Patch testing is designed for testing for substances that come in contact with skin and cause trouble. Patch testing is different than prick skin test as this is used to detect different kind of allergies. Patch tests identifies allergen specific T-lymphocytes as opposed to allergen specific IgE antibodies. The way patch test is done is different and more time consuming.
Most common way to do a patch test is by closed test. This involves applying any of the commercially available unites (True test, Allergeaze chamber, Finn chamber and few others). At Allergy, Asthma and Sinus Center, we use Allergeaze chambers. All of the patch test system involves applying panel or panels of 10-12 allergens under occlusion. Allergens are applied in standard amount to aluminum or synthetic material chambers mounted on non-occlusive tape strip. These are left in place for 48 hours and removed at that point.
Initial reading:To decide whether a patient is reacting to any of the chemicals, allergist looks at the application site for redness, swelling (papules), blisters, oozing crusting, blistering and spreading beyond the site where allergen was applied. Positive reactions are often associated with itching.
Second reading: Irrespective of any reaction at 48 hours or initial reading, patient is advised to return after 48-72 hours (3-4 days after initial application of patches) to see if the initial reaction has worsened or gotten better. Occasionally patient may show positive reaction only on the second visit. Some allergens are notorious for really delayed reactions (requiring reading on day 7 after initial placement of patches) like nickel, neomycin and corticosteroids.
The most crucial part of the patch testing is deciding if the identified allergen is responsible for patient current symptoms. For example, consider that a patient is positive to nickel and has had trouble with nickel containing jewelry in the past. The test correctly identifies nickel allergy but patient came looking for answers for a rash on the lower leg. Nickel allergy does not explain the lower leg rash, which could be due to eczema related to venous insufficiency.
Some patients may have positive test and patient cannot think of any source of allergen in what he or she is using. After a lot of searching, a patient may find the allergen in skin care products used by their spouse (this situation is called consort dermatitis).
Many time patient may never find out the allergen in their environment or even if they had contact with allergen, removal of that allergen doesn’t improve the symptoms. In this situation, patch test are considered but not clinically relevant.
Over the years, diagnosis for contact dermatitis has improved with the inclusion of more clinically relevant allergens. There is always a possibility that patient is allergic to an allergen that is not tested. Sometime testing a patient for body care products, cosmetics, clothing, or other items which come in contact with the skin may provide the answer. Sometime additional allergen/panel patch test may reveal the culprit.
However, at times, it is important to consider alternative explanation for patient symptoms. the common cause of rashes, which look like allergic contact dermatitis include irritant contact dermatitis, various kind of eczemas and other skin condition.
In general, irritant dermatitis looks very similar to allergic contact dermatitis, only difference being is that one can’t demonstrate any relevant allergen on patch tests. Patients usually improve when contact with known irritant i.e. use of harsh chemicals, frequent hand washing, contact with urine or stool etc. is removed.
Despite the negative patch test results, possibility remains that an allergen is missed during testing. these patients should proceed with what is called “Repeated open application test” or “usage test”.