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To diagnose food allergy a doctor must first determine if the patient is
having an adverse reaction to specific foods. This assessment is made
with the help of a detailed patient history, the patient’s diet diary,
or an elimination diet.
The first of these techniques is the most valuable. The physician sits
down with the person suspected of having a food allergy and takes a
history to determine if the facts are consistent with a food allergy.
The doctor asks such questions as:
– What was the timing of the reaction? Did the reaction come on quickly,
usually within an hour after eating the food?
– Was allergy treatment successful? (Antihistamines should relieve hives,
for example, if they stem from a food allergy.)
– Is the reaction always associated with a certain food?
– Did anyone else get sick? For example, if the person has eaten fish
contaminated with histamine, everyone who ate the fish should be sick.
In an allergic reaction, however, only the person allergic to the fish
becomes ill.
– How much did the patient eat before experiencing a reaction? The
severity of the patient¹s reaction is sometimes related to the amount of
food the patient ate.
– How was the food prepared? Some people will have a violent allergic
reaction only to raw or undercooked fish. Complete cooking of the fish
destroys those allergens in the fish to which they react. If the fish is
cooked thoroughly, they can eat it with no allergic reaction.
– Were other foods ingested at the same time of the allergic reaction?
Some foods may delay digestion and thus delay the onset of the allergic
reaction.
Sometimes a diagnosis cannot be made solely on the basis of history. In
that case, the doctor may ask the patient to go back and keep a record
of the contents of each meal and whether he or she had a reaction. This
gives more detail from which the doctor and the patient can determine if
there is consistency in the reactions.
The next step some doctors use is an elimination diet. Under the
doctor’s direction, the patient does not eat a food suspected of causing
the allergy, like eggs, and substitutes another food, in this case,
another source of protein. If the patient removes the food and the
symptoms go away, the doctor can almost always make a diagnosis. If the
patient then eats the food (under the doctor’s direction) and the
symptoms come back, then the diagnosis is confirmed. This technique
cannot be used, however, if the reactions are severe (in which case the
patient should not resume eating the food) or infrequent.
If the patient’s history, diet diary, or elimination diet suggests a
specific food allergy is likely, the doctor will then use tests that can
more objectively measure an allergic response to food. One of these is a
scratch skin test, during which a dilute extract of the food is placed
on the skin of the forearm or back. This portion of the skin is then
scratched with a needle and observed for swelling or redness that would
indicate a local allergic reaction. If the scratch test is positive, the
patient has IgE on the skin’s mast cells that is specific to the food
being tested.
Skin tests are rapid, simple, and relatively safe. But a patient can
have a positive skin test to a food allergen without experiencing
allergic reactions to that food. A doctor diagnoses a food allergy only
when a patient has a positive skin test to a specific allergen and the
history of these reactions suggests an allergy to the same food.
In some extremely allergic patients who have severe anaphylactic
reactions, skin testing cannot be used because it could evoke a
dangerous reaction. Skin testing also cannot be done on patients with
extensive eczema.
For these patients a doctor may use blood tests such as the RAST and the
ELISA. These tests measure the presence of food-specific IgE in the
blood of patients. These tests may cost more than skin tests, and
results are not available immediately. As with skin testing, positive
tests do not necessarily make the diagnosis.
The final method used to objectively diagnose food allergy is
double-blind food challenge. This testing has come to be the “gold
standard” of allergy testing. Various foods, some of which are suspected
of inducing an allergic reaction, are each placed in individual opaque
capsules. The patient is asked to swallow a capsule and is then watched
to see if a reaction occurs. This process is repeated until all the
capsules have been swallowed. In a true double-blind test, the doctor is
also “blinded” (the capsules having been made up by some other medical
person) so that neither the patient nor the doctor knows which capsule
contains the allergen.
The advantage of such a challenge is that if the patient has a reaction
only to suspected foods and not to other foods tested, it confirms the
diagnosis. Someone with a history of severe reactions, however, cannot
be tested this way. In addition, this testing is expensive because it
takes a lot of time to perform and multiple food allergies are difficult
to evaluate with this procedure.
Consequently, double-blind food challenges are done infrequently. This
type of testing is most commonly used when the doctor believes that the
reaction a person is describing is not due to a specific food and the
doctor wishes to obtain evidence to support this judgment so that
additional efforts may be directed at finding the real cause of the
reaction.
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