Testing for Food Allergies: Blood or Skin?
This a common question we are asked, what is the best way to test for food allergies? The best answer is, it depends. In some cases the blood test is the only test you need to do, other times it is the skin test and many times it is both. The problem is neither test is perfect or close to 100% accurate. When dealing with such an important condition as food allergies, we need as accurate a test and diagnosis as possible . An incorrect diagnosis has many implications. A false + test often leads to a false + diagnosis so a food is unnecessarily avoided and the family needlessly worried. We have seen complications of poor nutrition and poor weight gain when too many foods are avoided. A false negative test is more concerning as the family is not told to avoid a food that may trigger a serious allergic reaction. This is very dangerous as the family is not aware and is often not prescribed an epi pen.
The allergy skin test has been used to help diagnose food allergies for years and years. A small drop of a diluted form of the food is place on the skin, it is scratched and the results are ready in 15 minutes. A reaction that looks like a mosquito bite is measured in mm as to the bump known as the wheal and the surrounding redness called the flare. Results larger than the + skin test control are considered positive. The test is quick, the results available within minutes, and the wheal and flare response is a nice visual for the parents to see. A negative skin test response is highly accurate and often indicates the child is not allergic to the tested food. Unfortunately, in many cases there are false positive skin test s and the clinical significance can be tricky.
Blood tests on the other hand, while easy to order and obtain are often very difficult to correctly interpret. A strong positive blood test result tends to be very reliable that a food allergy exists. However in many cases, the scores are in the low positive range. The weak + blood scores are a clinical challenge. In one child with a blood score of 2 for peanut, he may eat it fine while another child with the same score may end up in the hospital with a severe reaction. To make matters more confusing, a blood score may give a false negative result 10-20% of the time. As stated above, a false negative result is a potentially very dangerous situation.
For both the blood and skin tests, the results only predict the likelihood of having a reaction, neither test will predict the severity of the reaction. The child with a peanut score 5, may react more severely and require the use of an epi pen while a child whose score is >100, may just need Benadryl to treat his reaction.
At The Food Allergy Specialists, we find a careful approach of obtaining an accurate clinical history and the proper utilization of blood and skin test results helps us to obtain the most accurate diagnosis which is essential to successful food allergy management.
See a recent interview on KVUE on egg allergy and flu shots
American College of Allergy, Asthma, and Immunology
The Food Allergy Network
American Academy of Allergy, Asthma, and Immunology
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