context: Millions of Americans have allergies. You might sniffle and sneeze as the seasons change or get itchy and teary-eyed when you dust the house or pet an animal. Perhaps you start wheezing when you eat a particular food. Allergy blood testing can help reveal what triggers your allergy symptoms and help your doctor choose the best treatment for you. Allergy blood tests detect and measure the amount of allergen-specific antibodies in your blood. When you come into contact with an allergy trigger, known as an allergen, your body makes antibodies against it. The antibodies tell cells in your body to release certain chemicals. These chemicals are what cause allergy symptoms. Immunoglobulin E ( Ig E) is an antibody that's strongly linked to the body's allergy response. Allergy blood tests usually screen for at least 10 of the most common allergy triggers, including dust, pet dander, trees, grasses, weeds, and molds related to where you live. They are also particularly helpful in diagnosing food allergies. Allergy blood tests may be referred to as immunoassay tests and include: Enzyme-linked immunosorbent assay ( ELISA, or EIA) Radioallergosorbent test ( RAST) The ELISA test measures the amount of allergen-specific antibodies in your blood. The RAST test also looks for specific allergen-related antibodies in order to identify your allergy triggers. Since the introduction of the ELISA test, RAST testing has not typically been used. Allergies can cause an increase in certain types of white blood cells. Blood tests to check your white blood cell counts, including a count of a type of white cell called an eosinophil, may also be done if your doctor thinks you have allergies. However, it is important to keep in mind that many other health conditions can cause an increase in white blood cells. Other blood tests may be ordered that measure the release of chemicals responsible for allergic reactions. Allergy skin testing is the preferred method, but in some cases blood testing may be ordered. Allergy blood testing is recommended if you: Are using a medicine known to interfere with test results and cannot stop taking it for a few daysl this would include antihistamines, steroids, and certain antidepressants. Cannot tolerate the many needle scratches required for skin testing Have an unstable heart condition Have poorly controlled asthma Have severe eczema, dermatitis, psoriasis, or another severe skin condition Might have an extreme reaction during skin testing or have a history of life-threatening allergic reaction called anaphylaxis Your doctor may also order blood testing to determine how well your allergy treatments (immunotherapy) are working. Blood testing may also show whether you have outgrown an allergy. Advantages of allergy blood tests include: Can be done at any time, regardless of any medications you are taking Requires only one needle stick (unlike skin testing); this may be more attractive for people who are afraid of needles. Allergy blood testing is the preferred test for infants and very young children. Disadvantages of allergy blood tests include: More expensive than skin testing; many health insurers do not cover allergy blood tests. May be less sensitive than skin tests Takes days or weeks to get results because the blood sample must be sent to a laboratory for evaluation; skin testing provides immediate results. A positive result means allergy-specific antibodies were detected in your blood. This is usually a sign of an allergy. The blood test will reveal what exactly you are allergic to. However, you can test positive for something but never have had an allergic reaction to it. A negative result means you probably do not have a true allergy. That means your immune system probably does not respond to the allergen tested. However, it is possible to have a normal (negative) allergy blood test result and still have an allergy. Allergy blood test results should be interpreted with caution by an allergy specialist. Your doctor will also consider your symptoms and medical history when diagnosing a specific allergy. Allergy blood testing is relatively safe. Side effects are usually minor and may include: Swelling and redness at the site where the needle was inserted Pain Bleeding at the site where the needle was inserted There is a potential that some people may faint during any type of blood testing.