Skin Prick or Scratch tests are the easiest and least expensive, but may not pick up all allergies (false negatives) or may show false positives. A person does not have to be actively eating the food for this test to be accurate, according to convential Allergists. Allergy medications (especially antihistamines like Benadryl, Claritin, Zyrtec, etc.) will interfere with accurate results of this test, since most allergy medications are designed to treat allergic skin reactivity (hives, rashes).
RAST IgE blood tests look for antibodies to IgE or immediate-reaction type allergens (pollens, foods, etc.) but can have false negatives or positives. A person may not have to be actively eating the food being tested for, but I do know food-specific IgE antibody levels can drop with avoidance, even though the person gets a reaction still when they eat the food. Conventional Allergists will tell you that a person usually doesn't lose IgE reactivity to foods, thus they believe that the IgE RAST is accurate and the antibodies persist long after avoidance. Allergy medications usually won't interfere with the results of this test, since the test is looking for antibodies instead of a specific reaction. However, Xolair may interfere with accurate results, since the drug binds directly to IgE antibodies, which may affect how the test turns out.
Intra-Dermal Skin Testing is not done commonly anymore, because it is believed to be more risky for triggering anaphylaxis from the test serums. But this test can have more accurate results than Skin Prick or Scratch testing. There can still be false negatives or positives with this test. The person does not have to be actively eating the food being tested for. Anti-allergy medications will interfere with accurate results.
Skin Patch Testing is a relatively new way to check for delayed food allergies, although it has been used for a long time by dermatologists to detect contact reactivity to chemicals, metals, etc. False negatives are probably more of a problem than false positives. Often, people react to the tape, which may skew the results. I don't know if the person has to have eaten the foods being tested for recently or not for this test to be accurate. Anti-allergy medications may interfere with the accuracy of this test.
Other less well-known tests:
Intolerance testing is usually done via ELISA blood tests looking for IgG class 4 antibodies to foods looking for delayed reactivity. This test also can have false negatives or positives. The patient must be actively eating the foods tested for, or the test is not likely to be as accurate. That said, I did have antibodies show up for foods I had not eaten in years, which confirms for me the severity of my reactivity to those foods. I don't know if anti-allergy medications will interfere with this test, but it is possible.
Stool IgA Tests can be used to look for autoimmune reactions to gluten, and for antibodies to diary, eggs, yeast, or soy. The only lab I know of doing this test is working on developing other food tests as well. False negatives can occur in the setting of IgA deficiency, and the person must have eaten the food(s) being tested for within the previous few months for the test to be accurate. Anti-allergy medications may not interfere with this test, but Prednisone and mast cell stabilizers may.
Muscle Testing/Kinesiology ~ subjective test, therefore less accurate than above tests but can show non-immune reactions to things. A related technique, called NAET, uses a combination of muscle testing and acupuncture to treat allergies to a wide variety of foods & other substances. False negatives & positives are common. I do not know if the person must be actively eating the foods tested for in order for this test method to be accurate. Anti-allergy medications do not interfere with this test.
Electro-Dermal Screening ~ uses acupuncture maridian energy flow in presence of test substance (food, pollen, etc.). Can be fairly accurate, but also may have false negatives (false negatives are way more common than false positives in my experience). False negatives may occur if the person hasn't eaten the food in a long time, or if the technician has poor technique. Metal allergy may also interfere with the results, since the person has to hold a metal bar in his/her hand to "complete the circuit" for testing purposes - however, a surrogate may be used in this situation. Anti-allergy medications do not interfere with this test.
ALCAT ~ blood test looks for any reactivity (any Ig class) to foods mixed with blood sample. Some think this is more accurate than IgG ELISA or IgE RAST blood tests, others think it is a bunch of bunk. I have no experience with this method. May have false positives or negatives. I don't know if the person has to have eaten the foods being tested for or not with this test. Anti-allergy medications probably would interfere with this test.
The Gold Standard is the double-blind, placebo-controlled, oral food challenge (DBPCOFC), in which the test substance is hidden in unmarked capsules mixed with capsules containing only placebo, and administered one at a time to the patient. The patient is then observed for ~20 minutes to see if any symptoms appear. This method can have accuracy problems, especially for detecting delayed reactivity. Anti-allergy medications would definately interfere with this test, since the whole point is to observe any reaction caused by the ingestion of the test food, and the whole point of allergy medications is to block such reactions from occurring, or at least decrease the severity of them.
Cooking the food vs. using the regular allergy extracts may result in different test results. Also, test food extract serums are not standardized, so you may get different results at different doctor's offices for the same food allergy skin or patch test. And also the test food extract serums can break down over time, and thus become useless for allergy testing. Test food serums made from fruits & vegetables are more prone to breaking down than serums made from other foods. Also, the test serums are usually made from organic foods to remove the possibility of additives, pesticides, preservatives, etc., from affecting the results. So as a result, you may be told you are not allergic to foods you do get reactions from, when in fact your reactions are caused by the chemical additives instead (this happened to my brother with oranges, so I know it can happen). Your best option then is to test reactivity to organic versions of the food to confirm whether the test was a false negative or whether the chemicals are really the culprit.
Also, most food allergy tests will not pick up pollen-food cross-reactivity, so you may need to keep a food journal and note any changes in symptoms to pinpoint these kinds of reactions. They are not usually life-threatening, but can be quite uncomfortable (personal experience strongly suggests that some foods that cross-react with pollens really should be avoided when possible during the pollen season).
I hope this information is helpful. Take care,
Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD (Lupus?), Osteoporosis, Anemia, T- & B-Cell Lymphopenia, malabsorption/malnutrition, Lyme Disease (Igenex Lab IgM WesternBlot positive/CDC negative), etc.
Meds: Pulmicort, Injectable Vitamin B12, Herbs, Nutritional Supplements, Homeopathy.