Some of the most valuable tool for diagnosing
food allergy include history, blood tes, skin test, and diet.
Food Allergy Diagnosis
After ruling out food intolerances and other health problems,
your healthcare provider will use several steps to find out if
you have an allergy to specific foods.
Detailed History
A detailed history is the most valuable tool for diagnosing
food allergy. Your provider will ask you several questions
and listen to your history of food reactions to decide if the
facts fit a food allergy.
• What was the timing of your reaction?
• Did your reaction come on quickly, usually within an
hour after eating the food?
• Did allergy medicines help? Antihistamines should
relieve hives, for example.
• Is your reaction always associated with a certain food?
• Did anyone else who ate the same food get sick? For
example, if you ate fish contaminated with histamine,
everyone who ate the fish should be sick.
• How much did you eat before you had a reaction? The
severity of a reaction is sometimes related to the amount
of food eaten.
• How was the food prepared? Some people will have a
violent allergic reaction only to raw or undercooked fish.
Complete cooking of the fish may destroy the allergen,
and they can then eat it with no allergic reaction.
• Did you eat other foods at the same time you had the
reaction? Some foods may delay digestion and thus delay
the start of the allergic reaction.
Diet Diary
Sometimes your healthcare provider can’t make a diagnosis
solely on the basis of your history. In that case, you may be
asked to record what you eat and whether you have
a reaction. This diet diary gives more
detail from which you and your
provider can see if there is a
consistent pattern in your
reactions.
Elimination Diet
The next step some healthcare
providers use is an elimination diet.
In this step, which is done under your provider’s direction,
certain foods are removed from your diet.
• You don’t eat a food suspected of causing the allergy,
such as eggs.
• You then substitute another food—in the case of eggs,
another source of protein.
Your provider can almost always make a diagnosis if the
symptoms go away after you remove the food from your diet.
The diagnosis is confirmed if you then eat the food and
the symptoms come back. You should do this only when
the reactions are not significant and only under healthcare
provider direction.
Your provider can’t use this technique, however, if your
reactions are severe or don’t happen often. If you have a
severe reaction, you should not eat the food again.
Skin Test
If your history, diet diary, or elimination diet suggests a
specific food allergy is likely, your healthcare provider will
then use either the scratch or the prick skin test to confirm
the diagnosis.
During a scratch skin test, your healthcare provider will
place an extract of the food on the skin of your lower arm.
Your provider will then scratch this portion of your skin
with a needle and look for swelling or redness, which would
be a sign of a local allergic reaction.
A prick skin test is done by putting a needle just below the
surface of your skin of the lower arm. Then, a tiny amount
of food extract is placed under the skin.
If the scratch or prick test is positive, it means that there is
IgE on the skin’s mast cells that is specific to the food being
tested. Skin tests are rapid, simple, and relatively safe.
You can have a positive skin test to a food allergen, however,
without having an allergic reaction to that food. A healthcare
provider diagnoses a food allergy only when someone has a
positive skin test to a specific allergen and when the history
of reactions suggests an allergy to the same food.
Blood Test
Your healthcare provider can make a diagnosis by doing a
blood test as well. Indeed, if you are extremely allergic and
have severe anaphylactic reactions, your provider can’t use
skin testing because causing an allergic reaction to the skin
test could be dangerous. Skin testing also can’t be done if
you have eczema over a large portion of your body.
Your healthcare provider may use blood tests such as the
RAST (radioallergosorbent test) and newer ones such as the
CAP-RAST. Another blood test is called ELISA (enzyme
linked immunosorbent assay). These blood tests measure the
presence of food-specific IgE in your blood. The CAP-RAST
can measure how much IgE your blood has to a specific
food. As with skin testing, positive tests do not necessarily
mean you have a food allergy.
Double-Blind Oral Food Challenge
The final method healthcare providers use to diagnose food
allergy is double-blind oral food challenge.
• Your healthcare provider will give you capsules containing
individual doses of various foods, some of which are
suspected of starting an allergic reaction. Or your provider
will mask the suspected food within other foods known
not to cause an allergic reaction.
• You swallow the capsules one at a time or swallow the
masked food and are watched to see if a reaction occurs.
In a true double-blind test, your healthcare provider is also
“blinded” (the capsules having been made up by another
medical person). In that case your provider does not know
which capsule contains the allergen.
The advantage of such a challenge is that if you react only to
suspected foods and not to other foods tested, it confirms the diagnosis. You cannot be tested this way if you have
a history of severe allergic reactions.
In addition, this testing is difficult because it takes a lot of
time to perform and many food allergies are difficult to
evaluate with this procedure. Consequently, many healthcare
providers do not perform double-blind food challenges.
This type of testing is most commonly used if a healthcare
provider thinks the reaction described is not due to a specific
food and wishes to obtain evidence to support this. If your
provider finds that your reaction is not due to a specific
food, then additional efforts may be used to find the real
cause of the reaction.
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