How useful is an allergy test?

Many companies now provide a service that is based on immunoglobulin G (IgG) testing. These tests are easy to perform and cheap and thus this is an attractive business model: the athlete does a simple home test, send the sample to a company and receives a report a few days/weeks later which will tell him or her which foods to avoid. Sometimes athletes also receive a shopping list of supplements they should get from the same company "to avoid further problems". These companies report IgG responses to multiple foods (usually around 100 foods with a single panel test) and claim that removal of foods with high IgG levels can lead to improvement of a range of symptoms. So what is this test and can we trust it?

IgG

As mentioned above these home tests measure IgG. IgG is one of the 5 classes of immunoglobulin (Ig) proteins - also known as antibodies - produced by cells of the immune system called B lymphocytes. The other 4 classes are IgA, IgD, IgE and IgM. Each class has distinct characteristics and roles in defending the body against pathogens, toxins and other foreign material that enter the body. IgG is by far the most abundant antibody in the blood. IgG is produced in increased amounts when immune cells encounter foreign molecules (known as antigens) which are usually proteins in the body fluids that come into contact with mucosal surfaces. IgG molecules specific to a particular antigen bind to it which acts as a signal for other immune cells to eliminate the foreign material.

After a meal, a small amount of some complete or partially digested dietary proteins in our gut can pass into the body fluids (e.g. blood or lymph). Here they act as antigens but they only very rarely cause allergic reactions because they are eliminated quickly by the IgG mediated mechanism described above.


IgE

There are, however, some proteins in cereal grains and nuts that can cause true allergies but these cases are very rare. Furthermore, these true food allergies are mediated by a specific IgE response and can produce a massive inflammatory response which can be fatal. This type of response does not involve IgG. Hence, only levels of IgE to specific antigens are a useful test of a suspected food allergy or any other allergy (e.g. to pollen or dust mites) for that matter.


There are some proteins in cereal grains and nuts that can cause true allergies but these cases are very rare

Increased levels of IgG to specific antigens can be considered as a normal response to the intake of certain food proteins and are thought to help with tolerance to the food items that contain them. IgG tests are therefore not useful in detecting potentially harmful, irritant or allergy-inducing food items. Rather high IgG levels actually indicate ongoing tolerance to any infiltrating food proteins.


IgG home test packs

An IgG home test pack for so-called food sensitivity usually contains sterile lancets to allow you to take a fingerpick blood sample which you apply to a collection card which you then place in a sealable bag and post back to the test pack supplier. They will then measure the IgG levels in your blood that are specific to around 100 different food items. The results are usually available within a few days and will be supplied in a qualitative manner indicating if your IgG is absent/low, medium or high for each food item. High levels are usually described as indicating an adverse reaction or sensitivity to a food item rather than an allergy followed by the suggestion that you should at least temporarily remove the “offending” food item from your diet (and, of course pay for another test a few weeks later to see if this has solved your “food sensitivity problem”).


Do they work?

As described above, high specific IgG levels are extremely unlikely to indicate any problem you may have with certain foods. Most professional societies stress that the presence of IgG is a normal response of the immune system to the exposure to food. The general scientific consensus is that the data obtained from IgG blood tests is not scientifically robust and most allergy specialists consider such te