Be Cautious with Allergy Tests
Reported January 3, 2012
(Ivanhoe Newswire) – Two leading allergists urge
clinicians to use caution when ordering allergy tests
and to avoid making a diagnosis solely based on the
results.
Robert Wood of the Johns Hopkins Children's Center and
Scott Sicherer of Mt. Sinai Hospital in New York warn
that blood tests and skin-pricking tests should be used
only to confirm suspicion and never to look for
allergies in an asymptomatic patient.
Test results, they add, should be interpreted in the
context of a patient's symptoms and medical history. If
a food allergy is suspected, Sicherer and Wood advise,
the patient should undergo a food challenge — the gold
standard for diagnosis — which involves consuming small
doses of the suspected allergen under medical
supervision.
Skin tests and blood tests are proxies that detect the
presence of IgE antibodies, immune-system chemicals
released in response to allergens. Skin testing involves
pricking the skin with small amounts of an allergen and
observing if and how the skin reacts. A large hive-like
wheal at the injection site signals that the patient's
immune system has created antibodies to the allergen.
Blood tests, on the other hand, measure the levels of
specific IgE antibodies circulating in the blood.
These tests can tell whether someone is sensitive to a
particular substance but cannot reliably predict if a
patient will have an actual allergic reaction, nor can
they foretell how severe the reaction might be, the
scientists say. Many people who have positive skin tests
or measurably elevated IgE antibodies do not have
allergies, they caution
"Allergy tests can help a clinician in making a
diagnosis but tests by themselves are not diagnostic
magic bullets or foolproof predictors of clinical
disease," Wood was quoted as saying. "Many children with
positive tests results do not have allergic symptoms and
some children with negative test results have
allergies."
Undiagnosed allergies can be dangerous, even fatal, but
over-reliance on blood and skin tests can lead to a
misdiagnosis, ill-advised food restrictions or
unnecessary avoidance of environmental exposures, such
as pets.
In addition, the researchers caution, physicians should
be careful when comparing results from different tests
and laboratories because commercial tests vary in
sensitivity.
In their report, the scientists say, skin and blood
tests can and should be used to confirm a suspected
allergic trigger after observing clinical reactions
suggestive of an allergy. Also, monitor the course of
established food allergies via periodic testing. Levels
of antibodies can help determine whether someone is
still allergic, and progressively decreasing levels of
antibodies can signify allergy resolution or outgrowing
the allergy. Skin and blood tests can confirm an allergy
to insect venom following a sting that causes
anaphylaxis, a life-threatening allergic reaction marked
by difficulty breathing, lightheadedness, dizziness and
hives. Lastly, skin and blood tests can determine
vaccine allergies (skin tests only).
SOURCE: Pediatrics, published online December, 2011
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