Sometimes, even after performing skin prick and blood tests, an allergist is unable to arrive at a definitive diagnosis. In this case, he or she may recommend an oral food challenge (OFC). This is a highly accurate diagnostic test for food allergies.
Because this test can cause a serious allergic reaction, only an experienced allergist should conduct it. It should happen at a medical facility with the appropriate medications and equipment.
The medical community has developed thorough guidelines and standards for OFCs. Studies have shown that these tests are safe when handled in this way. Indeed, thousands of OFCs have been performed worldwide with an excellent safety record.
During the food challenge, the allergist feeds you the suspect food in measured doses. He or she will start with very small amounts that are unlikely to trigger symptoms. With each dose, the healthcare professional will watch you for a period of time for any signs of a reaction.
If you have no significant symptoms, you will gradually receive larger and larger doses. If you show signs of a reaction, the food challenge will stop.
With OFCs, most reactions are mild, such as flushing or hives. Severe reactions are uncommon. You will be given medications to relieve the symptoms if necessary.
If you have no symptoms by the end of the procedure, your allergist will rule out the food allergy. If the test confirms that you do have an allergy to the food in question, he or she will discuss ways you can avoid the food. Your doctor will also prescribe appropriate medications.
In some cases, OFCs are given to people who already have a known food allergy. For example, an allergist may order the test to find out if a patient has outgrown the allergy. In clinical trials, OFCs help researchers learn how well participants are responding to the treatment under study.
Double-Blind, Placebo-Controlled Food Challenge (DBPCFC)
This test is considered the “gold standard” for diagnosing a food allergy. The patient receives increasing doses of the suspected food allergen and a placebo (a harmless substance). The food allergen and placebo are given separately, either hours apart or on separate days. Because the allergen and the placebo look alike, neither you nor your doctor will know which one you are receiving—hence the term “double-blind.”
For example, if you are being tested for milk allergy, you may eat a piece of hamburger that contains milk powder, or a lookalike that has no milk in it. This process ensures that the test results are objective. Neither the patient’s anxiety nor the allergist’s preconceptions can sway the outcome.
Single-Blind Food Challenge
In this test, the allergist knows when you are receiving the allergen, but you do not know if the allergen is or is not in the food you are eating. Similar to the DBPCFC, the food allergen or placebo will be given either hours apart or on separate days.
Both you and your doctor know whether you are receiving an allergen in this type of challenge. It is most often done when a patient’s nervousness is unlikely to affect the results. An open food challenge is administered in one clinic visit. A single serving size of the allergen is divided into gradually increasing portion sizes over approximately one hour. You will then be observed for an additional one to three hours, to monitor for a reaction.