A skin prick test or a blood test are designed to measure a protein called IgE that is involved in many types of food allergic reactions. The tests detect the IgE antibodies that are specific for a particular food. The blood tests are most properly called “serum food-specific IgE tests” but are often called “RAST” (radioallergosorbent test) or referred to by various brand names.
A skin prick test is usually less expensive and can be done in the doctor’s office. The doctor places a drop of the substance being tested on the patient’s forearm or back and pricks the skin with a probe, allowing a tiny amount to enter the skin. If the patient is possibly allergic to the substance, a wheal (mosquito bite-like bump) will form at the site within about 15 minutes. The test is only slightly uncomfortable, generally not painful and there is no bleeding.
The blood test is sent to a medical laboratory, where tests are done with specific foods to determine whether the patient has IgE antibodies to that food. The results are usually received within one week.
Which test is better?
Although both tests are reliable, there are instances where your doctor may prefer one or another, and sometimes both are performed. A doctor might choose the blood test, for example, with a patient who has eczema or other skin problems that would make it difficult to read the results of a prick skin test. The results of either test are combined with other information, such as a history of symptoms to determine whether a food allergy exists. Sometimes, when tests and history are inconclusive, a supervised feeding test is performed under direct medical supervision (a food challenge) to definitively determine if there is an allergy.
It is important to know that allergy skin and blood tests do not necessarily show whether an allergy exists. Some allergies occur despite negative tests in part because some allergies do not involve IgE antibodies. Many times, a test is slightly positive but the person can eat the food without symptoms. Lastly, the tests do not tell how severe an allergy may be. This is why it is so important to work with your doctor (typically an allergist) to put together the history and test results to obtain a proper diagnosis. -FAAN
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