcal throat infection.

Medically Reviewed by a Doctor on 5/30/2014

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