A completely accurate and reliable diagnostic test for any food allergy does not exist. Skin tests and RAST can give false results, but they can be useful as an aid to diagnosis. The only realiable way to investigate food allergy is by the exclusion / re-introduction of the suspected foods in the diet under medical supervision
Skin test
When cow’s milk allergy is suspected, a skin test can be done. Most allergic patients react positive to a skin test and specific IgE antibodies can also be found in the blood. Unfortunately, these tests can provide false results and they do not provide a decisive explanation for the appearance of the symptoms.
Oral provocation test
The only reliable test to confirm the diagnosis is the oral provocation test. In the first phase, all suspect foodstuffs, those with a high risk for allergic reactions, are excluded from the diet for a number of weeks. During the second phase the suspect foodstuffs are, one by one, introduced in the diet and the reactions are closely observed. When symptoms appear after the reintroduction of cow’s milk and again disappear after having been retracted, it’s possible to confirm a diagnose of cow’s milk protein allergy. If there is no reaction, another cause for the allergy must be searched for.