Allergy Testing-TOPICS
| Skin prick testing | Intradermal skin testing | Measuring Total IgE |
| RAST Blood testing | Eosinophil counts | Patch testing |
| Challenge testing | Unproven diagnostic tests |
INTRODUCTION
Avoiding allergic triggers is an important part of
allergy and asthma management, as well as controlling allergic
reactions to venomous insects and foods. Your doctor will normally
ask you a series of questions in order to identify factors in
your environment that may be triggering your symptoms. Skin
testing helps your doctor to confirm your sensitivity so that
appropriate avoidance advice can be given. This may be followed
by allergy testing (using skin tests or RAST) to help confirm
or exclude the presence of sensitivity. Results are available
within 15 20 minutes. It is important to note that all test
results must be interpreted in the context of a careful history.
Preparations for skin testing
Medications with antihistamine-like actions (antihistamines,
some cold remedies, tricyclic antidepressants) are normally withheld
for 3-5 days before testing as these will interfere with the results
of testing. You may also be advised to avoid creams and moisturisers
on your forearms for a similar period of time to reduce the likelihood
that allergen extracts will "run" into each other.
How testing is done
Skin prick testing is most commonly performed on the forearm,
although the back is sometimes used. The arm is first cleaned
with alcohol, then a drop of commercially-produced allergen extract
is placed onto a marked area of skin. Using a sterile lancet,
a small prick through the drop is made. This allows a small amount
of allergen to enter the skin. If you are allergic, a small mosquito-like
lump will appear at the site of testing over 15-20 minutes. Skin
tests are slightly uncomfortable, but is usually well tolerated
and accurate, even in small children and infants.
Skin test lancet (scale in mm)
Skin
testing in infants and children
Skin tests are slightly uncomfortable, but is usually well tolerated
and accurate, even in small children and infants.
Rationale for skin prick testing
Underneath the lining of the skin, gut, lungs, nose and eyes
are mast cells. These are designed to kill worms and parasites
and contain granules filled with irritant chemicals (including
histamine). Mast cells are also armed with proteins called IgE
antibodies, which act as remote sensors in the local environment.
A person allergic to house dust mite, for example, will have IgE
antibodies capable of recognising the shape of dust mite allergens,
in much the same way that a lock "recognises" the shape
of a key. When this happens, mast cells are triggered to release
their contents into the tissues, triggering an allergic reaction.
Mast cells showing irritant granules
Selecting
the allergens
In patients with hay fever or asthma,
testing usually includes house dust mite, cat and dog dander (perhaps
other animals if contact occurs), mold spores and relevant grass
pollens, weed and tree pollens and sometimes occupational allergens
in those with hay fever or asthma. Testing can also be used to
confirm suspected food allergy and stining insect venom allergy.
Skin testing has been shown to improve the accuracy of diagnosis
in clinical studies.
While food allergy is more common in people with asthma than the general population, it is a rarely triggers asthma alone. Instead, asthma is usually accompanied by severe itchy hives, swelling or dizziness or gut cramps. Since irrelevant small false positive reactions to food occur in up to 5 per cent of subjects, and as such results are often misleading, testing for food sensitivity is not considered routine in the absence of a history suggesting food allergy as well.
Skin testing is not a reliable way of confirming suspected reactions to aspirin or food additives, and you will need to discuss such concerns with your doctor.
Reasons for testing
Skin testing should be performed in all patients suspected
of suffering from severe episodic or persistent asthma, as well
as in those with suspected hay fever, or suspected allergic reactions
to stinging venomous insects or food allergy. There are no age
limitations for skin testing, although the very young and the
elderly may have diminished skin test reactions compared to other
subjects, and the very young (< 2 years) are only occasionally
sensitised to inhaled allergens. Since pregnant subjects may experience
uterine (womb) contractions if they suffer a severe allergic reaction
to testing, testing is usually only be performed in this group
if the immediate benefit of testing is considered to outweigh
the risk.
Advantages of skin testing
Skin prick testing is the most convenient and least expensive
method of allergy testing and results are available within 20
minutes. This allows you to discuss the results with your doctor
at the time of testing.
Side-effects and risks of skin testing
Skin tests are slightly uncomfortable, but usually well tolerated,
even by small children. Local itch and swelling normally subsides
within 1-2 hours. More prolonged or severe swelling may be treated
with an oral antihistamine, topical corticosteroid cream and an
ice pack. Occasional patients will experience feel dizzy or light-headed
and need to lie down. Severe allergic reactions from allergy testing
in asthma are very rare.
Alternative methods such as scratch testing have generally been abandoned because of poor reproducibility, and greater patient discomfort.
Intradermal skin testing is practised in some countries. A small amount of very dilute allergen is injected into the upper layers of the skin, normmly using a diabetic insulin syringe. It is a more uncomfortable test than skin prick testing. While it is more sensitive, is more likely to lead to false positive and clinically irrelevant results. For this reason, it is more commonly used for evaluation of patients with sensitivity to antibiotics or insect venom.
MEASURING TOTAL IgE
Levels of total IgE antibody can be estimated from a blood sample.This
is often (but not always) raised in people with allergies and
in those with internal parasites. It is often very high in people
with eczema an a condition known as allergic bronchopulmonary
aspergillosis. Nevertheless, an elevated IgE does not prove that
symptoms are due to allergy, and a normal IgE level does not exclude
allergy. Thus measuring IgE levels has a limited role to play
in assessing patients with possible allergic conditions.
RAST BLOOD TESTING
The amount of IgE directed against specific allergens can be measured
with a blood test. In general, it is less likely to accurately
detect allergies than the skin test, and may give misleadingly
falsely positive or false negative results. Australian Medicare
rebates are also only available for a total of 4 individual allergens
or allergen mixes. For this reason, testing is often performed
when skin testing is not easily available, when skin condition
such as severe eczema or dermographism prevent accurate testing,
or when the patient is taking medications (such as antihistamines
or tricyclic antidepressants) that interfere with accurate testing.
EOSINOPHIL COUNTS
Eosinophils are specialized white cells that are designed to kill
worms and parasites. They also can cause inflammation in the tissues
in allergy. High levels are sometimes seen in blood samples from
people with hay fever, asthma and atopic eczema, as well as in
a number of less common conditions. Nevertheless, an elevated
eosinophil count does not prove that symptoms are due to allergy,
and a normal eosinophil count does not exclude allergy. Thus measuring
eosinophil counts has a limited role to play in assessing patients
with possible allergic conditions.
PATCH TESTING
Patch testing is useful for the assessment of contact
allergic dermatitis, such as that triggered by nickel metal,
cosmetic preservatives or various plants. Using hypoallergenic
tape, commercial standardised allergen paste is applied to a rash
free area of skin, most commonly the back. The tapes are normally
left in place for 48 hours, being kept dry the entire time. The
test site is then "read" at various time intervals,
looking for evidence of an eczema-like rash that might indicate
sensitivity to a particular allergen.
Patch test allergens (left) and patch tests applied to the back (right).
CHALLENGE TESTING
To establish a diagnosis where there is doubt as to the cause
of a severe allergic reaction, challenge testing may sometimes
be required. This will normally only be performed using foods
or medication under the supervision of a specialist in allergy
and clinical immunology with appropriate resuscitation facilities
available.
UNPROVEN DIAGNOSTIC TESTS
Mainstream allergy diagnosis and treatment is based on a sound
scientific knowledge of the components of the immune system and
how they work together to sometimes trigger allergies. Conventional
allergy testing involves well validated diagnostic methods and
proven methods of treatment. By contrast, a number of unproven
tests have been proposed for evaluating allergic patients including
cytotoxic food testing, Alcat tests, kinesiology, Vega testing,
electrodermal testing, pulse testing, reflexology and hair analysis.
There is little or no scientific rationale for these methods.
Results are not been reproducible when subject to rigorous testing
and do not correlate with clinical evidence of allergy. As results
may lead to misleading advice or treatments, their use is not
advised. No rebate is available under the Australian Medicare
system for these tests. This topic will be the subject of a separate article. These tests
are also discussed at the web site Quackwatch.
References
