When suspicion is raised about a possible allergy, your allergist will first get more details from your history. Do the symptoms suggest an environmental allergy such as sneezing, nasal congestion and itchy eyes? Perhaps symptoms developed after ingestion of food or after an insect sting. He or she will want to know about any symptoms of asthma as well as family history. The medical history serves as a basis for the allergy diagnosis and may then lead to confirmatory testing. There are 2 primary forms of testing, the skin test – which gives the fastest and most accurate results – and the allergy blood test.
A skin prick test, also called a puncture or scratch test, checks for immediate allergic reactions to as many as 50 different substances at once. This test is usually done to identify allergies to pollen, mold, pet dander, dust mites and foods. In adults, the test is usually done on the forearm. Children may be tested on the upper back. Skin prick testing provides results in about 20 minutes. A liquid containing a tiny amount of the food allergen is placed on the skin. Your skin is pricked with a small, sterile probe, allowing the liquid to seep under the skin. Multitest devices now allow for 8 or 10 tests to be applied simultaneously, making the process even easier. To see if your skin is reacting normally, two additional substances are scratched into your skin’s surface. Histamine (positive control) – in most people, this substance causes a skin response. If you don’t react to histamine, your allergy skin test may not reveal an allergy even if you have one. Glycerin or saline (negative control – in most people, these substances don’t cause any reaction. If you do react to the negative control, you may have sensitive skin. Test results will need to be interpreted cautiously to avoid a false allergy diagnosis. The test isn’t painful. You won’t bleed or feel more than mild, momentary discomfort. The test is considered positive if you develop a raised, red, itchy bump (wheal) that may look like a mosquito bite. The nurse will then measure the bump’s size and record the results. Next, he or she will clean your skin with alcohol and may apply a topical steroid. When the test is over, the doctor or nurse will clean your skin and put some cream on it to help with any itching. Any swelling from a reaction usually goes away within 30 minutes to a few hours. Some medicines can get in the way of the tests. Check with your doctor to see if you need to stop taking any medication before the test.
If you take medicine that could affect allergy test results, have sensitive skin, or have had a bad reaction to a skin test, your doctor might do a blood test instead. A sample of your blood is sent to a lab, and your doctor will typically get the results in about a week. Allergy blood tests, which are a bit less exact than skin tests, measure the amount of IgE antibody (“allergic antibody”) to the specific allergen being tested and are reported as a numerical value. This is typically more expensive than a skin test.
Your allergist will use the results of these tests in making a diagnosis. If an allergy test is negative, you aren’t allergic to that substance. It’s rare to get a false (incorrect) negative allergy test result (meaning the test says you don’t have an allergy when you actually do). A positive test however does not mean you have an allergy – or at least react to that allergen. A false positive test result is possible, especially from a blood (IgE) test. A false positive means the results show you have an allergy when in fact you don’t.
In some cases, a skin or blood test is not available, or the results are not conclusive. In those cases, an allergist may recommend an oral challenge. This is considered the most accurate way to make a food allergy diagnosis. During an oral food challenge, which is conducted under strict medical supervision, the patient is fed tiny amounts of the suspected trigger food in increasing doses over a period of time, followed by a few hours of observation to see if a reaction occurs. Because of the possibility of a severe reaction, an oral food challenge should be conducted only by experienced allergists in a doctor’s office or at a food challenge center, with emergency medication and equipment on hand.