PATCH TESTING

PATCH TESTING

Art of Diagnosis

Once it has been determined that you are likely to have ACD by history, then you will be instructed on whether or not you will need a patch test. Since its discovery in 1895, the non-invasive ‘epicutaneous patch test’ has been the gold standard test, the best test to confirm that a patient is sensitized to a chemical.

Who needs a patch test?

In order to confirm the diagnosis of ‘ACD’, your provider will take a thorough history and visualize (see) your rash. Sometimes the association with the allergen chemical is obvious. For example a round patch of rash on the wrist which corresponds to the point of where the back of the watch is releasing nickel suggests the role of the watch, and confirmatory testing will not be needed. In a significant number of cases, however, the rash will come and go and it will be difficult to know what is causing it without a patch test. Many rashes can look like ACD and it is important that your seek evaluation by someone properly trained in the evaluation and management of ACD to determine if a patch test is needed.

How is the patch test done?

The test is not invasive, not painful, and takes up 5-7 days to complete. Many insurance companies cover this test and you should check with your insurance carrier before you have the test. The chemicals are applied to the skin on stickers for up to 48hours. They are then removed and the sites checked for ‘early reactions’. The patient then returns in 48-96hours after the removal to have the final reading of the patch tests. All positive reactions are then recorded. The patch test expert will then re-review the patient’s history and correlate it with the positive reactions to see if they can help the patient discover the source of the chemical to which they are reacting. The potential outcomes for patch testing are as follows: negative, irritant reaction, equivocal/uncertain, weak positive, strong positive, extreme reaction. Symptoms that may result from patch testing include redness, severe itching, blisters, or ulcers. Positive results must be differentiated based on significance. For example, you may have a positive result to a specific allergen, but if you have no contact with such an allergen on a regular basis, the allergen may not be significant. In some cases, an individual with allergic contact dermatitis may have negative results to the placed chemicals. However, this simply means that you were not allergic to the tested chemicals, and an additional round of patch testing with a greater number of allergens, may by necessary. One the source of the chemical is identified and avoided, the rash will go into remission (go away). 

What happens after patch testing?

Avoidance of allergen chemicals that the person has reacted to on their patch test (been noted to be sensitized to) is the best method of treatment. However, since the chemicals can activate the skin for weeks after exposure, it is important to maintain strict avoidance for at least 6 weeks. In the meantime, cool compresses, topical anti-inflammatory creams (steroids, topical immune modulators), and antihistamines may be needed, while the rash subsides.