Bronchial challenge
test
Skin
prick and blood tests are most commonly used to identify allergy to
inhaled substances, as they generally correlate well with bronchial
provocation tests. However, there may be special instances such as
in occupational asthma or in investigative work, when specific challenge
is indicated. For example, when it is suspected that asthma is being
caused by a substance encountered at work, and when skin and blood tests
are negative, suggesting that mechanisms other than the normal type are
operating, then a challenge test may be the only way of confirming the
allergy.
Conjunctival challenge
test
Skin
prick and blood tests are most commonly used to identify allergy to
airborne allergens that cause eye symptoms. However, there may be
special instances when an allergy is expected that predominantly affects
the eyes, but the standard tests fail to show it. For example, when
it is suspected that asthma is being caused by a substance encountered at
work, and when no commercial extract of that substance is available, then
a challenge test may be the only way of confirming the allergy. In
other situations the standard allergy test may fail to show up an allergy
to a substance strongly suspect of causing the symptom possibly because a
different allergic mechanism is operating, or possibly because the
sensitivity is confined only to the eye and therefore does not show up in
the skin or blood.
Conjunctival
provocation begins with administration of a drop of diluent solution into
one conjunctival sac; this challenge will detect non-specific responses.
Over the next 15 minutes eye and nose symptoms are recorded. If
there are no clinical symptoms then an allergen-containing solution is
deposited into the nose. The dose of allergen is increased at
15-minutes interval until symptoms or signs develop, the strength of
solution that first provokes symptoms then giving a clue as to the degree
of sensitivity to that substance.
Drug challenge
test
If
it is suspected that a particular drug or group of drugs is causing a set
of allergic symptoms, and if the usual tests do not confirm allergy (in
the case of drug reactions this is often so) then a drug challenge test
might be the only way of either confirming the reaction, or, more
commonly, confirming that a particular drug can be tolerated without
problem.
This
kind of testing is only really needed when a particular drug is very
important to the patient. For example, a patient on insulin and
encountering possible insulin allergy needs to establish which insulin can
be used without problem. A patient who has had a reaction after
local anaesthetic needs to know which local anaesthetic is safe if a
doctor or dentist needs to use one in the future.
Food challenge
test
When
symptoms such as mouth itching or swelling, itchy rash, wheezing, vomiting
etc. occur quite shortly after eating a troublesome food, the likely
diagnosis is food allergy. A careful examination of the dietary
history usually suggests the likely food responsible. It may be
helpful to get the patient to keep a diet diary where symptoms and all
foods eaten are recorded side by side.
Undertaking
skin prick or allergy blood tests can in most cases be used to confirm
this suspicion. However, these tests are not 100% accurate and both
false positive (the test is positive, but the food in question causes no
symptoms) and false negative, (the food causes symptoms, but the test is
negative) results sometimes cause confusion.
On
occasions it is therefore necessary to use a food allergy test to confirm
that a food or foods is responsible for the symptoms. One simple
approach might be to ask the patients to avoid eating the suspect food or
foods for a while and see if symptoms subside. However, this kind of
evidence (particularly in the absence of supporting evidence from blood or
skin tests) may not be conclusive. For example, patients will often
attend an allergy clinics mistakenly believing that their freedom from
symptoms is the result of avoiding either one or a number of foods, when
the real answer is that no food was ever causing their symptoms. In
this situation a food challenge test may be the only way of reaching the
conclusion that it will be safe to allow the food to return to the diet.
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Medical
supervision
Please
note that the existence of food allergy (the
immediate kind) should only be tested for with a
food challenge test if the patient is under the care
of a doctor and in a hospital or clinic setting.
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This
kind of test will be carried out in hospital under
careful supervision. The food under suspicion will
first be wiped across the patients lower lip.
For example, if peanut is being tested, a small
quantity of crushed peanut will be used. The
patients will then be monitored for 20 minutes to
see if any redness, itching, swelling or eye or nose
symptoms occur. If not, a small amount of
peanut the size of a rice grain) will be introduced
into the mouth. Again the patient is checked
for twenty minutes. Next a whole peanut is
tested, then two peanuts and then eight peanuts.
Finally,
if there has been no reaction at all following a cumulative dose that adds
up to a normal portion, the patient goes home, but watches out to see if
any delayed reaction occurs in the following 24 hours.
