Peanut Allergy Diagnosis and Testing

A common question people have is "How do I know if I have a nut allergy?", and people often expect to have formal peanut allergy testing by their doctor. This may not always be necessary.

The history is the most important factor here. If someone consistently gets symptoms of an allergic reaction - a rash, facial swelling, difficulty breathing - shortly after being exposed to nuts then that is very suggestive of a reaction and formal peanut allergy testing may be unnecessary unless there is doubt as to what substances are involved. It should be noted that reactions are unpredictable, and it is possible for someone to have a mild reaction at first and then a much more severe reaction the next time, so you should never 'taste test' food, and an apparently mild allergy to nuts should be taken seriously as they do have the potential to develop over time.

Peanut allergy testing can be worthwhile even in situations where an allergy seems very likely as it allows monitoring over time; this is more useful in children who some proportion may outgrow the disease, although that is rather controversial.

More formal medical testing involves Skin Prick Tests (SPT) and IgE tests (a blood test). Looking at the skin prick test first, a needle is used to introduce a small amount of the substance just under the skin, and the size of the lump raised is used as an indication of the degree of allergic response to it. Interpretation of this test can be a little subjective, and it can be affected by medications such as antihistamines. Studies have been done to identify the sizes of lump that mean that a significant allergic reaction is likely.

IgE testing measures the levels of antibodies in the blood to allergenic substances, such as peanut protein. This is not affected by antihistamines, and produces a count of antibodies which is divided into a 'grade' from 0-6, with 0 being no significant number of antibodies and 6 being a large amount. Sometimes the actual number of antibodies is given instead.

However, both skin prick tests and IgE peanut allergy testing give an indication of the likelihood of a reaction, and it doesn't necessarily mean that a reaction will be a severe one, even if someone has a grade 6 IgE to peanut.

Finally, if results don't mesh with the history, a formal challenge can be done in hospital. These involve a small amount of the substance being touched to the skin, and then to the lip, then a drop being consumed, then more, and so forth. This is done very slowly over a period of hours, and the test is abandoned if a reaction clearly occurs. These should only be done in hospital because, as mentioned before, there is a risk of a significant reaction. They are more commonly used to prove that someone can now eat a food safely.

In summary, the history is the most important factor in determining if someone is likely to have an allergic reaction to a substance. Blood tests or skin prick tests can be used to give an indication of the likelihood of a reaction but not necessarily how severe the reaction is. Food challenges are used to identify if someone has grown out of an allergic reaction to a substance, and should only be performed in supervised conditions such as in a hospital.