Interpreting the allergy test results

When a person presents with symptoms suggestive of allergy, then tests can either narrow down or rule out other causes. There are three medically accepted tests, which can be helpful to evaluate a patient who may have allergy. They are the skin prick tests (SPT), blood tests and challenge testing. However, the history of symptoms is very important and this needs to be taken into account when any of these tests are interpreted.


Blood tests and skin prick tests are methods that detect IgE antibodies (these are the allergic antibodies). Blood testing for IgE antibodies (also known as RAST testing) measures the levels of allergic antibodies directly in the blood. A positive skin test indicates indirectly the presence of IgE antibodies in the skin. Both methods complement each other and are often used together to help diagnose allergies. There are however some circumstances where skin testing is more suitable than blood testing and vice versa.




Skin test results are expressed as the diameter of the allergic wheal (in mm). Blood tests levels are expressed as concentrations of allergic antibodies in the blood (International Units/mL). The size of the blood test and the skin test do not indicate the severity of the allergy. They do however indicate the probability of being allergic


For example, a large skin prick test diameter to peanut of 10 mm and a high blood test level of 30 IU/mL means that the child almost certainly has peanut allergy


A skin test to peanut of 0 mm and a blood test level of 0 make it highly unlikely that the child is allergic. However, there is a possibility that your child is allergic despite negative tests, especially if your child has had an allergic reaction previously.