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