How can you find out if you suffer from a food allergy?

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 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.

1. 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.
   
2. 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.
   
3. 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:

1. 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.
   
2. Careful planning the doses to be used and anticipating the medical responses to reactions before starting. Ensuring resuscitation equipment is standing by.
   
3. Ensuring the patient and if a child the parent understand what is going to happen. They will have to sign a medical consent form.
   
4. 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.
   
5. 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.
   
6. 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.
   
7. Adequate observation for up to four hours after the challenge.
   
8. A nurse or doctor assessing any allergic reaction. Medication may - or may not - be necessary.

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.