What are reliable food allergy testing methods?

To properly diagnose allergies to tree nuts, such as almonds, cashews and walnuts, physicians should having patients try to eat tiny amounts  of the food, found a study this month in the Annals of Allergy and Immunology.

Researchers analyzed data from over 150 tree nut oral food challenges conducted at one medical center. Patients were classified as the following: allergic to tree nuts if they had symptoms after tree nut ingestion; sensitized to tree nuts if they only had positive results on skin prick or serum-specific immunoglobulin E testing; or avoiding nuts despite no sensitization or clinical history. Patients weren't given an oral challenge to nuts for which they'd had an allergic reaction.

Overall, 86 percent of oral challenges were passed. Among those with a previous reaction to ingesting tree nuts, the passage rate of the oral challenge for other nuts to which they were sensitized was 76 percent. For those without a history of tree nut ingestion, the passage rate was 91 percent. Among patients who were allergic to peanuts (which are not tree nuts) and sensitized to tree nuts, 96 percent passed the tree nut oral challenge.

What about blood testing? Known to allergists for many years, food allergy testing, whether by skin prick testing or by blood testing, is by itself an inadequate test to diagnose food allergies.

The issue is that skin prick testing or specific IgE blood testing is fraught with false positives and false negatives. The sensitivity and specificity of these tests is known to be relatively low, meaning that many people with a positive skin or blood test to a particularly food may indeed be able to tolerate that food, while the reverse is also true, that a patient with a negative skin or blood test may still be allergic to that food. The reason for this is that these tests are really merely surrogate markers of allergy. In the case of a blood test, we are merely measuring the amount of a type of antibody that Is associated with allergies in the blood.

The gold standard for diagnosing a true food allergy is what we call a double blinded placebo controlled food challenge (DBPCFC). This means challenging a patient to a food in incremental doses whereby the patient is blinded to what he or she is getting, and the person administering the food is blinded as well. Besides the actual food, a placebo may instead be offered, but neither the tester nor the patient know which the patient is receiving.

During the test, the patient may demonstrate symptoms consistent with an allergic reaction or anaphylaxis, such as hives, itching, or may not develop symptoms at all. Once the test is completed, an analysis can show if there is indeed a food allergy.

However, the DBPCFC is difficult and sometimes impossible to administer in real life. It takes too much time and staff, the taste of the food has to be disguised (sometime using a medicinal capsule) so that the testee cannot tell the difference between food, and the ingestion of an adequate amount of food is needed for a valid test.

So in lieu of the DBPCFC, many food allergy centers or allergy practices offer open food challenges. This is where the food administered is known, and if the patient develops an allergic reaction, then there is a fairly high reliability that the food is the culprit. Food challenges, as illustrated in the article, confirm that other forms of testing often yield erroneous results.

One additional note of caution must be made. There are many reactions that are not consistent with an allergy such as memory loss, difficulty concentrating, headaches, hyperactivity or fatigue. These symptoms do not reflect an allergy.

In some cases, patients will resort to unreputable laboratories that offer IgG or IgG4 testing for foods. It should be emphasized that the presence of IgG to a particular food has no significance when it comes to allergy. These are bogus tests that exploit patients who are convinced that they have a food allergy, even though clinical history would refute this. These types of tests have no diagnostic value, and they should be avoided at all costs. They are often very expensive, not covered by insurance, and are a waste of the patient's money.