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Rash (cont.)

What Specialists Treat Rashes?

Dermatologists (skin specialists) are best equipped to diagnose and treat most rashes, especially those that require biopsy or special tests. Unfortunately, there may be quite a delay in obtaining a dermatologist consult in some areas. For that reason, most primary-care physicians (family physicians, internists, and pediatricians), as well as those who work in urgent care and emergency departments, will be the first to see a patient's rash and, in many cases, can make accurate diagnoses and advise effective treatment. Allergists treat many rashes also, especially those related to hives. Occasionally, an infectious-disease specialist may be involved in the treatment of rashes caused by severe or unusual infections. Medical or surgical oncologists may have a role when a skin rash is due directly or indirectly to an underlying cancer.

What Tests Do Doctors Use to Diagnose a Rash?

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;
  • rashes that come and go can sometimes be diagnosed by high-quality digital pictures; and
  • some blood tests are less useful in diagnosis. This includes blood testing for herpesvirus and Lyme disease. The problem in both cases is that a "positive test" usually means only that the person has had or been exposed to the disease in the past and says nothing about active or current infection

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 7/27/2016
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