As part of a series of problems common in primary care (see p 1), Aziz Sheikh and Samantha Walker take you through food allergy

A newly qualified teacher requests investigations for possible food allergies. She has been troubled with symptoms of tiredness, nausea, bloating, and intermittent diarrhoea. Bread and chocolate have been identified as possible triggers, and reducing intake of these foods has resulted in some improvement of symptoms. General physical examination is unremarkable.

What issues you should cover

What does she mean by "food allergy"? Patients often use food allergy as a generic term that encompasses a broad range of symptoms triggered by certain foods. In contrast, clinicians reserve the term for immunologically mediated abnormal reactions to foods. Although about a fifth of the general population believe they have a food allergy, less than 1% of reactions can be confirmed on double blind, placebo controlled food challenge.

Differentiate between IgE mediated allergic reactions and non-allergic food reactions. The former may require meticulous avoidance of the foods implicated (often for life) to minimise the risk of potentially life threatening reactions. The latter, although not life threatening, may result in dietary deficiencies.

In IgE mediated food allergy, common triggers include eggs, milk, peanuts, and fish (including seafood); less common triggers include fruit, vegetables, and tree nuts. Reactions typically occur within minutes of ingestion of the offending food(s) and provoke predictable reactions, which are typically local (angio-oedema, perioral itching, and laryngeal oedema) and systemic (urticaria, rhinoconjunctivitis, wheezing, diarrhoea and vomiting, and, in some cases, anaphylaxis).

Self help groups

• British Allergy Foundation (tel 020 7600 6127; www.allergyfoundation.com)
• Leatherhead Food Intolerance Databank (tel 01 372 376 761)
• Medic-Alert Foundation (tel 020 7833 3034)
• Anaphylaxis Campaign (tel 020 8554 5579; www.anaphylaxis.org.uk)

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In food intolerance, symptoms are typically non-specific and may occur in response to a range of foods. A temporal relation between food intake and onset of symptoms is often difficult to establish. Detailed questioning typically shows that the offending foods are sometimes well tolerated.

Has she previously been investigated for food allergy? Inquire about results of food specific skin prick tests and tests for specific IgE; verify concordance between symptoms and allergy tests. In some patients, food allergies will have been diagnosed on the basis of investigations of spurious value--for example, kinaesiology.

Ask about the worst reaction. Is she at risk of food induced anaphylaxis? Is her diet nutritionally adequate? In children, assess and monitor height and weight gain.

What you should do

No allergy tests are necessary in those with a history that strongly suggests food intolerance. Other differential diagnoses must, however, be considered and, if suspected, investigated appropriately.

Reassure patients with a food intolerance that they do not have a food allergy. Advise them to abstain from the offending foods for a while, but encourage them to try, from time to time, to reintroduce them into their diet. In those with weight loss or dietary deficiencies, consider referral to a dietitian.

For those in whom IgE mediated food allergy is suspected, or in those in whom you cannot safely exclude this diagnosis, request serum specific IgE tests to the foods implicated. Indiscriminate testing to a range of foods is not recommended as these tests have low specificity.

IgE mediated food allergy will require avoidance of the provoking foods. Help from a dietitian with detailed written advice on avoidance strategies is often useful.

Refer patients with a history of IgE mediated anaphylaxis to an allergy specialist. Consider requesting serum specific IgE tests to the foods implicated while awaiting assessment. Advise the patient to totally avoid the food trigger(s) identified. In patients with life threatening symptoms, prescribe self administered adrenaline (epinephrine). Give patients a written management plan advising exactly when, where, and how to administer the adrenaline auto-injector.

Most fatal reactions to food occur in people with asthma. In those with both asthma and food induced anaphylaxis, ensure that asthma is optimally controlled.