An allergy test helps identify specific substances, known as allergens, that trigger an immune system response. These tests pinpoint what causes allergic reactions, from mild symptoms like sneezing to more severe responses. By identifying these triggers, individuals and their healthcare providers can better manage allergies.
Different Types of Allergy Tests
The skin prick test, also called a scratch or puncture test, is a common method. A healthcare provider places small drops of allergen extracts onto the skin, typically on the forearm or back, then lightly pricks the skin through each drop with a thin needle or lancet, allowing the allergen to enter just beneath the surface.
Another type is the intradermal test, which is sometimes used if skin prick results are unclear or negative, especially for insect venom or penicillin allergies. In this test, a small amount of the allergen is injected directly into the outer layer of the skin using a fine needle. Unlike skin tests, blood tests, specifically IgE tests, involve drawing a blood sample. This sample is then sent to a laboratory to measure the levels of specific immunoglobulin E (IgE) antibodies, which are produced by the immune system in response to allergens. Finally, the patch test helps identify causes of contact dermatitis, where patches containing suspected allergens are applied to the skin and worn for 48 to 96 hours.
The Allergy Testing Experience
During a skin prick test, the healthcare provider first cleans and marks the testing area, often the forearm or back. Small drops of various allergen extracts and control solutions are then placed on these marked spots. You might feel a brief, mild pricking sensation as the skin is gently punctured through each drop.
Within about 15 to 20 minutes, the skin is observed for any reactions. During an intradermal test, the injection of the allergen feels like a small pinprick, and the site is then monitored for a reaction, which also appears as a wheal. While skin tests focus on visual reactions, blood tests involve a standard blood draw, feeling like a brief pinch as the needle is inserted into a vein, with no immediate visual reaction on the skin.
Understanding Your Allergy Test Results
After a skin prick test, a positive reaction manifests as a raised, red, and itchy wheal, similar to a hive or mosquito bite, surrounded by a red flare. The size of the wheal, measured in millimeters, indicates the strength of the allergic response, with larger reactions often suggesting a higher likelihood of clinical allergy. A negative result means no significant wheal forms, indicating that you are likely not allergic to that specific substance.
For blood tests, results are presented as numerical values, often in kilounits per liter (kU/L), representing the concentration of allergen-specific IgE antibodies in your blood. Higher numbers generally indicate a greater amount of IgE antibodies and a higher probability of an allergy. However, a positive blood test indicates sensitization, meaning the presence of IgE antibodies, but does not always perfectly correlate with actual symptoms. Patch test results, which identify contact dermatitis, are typically evaluated after 48 to 96 hours, with positive reactions appearing as a rash, redness, or small blisters at the site where the allergen was applied.
Preparing for Your Allergy Test
Before an allergy test, your healthcare provider will provide specific instructions, particularly regarding medications. It is recommended to stop taking certain antihistamines, such as fexofenadine, loratadine, or cetirizine, for three to seven days prior to a skin test, as these can suppress allergic reactions and lead to inaccurate results. Some tricyclic antidepressants and certain stomach acid reducers may also need to be discontinued for a week or more before testing. For blood tests, discontinuing medications is usually not necessary.
You should discuss all current medications with your provider. For skin tests, wearing loose-fitting clothing that allows easy access to your forearms or back is advisable. There is no need to fast or follow a special diet before an allergy test.