"Anaphylaxis is a severe and sudden allergic reaction when a person is exposed to an allergen. The most common allergens are eggs, peanuts, tree nuts (e.g. cashews), cow's milk, fish and shellfish, wheat, soy, certain insect stings and medications."
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FEATURE ARTICLE: How can you find out if you suffer from a food allergy?
Signs and symptoms
The clinical manifestations of IgE-mediated allergy to foods are extremely diverse.
They may involve the skin (urticaria), the respiratory system (asthma), and the oral
mucosa (swelling, redness) as well as the gastro-intestinal tract (nausea, vomiting,
pain and discomfort). Atopic dermatitis is also associated with food allergy.
Reactions can occur minutes to hours after exposure to the allergen. The most severe
of these reactions is anaphylactic shock, which combines several of those symptoms.
Typical signs can include peripheral vasodilatation leading to circulatory collapse,
accompanied by swelling of the skin and mucosal tissues (angioedema), which results in
breathing difficulties. Untreated anaphylactic shock can rapidly result in death. A
second severe, and potentially fatal, manifestation is Quincke's oedema, the typical
signs are swelling of the deep layers of the skin and mucous membranes, such as those of
the throat. Technically, it has been described as localised anaphylaxis.
Diagnosis
Diagnosis of food allergy requires a combination of clinical history, laboratory
(specific IgE measurements) or outpatient (skin prick tests) tests and challenges with
the food.
Diagnosis of an allergy involves a clinical doctor drawing together three different
kinds of information.
- A detailed history of past allergic reactions and
other allergic conditions, such as asthma, eczema and hayfever, and consideration of any
seasonal or environmental symptoms.
- A thorough medical examination, involving peak flow measurements if the patient is
asthmatic, and a close look to see if there are allergic symptoms in the skin, eyes and
nose. This information will help the specialist decide which tests are appropriate.
- Results of allergy tests -usually skin prick testing or blood
testing.
So what do these tests involve? Is skin prick testing dangerous? At what age can
testing be given? Are the tests that are offered reliable? Here, we attempt to answer
these and other common questions.
Skin Prick Testing
This is suitable for any age group. Even babies under a year old are tested at some
clinics in this way. A tiny prick is made with a lancet through a drop of allergen
placed on the skin, usually on the forearm.
A positive reaction will be indicated by itching within a few minutes. The site where
the allergen was introduced then becomes red and swollen, with a raised weal in the
centre that looks like a nettle sting. The weal enlarges and reaches its maximum size
within 15-20 minutes, when the measurements of the weal are recorded. The reaction fades
within an hour.
This method introduces such a small amount of allergen into the skin that testing is
usually safe, even in cases of severe nut allergy. Dr. Bill Frankland, consultant
allergist at the London Allergy Clinic, knows of only three cases reported in the world
medical literature where skin prick testing caused anaphylaxis. This is out of millions
of tests performed.
A negative response usually means the patient is not sensitive to that
allergen. But skin prick testing for food allergens may be unreliable and "false
negatives" can occur where the reaction to food is not immediate. A negative response
may occur if the patient is taking antihistamines. These should be stopped five days
before testing. Another cause is the fact that some allergens are very unstable; thus
for many fruits and vegetables a doctor might use a "prick-prick" test, where a lancet
is used to pierce the fruit or vegetable and then used to make the skin prick.
A positive response usually means the patient is allergic to that
allergen. However, a patient may have a positive skin test but suffer no symptoms when
coming into contact with the allergen. Positive skin tests may also occur before an
individual has experienced allergic symptoms to a food. People may also still have
positive skin tests to foods and inhalants, even when they have grown out of the allergy
and no longer have an allergic reaction on eating a problem food.
Blood Tests
RAST (Radioallergosorbent test) and CAP-RAST are the most commonly used blood tests
in the UK (the CAP-RAST seems to be superseding the RAST test because it appears to be
more reliable and more sensitive). Other immunological blood tests not using radioactive
material but enzymes are now superseding the original RAST methodology. They work by
detecting the presence of a molecule, called IgE, which is involved in allergic
reactions (link to appropriate web page), which can bind a particular allergen.
Blood tests give graded results from 1-6, with 6 being the most positive. Blood tests
are not affected by antihistamines, and can be used in patients with severe eczema. A
blood sample is taken and sent of to a specialised laboratory for analysis; as a result
they are quite costly and the results are not available immediately. These tests also
only indicate an allergy if the IgE is present in the blood, and there is not a clear
relationship between the presence of IgE and the severity of the allergy. In some cases
the blood test is negative, but an individual still experiences symptoms on consuming a
food.
Challenge Testing
(The following was compiled with the help of Dr Jonathan Hourihane).
A patient who has had a genuinely life-threatening allergic reaction, like
anaphylaxis, should probably not be challenged with the food that caused it. Past
symptoms must be discussed thoroughly with the medical team offering the
challenge.
Challenge testing MUST always be given under strict medical supervision, and by a
specialist with a high degree of knowledge about allergy. It involves giving a
patient increasing doses of the suspected allergenic food, allowing ample time between
doses for a response to occur. Challenges are often conducted in a double-blind manner.
This is when neither the patient, nor the investigator knowing whether the food being
given has any allergen hidden in it or not. A safe challenge involves the following
course of action by the medical team performing the challenge:
- They must ensure the patient or person is fit and
well before challenge. In particular, there must be an absence of asthma or wheezing.
Antihistamines must be avoided during the week leading up to the challenge.
- Careful planning the doses to be used and anticipating the medical responses to
reactions before starting. Ensuring resuscitation equipment is standing by.
- Ensuring the patient and if a child the parent understand what is going to happen.
They will have to sign a medical consent form.
- Increasing the dose very gradually. For example, in a peanut challenge, the doctor
or nurse might choose to start with a small piece of peanut (or peanut butter) rubbed on
the lip. If there is no reaction after 10-15 minutes, they can proceed cautiously to the
next stage.
- The next stage might be allowing the patient to eat, for example, a tiny smear of
peanut butter spread thinly on a small piece of bread.
- Gradually increasing the dose until, for example, 8-16 nuts have been eaten. Many
challenges are stopped too early due to anxiety, but it is necessary to proceed if the
true picture is to be obtained. A negative challenge is valid only if no symptoms are
observed following exposure to a large dose of the problem food.
- Adequate observation for up to four hours after the challenge.
- A nurse or doctor assessing any allergic reaction. Medication may - or may not - be
necessary.
Conclusions
A detailed history and examination alongside the chosen test is required in order to
give a complete picture, and make a correct diagnosis. As a consequence of the
unreliability of many blood tests in predicting allergic reactions, skin prick testing
remains the primary tool to confirm an allergic diagnosis, and gives the most reliable
results. However, none of these tests are infallible. This is because the sensitivity
and predictive ability of skin prick tests and specific IgE measurements varies
considerably from food to food. It is very high for fish, for instance, but very poor in
the case of apple allergy. Food challenges remain the "gold standard" for diagnosis,
except where the patient has suffered an anaphylactic shock.
Treatment
After food allergy has been diagnosed, the only treatment measure that can be offered
is to avoid the offending food. This makes it very important to provide consumers with
clear information about the composition of foods. Food avoidance can also have serious
nutritional consequences when it removes an important food group from a person's diet.
For these reasons someone with a food allergy should consult a trained dietician before
implementing a restricted diet.
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