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Rash (cont.)

Rash Diagnosis

There are many useful laboratory and special examinations that can be helpful in the diagnosis of rash, such as

  • 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), to look for lupus, complete blood count (CBC), liver function tests (LFT) to look for rashes related to hepatitis, and thyroid function tests;
  • blood test for EBV (mono) or a rapid plasma reagin (RPR) or other blood tests for syphilis may be appropriate;
  • 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.

Unfortunately, the skin biopsy results of viral rashes and drug rashes may be similar enough that a definite diagnosis cannot be made. Nor can a biopsy indicate which drug is the cause of a drug rash.

Sampling skin material and viewing under direct microscopy is a fast and simple way to help confirm or eliminate fungus as a cause of the 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. Prior treatment with antifungal creams may cause a false-negative test.

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 that 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 recent streptococcal throat infection.

Medically Reviewed by a Doctor on 7/21/2015
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