Allergy testing is a method to determine if a patient is allergic to various foods or substances in the environment and may be done by means of a skin test or by using a blood test.
Skin Tests
In a skin test, a small amount of a substance of concern is placed on the patient’s skin, either on the forearm or on the back. This is repeated for many substances such as foods, grasses, molds, animal dander, etc., and a record of the identity and location of each drop of liquid is kept. In addition a non-allergenic substance and a drop of histamine are added as controls. Following this a tiny metal spike is used to scratch the skin at the location of each droplet. After a designated length of time each area is examined to measure the size of any raised and red reaction of the skin. This test is effective for children or adults of any age.
Piercing the skin allows the potential allergen to pass into the body where it will produce an inflammation response and release histamine if the patient is allergic to that substance. The type of immune reaction that is being tested here is the IgE reaction where IgE is present and already bound to mast cells. Food intolerances that do not involve an inflammatory immune response will not give a positive test result. In addition, just because a positive result is observed with a skin prick test doesn’t mean that anaphylaxis or a severe allergic symptoms will occur when consuming that food.
Skin allergy testing has a high negative predictive value, which means that if no reaction is obtained then it’s almost certainly true that the patient is not allergic to the given substance. However these tests also have poor positive predictive value, meaning that just because a reaction is found during the allergy test it does not mean that the patient is for certain allergic to the substance. Therefore the results are most useful for finding out to which substances a patient is certainly not allergic. A positive test result for food allergy is not, in and of itself, diagnostic for food allergy.
It is important to stop taking any oral antihistamines for up to a week prior to conducting this test. The skin prick test is a safe and simple procedure, and the only physical downside is that it results in a lot of itchy spots on the body which may need to be treated with topical antihistamines.
Blood Tests
IgE blood tests look for the presence of Immunoglobulin E in the blood that is adapted to recognize a particular antigen. This test also has high negative and low positive predictive value, which means that a negative test will almost certainly rule out an allergen, but a positive result will not prove that a substance is problematic. Mildly elevated results are often encountered, especially in people with a lot of allergies. Positive results might be useful to consider in the context of an elimination diet but should not be relied upon, otherwise a patient may end up eliminating important sources of nutrition.
Various healthcare providers sometimes order extensive banks of blood allergy tests which might also include IgG and IgA results. The problem is that such testing leads to confusion and elimination of extensive lists of foods and may encourage malnutrition.
Most allergists and experts in IBS would say that IgG or IgA testing is meaningless and there is no evidence that this provides any useful information about true allergies. Naturopaths are particularly keen on such tests which are often aggressively marketed by testing laboratories. Evidence seems to support the idea that the presence of IgG antibodies indicates that the immune system has been exposed to a particular substance and has become adapted to it in a non-pathogenic way. This fits with the observation that people often find that their IgG blood tests are positive for many foods that they commonly eat. It might be the case that elevated IgG levels indicate that certain food proteins have at some point crossed through the gut barrier and the immune system has been exposed to them. But dietary intervention on the basis of these tests is very controversial.