What kind of allergic reactions can patch testing diagnose?
Patch testing is only used for one type of allergic reaction: allergic contact dermatitis. This is the type of allergy that may occur after poison ivy exposure. Following exposure to a substance, 48 to 72 hours later (classically) a rash will develop on the skin. Poison ivy is probably the most well-known trigger of ACD. Other triggers are nickel in jewelry, fragrance in perfumes, and additives and preservatives in makeup and skincare products.
Can patch testing diagnose allergy to pollen, molds, trees, or pets?
No. Those substances usually trigger a type of allergy called IgE-mediated allergy. They may cause a runny nose or itchy eyes, or symptoms of hayfever. This type of allergy is diagnosed by skin prick testing, which would be performed in an allergist's office.
Can patch testing be used to diagnose food allergies?
When most people think about food allergies, they think about eating foods such as peanuts or shellfish and then developing swelling of the lips or tongue.
Patch testing does NOT test for this type of food allergy. For these symptoms, we recommend that you see an allergist to discuss the use of skin testing or blood testing for food allergies.
There are very few foods that can be tested for via patch testing. In some people who are allergic to fragrance additives, eating tomatoes, citrus, or cinnamon can worsen their skin rash. This is known as systemic allergic contact dermatitis, and patch testing can test for this very specific type of food allergy. For other types of food allergies, you would be referred to an allergist.
Can patch testing be used to diagnose the cause of hives?
No. Patch testing is not used in the evaluation of hives. Hives occur due to a different type of reaction – not allergic contact dermatitis. That is why patch testing will not be helpful.
The medical term for hives is chronic urticaria. In this condition, individual raised red areas, called "wheals", come and go on different parts of the body. One of the keys to the diagnosis is that each individual wheal lasts for less than 24 hours in any one spot.
Some people have wheals that come and go for weeks. Episodes that last less than 6 weeks are known as acute urticaria. The major triggers of this reaction are medications (such as antibiotics), illness (such as an upper respiratory infection or a gastroenteritis), or foods (such as shellfish or nuts).
If the wheals last for more than 6 weeks, this is known as chronic urticaria. Patients with chronic urticaria should see their primary care physician for a physical exam. Your physician may order additional blood tests or other tests depending on your symptoms and physical exam. For example, they may ask questions to look for signs or symptoms of a sinus infection, urinary tract infection, dental infection, thyroid disease, or other infections or conditions.
However, and this is a very important point, in the majority of cases of chronic urticaria, no underlying trigger is found. That can be very frustrating for people who have been dealing with outbreaks on their skin for months.
We counsel our patients that if you are among the majority of persons with chronic urticaria for which no underlying trigger can be found, then you need to focus on treating the hives. Your physician will begin treatment with antihistamines by mouth. Many people need a combination of different antihistamines. They may require treatment for months, or even longer in some cases. Some people will even require additional treatments because their hives are not controlled on antihistamines.
If you have chronic urticaria that requires systemic treatment (beyond antihistamines by mouth), then you may need to locate allergists or dermatologists who have expertise in using these additional medications.