An allergy diagnosis is made by a very careful history, skin prick
tests, blood tests (Immuno-CAP RAST), Patch tests, Atopy Patch test & Oral Challenges.
A Double-Blind Placebo-Controlled (DBPC) Oral food Challenge is considered the “gold
standard” in the diagnosis of food allergies and some drug allergies.
The importance of taking a careful (thorough) history cannot be stressed enough,
because not only can a good history give the diagnosis on its own, but it guides
the Allergist towards stream-lined allergy testing. Random (battery) testing is
financially wasteful and also time consuming and potentially misleading. The allergy
tests should only be used to confirm a diagnosis that has been made based on the
The aim of oral food challenge is to study the consequences of food ingestion in
an objective way. It is well known that the perceived prevalence of food allergies
is much higher than the true prevalence, and this is due to the subjective symptoms
that are often associated with food ingestion. This is due to a conditioned (Pavlovian)
Clinical uses of oral challenges
To confirm a diagnosis of food allergy based on history and equivocal skin prick
tests & / or Immuno-CAP RAST-type tests. The new Immuno-CAP tests are so sensitive
& specific for diagnosing food allergies to milk, egg, peanut, fish, and wheat
that oral challenges to these foods are very rarely done to diagnose allergy.
To diagnose Food Intolerance, where skin tests & blood tests are expected to
be negative (not useful).
To see if a child has grown out of a food allergy, especially when the Immuno-CAP
tests are equivocal.
To re-assure patients who, would otherwise be unconvinced that their symptoms are
not attributable to the foods that they have developed a conditioned response to.
- To confirm drug allergies.
Principles of Oral Challenge
The food challenge should replicate normal food consumption in terms of amount and
state of the food. The period of observation after the ingestion should match the
time period between ingestion & onset of symptoms based on the history.
If co-factors like exercise or ingestion of drugs like NSAIDs are required for the
reaction, this has to be replicated in the challenge. For example in patients with
food-related exercise-induced anaphylaxis, the ingestion of the suspected food has
to be followed by exercise.
Risk of Anaphylaxis during oral challenges
- With a history of previous anaphylaxis related to the food being tested.
Foods like peanuts, nuts, milk, egg, and fish carry a greater risk of anaphylactic
shock than others.
Patients with atopic eczema are at increased risk for anaphylaxis after strictly
eliminating foods that they are allergic to, even though they never had anaphylaxis
to that food previously.
Because of the risk of anaphylaxis oral challenges should be done in an environment
that is set up to offer full resuscitation.
Pitfalls of Double-blind Placebo-Controlled (DBPC) oral food challenges
DBPC food challenges are considered the “gold standard” for diagnosis
of adverse reactions to foods, but do they mimic real-life exposure?
To stop a DBPC food challenge & declare it positive or negative symptoms should
be objective and /or repetitive.
Causes of false negative DBPC oral food challenge include:
- Inadvertent drug use during the challenge.
a short-term specific oral tolerance induction (SOTI) may be induced as increasing
amounts of the offended food are administered during a titrated oral food challenge.
Causes of false positive DBPC food challenge include:
- Difficulty in maintaining a strict diet throughout the oral challenge procedure.
The elimination diet implemented before the challenge might be responsible for the
“new” symptoms seen during the trial.