Patch testing is a common diagnostic tool used to identify the specific substances that trigger contact dermatitis reactions. Contact dermatitis is a localized irritation of the skin in response to a certain substance that is otherwise harmless. Common trigger substances include poison ivy, jewelry, hair care products, cleaning solution, detergent, cosmetics, perfumes, and latex rubber.
In allergic contact dermatitis, typically harmless substances cause an immune system reaction when they come in contact with the skin. The reaction to trigger substances causes localized symptoms about one to two days after exposure but sometimes as late as 1 week after exposure. These symptoms may include:
These symptoms can last for up to four weeks, and can usually be treated at home through medications and topical creams. The best protection against contact dermatitis is to identify the specific triggers that cause reactions, so that patients can avoid these substances as often as possible.
Although allergic contact dermatitis is similar to other allergies in that it triggers an immune system response, these triggers are external rather than internal. So testing for this condition is also done externally, and does not require the use of any needles or injections.
Patch Test Procedure
Before undergoing patch testing, patients should stop using corticosteroids in the area to be tested, and should not expose the test area to the sun for at least three weeks. Most dermatologists use the TRUE Test which is a very good screening tool but is limited to testing less than 30 different chemicals. As the first dermatology group in the St. Louis region to offer expanded patch testing, we have access to more than 200 antigens known to cause contact dermatitis.The patch testing procedure is simple and painless. In order to identify specific triggers, your doctor will tape small metal cups that contain various potential chemicals to the skin on the back. These chemicals are then left on the skin for 48 hours, and must remain dry during this time. Patients are advised to take only sponge baths and avoid excessive sweating. After 48 hours, the patch test is removed and an initial reading is taken to observe any reactions. An additional reading is taken after another 24 to 48 hours. Patients can bathe in between these readings, but should avoid scrubbing the back. Once the final results of the test have been read, normal bathing can be resumed.
When reading the results, your doctor may classify each spot on a scale from negative (meaning no reaction) to extreme reaction (meaning positive results for substance). Strong results may cause blisters or ulcers on the skin, which can be treated once the test is complete.
Once positive results have been determined, patients can take steps to avoid their triggers and prevent contact dermatitis from occurring. Your doctor will provide you with specific information on your individual triggers and how to prevent contact. Since we are involved with the American Contact Dermatitis Society, we have access to a database allowing us to provide a comprehensive list of products a patient may use based on their individual test results.
Although patch testing is considered a safe and effective diagnostic tool, these tests do cause a small area of contact dermatitis at the site of the substance, so patients often experience mild itching and irritation on the their back. Positive test results may result in an area on the skin that has redness, bumps, swelling and even a small blister. After the test is complete, patients can use a topical steroid on affected areas to help relieve symptoms. If this cream is used before the end of the test, results may be inaccurate.
Since contact dermatitis is an immune system condition, your body may react to the chemical substance in the same area that they originally contact dermatitis occurred, so patients may experience a mild recurrence of their original condition. These areas can be treated as need during the test, as only the test area is examined afterwards. The most significant potential adverse outcome of patch testing is finding results that do not help to resolve your rash.