Many patients who suffer from allergies have tried every medication on the market at some point and found no relief from their symptoms. Others have gotten some relief with medications, but they do not like the prospect of taking medications every day for the foreseeable future and want to consider immunotherapy. These, and other reasons, motivate us to choose to be tested for allergies.
The classic allergic response is an IgE mediated response of the immune system to a foreign antigen, such as pollen, dust mites, or molds. There are other immunologic reactions that manifest with similar symptoms, like sneezing, nasal congestion, and headache. For the purposes of this article, we will limit our discussion to classic allergy. Allergy testing is a way to determine if you are having an allergic response to a particular antigen. An antigen is usually a protein molecule on some foreign substance. The body recognizes these substances as foreign but not as common environmental contaminants, so the immune system kicks in to mount an attack on the antigen. This immune response can be measured indirectly with a blood test or triggered in a controlled fashion and measured directly with skin testing.
Most people are familiar with skin testing. It is the oldest form of allergy testing. In general, skin testing involves introducing the antigen of interest into the skin and observing the skin for a reaction. The exact technique can vary, but in our office, we use a technique called MQT. First, a skin prick panel is applied to the skin of the forearm or upper arm or back. The skin prick delivers the antigen to the epidermis, or top layer of skin only. There are immune system cells in that layer that will trigger a response if your system is primed to do so. The response is a local wheal, or raised, red bump on the skin. The wheal is measured and recorded. If there is a very brisk reaction, the end point is set at the highest level. The endpoint tells us how dilute to make the antigen in an immunotherapy treatment vial. Less severe reactions require further testing to know where the end point is. This is done by intradermal testing. A small amount of antigen is injected into the dermis, or the deep layer of the skin. The reaction is measured and an end point is reached. Non-reactive scratch tests are taken to mean that those substances are not causing allergy in that patient. The advantage of skin testing is that the results are immediate and a reflection of what actually happens when your body is exposed to an allergen. The disadvantages of skin testing are that the results are influenced by medications which may need to be stopped before testing, and patients with certain skin conditions cannot be tested.
For those who cannot tolerate skin testing or cannot complete skin testing for some other reason, there is the option of doing a blood test (called in vitro testing). In vitro testing has the advantage of being easy for the patient – she only needs to have one needle stick for blood to be drawn. The blood test does not require stopping any medications. The disadvantage of blood testing is that you are measuring levels of reactive immunoglobulin in the blood stream, not a direct response. Sometimes elevated IgE does not correlate to symptoms in the patient. With recent advances in the technology used for in vitro testing, the correlation between blood and skin testing results is much better and we generally consider the tests to be equivalent.
A discussion with your physician can clarify if allergy testing is for you and together you can use the results to craft a treatment plan that works for you.