Patch Tests For Allergy Diagnosis- Allergy/Immunology, Dermatology

Doctor: Allergist, Dermatologist

What is patch test?

Patch test is a diagnostic test applied on the surface of the skin. Patch tests are used to determine the specific cause of allergies to skin (contact dermatitis).  Patch tests can also detect delayed allergic reactions, which could take several days to develop.

The procedure:

Before undergoing a patch test, the patient would be asked to abstain from taking certain medicines or using a tanning bed, particularly on the back. The patch test is generally performed on the back. The patch contains various allergens which are applied in little dots on an adhesive sheet. Patch tests do not use any needles.

During the test, patches with different allergens are placed on the skin. At a time around 20 to 30 extracts of substances (responsible for causing contact dermatitis) may be applied on the skin such as medicines, metals, latex, resins, fragrances, hair colors, preservatives etc. Patches are kept on the skin for 48 hours. During this time, patient is asked to avoid taking a bath or shower. Patient is asked to avoid any exercise which may lead to excessive sweating.  

After 48 hours, the patches are removed. Before removing the patches, the location of each patch is marked with a surgical marker. This will provide the allergist or dermatologist a reference for the final evaluation.

The final evaluation is done anywhere from 72 to 96 hours. Irritated skin at the patch site might indicate an allergy. All types of reactions are recorded. Once all of the readings of the patch test are noted, a topical steroid might be prescribed to treat any itching or rash.

References:

• https://www.fda.gov/BiologicsBloodVaccines/Allergenics/ucm391311.htm
• https://www.mayoclinic.org/tests-procedures/allergy-tests/about/pac-20392895
• https://www.verywellhealth.com/patch-testing-for-contact-dermatitis-82758

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