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The allergy tests normally used by doctors are the Skin Prick Test and the Specific IgE Blood Test, previously known as RAST. These tests normally give the same result although one advantage of skin prick testing is that the test result is immediately available whilst the patient is still in the clinic. Although called allergy tests, a positive result to either test only shows that the immune system is sensitised to the allergen, it does not diagnose the allergy. It sometimes happens that people with positive allergy tests are completely well and free from allergy.
For this reason, an allergy focused medical history is essential when diagnosing an allergy. After all, unless the correct test has been requested in the first place, allergy test results are meaningless. This is especially important in the case of suspected food allergy when an inaccurate diagnosis may commit the patient to lifelong, but unnecessary, food avoidance.
This is why allergy tests are unsafe if interpreted by people who have had no training in allergy. It also explains why it is important not to test for every known allergy without reference to an allergy-focused medical history, an approach that would inevitably lead to errors in diagnosis.
A positive result in someone who does not have an allergy is called a false positive whilst a negative result in someone who does have an allergy is called a false negative.