A double-blind placebo-controlled food challenge (DBPCFC) is among the most valuable diagnostic tools available for confirming potential food allergies that have been identified by other testing methods (like prick tests or RAST tests). It involves ingesting some of the likely allergen and observing for signs of a reaction.
In a placebo-controlled test, patients will ingest some foods that include the likely allergen and some that don't. "Double-blind" refers to a method of testing in which neither the allergist nor the patient is certain whether the substance ingested contains the likely allergen.
Why and How Food Challenges Are Used to Confirm Other Diagnostic Tests:
Food challenges are used for many of the same reasons as elimination diets. Because many diagnostic tests can return false positives, a positive result on a food challenge is considered proof of a food allergy.
Food challenges are placebo-controlled, so neither the allergist nor the patient knows whether the capsule being administered contains the likely allergen. This means that if a patient reacts consistently to the allergen (but not the placebo), the results are reliable.
Food challenges are especially useful in determining whether a child has outgrown a food allergy.
Differences Between Food Challenges and Elimination Diets:
While elimination diets are better suited for narrowing down potential food allergies, or as a diagnostic tool for food intolerances, DBPC food challenges have a few advantages over elimination diets for confirming food allergies:
- Capsules can be administered in increasing dosages, to allow doctors to pinpoint at which dosage a reaction may be triggered.
- It is far easier to administer a food challenge in a medical setting, thereby allowing doctors to assist in the event of a complication.
- There is less potential for cross-contamination in a food challenge, leading to less ambiguous results.
How Double-Blind Placebo-Controlled Food Challenges Work:
Probable allergens for food challenges are administered in a form that conceals their texture and flavor. Usually this is a capsule bound in an allergen-free matrix. Placebo capsules, administered about half of the time during testing, are sugar pills. Patients may be given capsules every 30 to 60 minutes or even more, at the tester's discretion.
During the test, the patient is observed for symptoms. The test ends if symptoms appear or when the allergist decides that enough allergen has been ingested, and enough time has passed, that no reaction is likely to occur. Testing usually takes four to eight hours.
Your allergist will discuss with you the risks and benefits of a DBPC food challenge, but the major risk is a severe reaction. The chances of a reaction serious enough to require asthma medication or epinephrine occurring may range from 1% to as high as 10%. However, because food challenges generally are administered in hospitals or medical offices, the risk of life-threatening reactions is considered low.
Allergists generally do not administer food challenges to patients with a history of anaphylaxis, or with poorly controlled asthma. This is because the risk of severe reactions is considered too great.
Accuracy of Food Challenges:
Food challenges are the "gold standard" of food allergy diagnosis. But they are not 100% accurate. It is possible for DBPC food challenges to return false negatives, false positives, and ambiguous results.
False negatives --- that is, no reaction to a food that is actually an allergen --- can occur for several reasons:
- Some allergic reactions are exercise-induced or require another circumstance (a psychological, physical, or medical factor) to show up. These circumstances are called augmentation factors and are difficult to test for in a food challenge.
- Certain drugs (like antihistamines) the patient may have been taking prior to the test may interfere with the test, causing a false negative.
- Patients can, in some cases, develop a short-term oral tolerance induction, or SOTI, to the food they're being tested for. This means that the body may become accustomed to the small amounts of food delivered in the capsules and may not react as the dosage increases. SOTI, however, lasts for short periods of time, and is by no means a "cure" for food allergies.
False positives generally occur due to ingestion of another food during the course of the test. If the test is taken over a long period of time, the person being tested will need to eat. While food taken during a long test should be prepared to be free of allergens, mistakes can alter the test results.
Ambiguous results occur when:
- The patient experiences symptoms that are not typical allergy symptoms (like vomiting or heart palpitations);
- Symptoms occur several hours or more after the allergen capsule was administered; or
- The patient experiences a mild reaction (like hives around the mouth), especially if it cannot be repeated or does not get more severe with higher doses.
Despite these potential issues, DBPC food challenges are considered far more accurate than blood tests or prick tests.
Niggemann, Bodo, and K. Beyer. "Pitfalls in Double-Blind Placebo-Controlled Food Challenges." Allergy July 2007 62(7):729-32. 25 Jun 2007. Roberts, S. "Challenging Times for Food Allergy Tests." Archives of Disease in Childhood 2005(90):564-66. 25 Jun 2007. Roberts, S. "Food Challenge Tests." Archives of Disease in Childhood 2005(90):1207-08. 25 Jun 2007.
Niggemann, Bodo, and K. Beyer. "Pitfalls in Double-Blind Placebo-Controlled Food Challenges." Allergy July 2007 62(7):729-32. 25 Jun 2007.
Roberts, S. "Challenging Times for Food Allergy Tests." Archives of Disease in Childhood 2005(90):564-66. 25 Jun 2007.
Roberts, S. "Food Challenge Tests." Archives of Disease in Childhood 2005(90):1207-08. 25 Jun 2007.