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Medical Author:

Nili N. Alai, MD, FAAD

Nili N. Alai, MD, FAAD

Dr. Alai is an actively practicing medical and surgical dermatologist in south Orange County, California. She has been a professor of dermatology and family medicine at the University of California, Irvine since 2000. She is U.S. board-certified in dermatology, a 10-year-certified fellow of the American Academy of Dermatology, and Fellow of the American Society of Mohs Surgery.

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Medical Editor:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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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.

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