I am sure we have had similar questions on this subject relatively recently, but can find no reference on a quick search.
This previous post might be of some use;
http://www.theanswerbank.co.uk/Body-and-Soul/Health-and-Fitness/Question1130287.html
The biggest single problem with the York Testing protocol is that it tests for IgG antibodies from blood, whereas a true food allergy, and certainly the more serious IgG- mediated allergic responses, will not show up by this method.All an IgG test can reliably demonstrate is an exposure to certain foods or food groups - not an allergic response.
So IgG testing is considered unproven as a diagnostic tool and the results of such tests lack diagnostic utiiity.
This is what the European Academy of Allergy and Clinical Immunlogy have to say about IgG (4) testing;
"Testing for blood IgG4 against different foods is performed with large-scale screening for hundreds of food items by enzyme—linked immunosorbent assay-type and radioallergosorbent-type assays in young children, adolescents and adults. However, many serum samples show positive IgG4 results without corresponding clinical symptoms. These findings, combined with the lack of convincing evidence for histamine-releasing properties of IgG4 in humans, and lack of any controlled studies on the diagnostic value of IgG4 testing in food allergy, do not provide any basis for the hypothesis that food-specific IgG4 should be attributed with an effector role in food hypersensitivity.
In contrast to the disputed beliefs, IgG4 against foods indicates that the organism has been repeatedly exposed to food components, recognized as foreign proteins by the immune system. Its presence should not be considered as a factor which induces hypersensitivity, but rather as an indicator for immunological tolerance, linked to the activity of regulatory T cells. In conclusion, food-specific IgG4 does not indicate (imminent) food allergy or intolerance, but rather a physiological response of the immune system after exposition to food components. Therefore, testing of IgG4 to foods is considered as irrelevant for the laboratory work-up of food allergy or intolerance and should not be performed in case of food-related complaints."
(IgG(4) is the basis of the York Test)
An excellent article covering the whole area of IgG Food Allergy Testing can be found here;
http://www.sciencebasedmedicine.org/index.php/igg-food-intolerance-tests-what-does-the-science-say/
Personally, I think they have no diagnostic value and can only serve to lighten your wallet... :)