People seem to be very ready to attribute many symptoms to either a specific food or a range of foods. Studies suggest that at most 50% of patients suspected of having a food hypersensitivity will have a positive double-blind placebo-controlled food challenge.20 This statistic is from selected populations derived from allergy clinics. These people are almost certainly at higher risk of definite food allergy than an individual selected from the general population in a more general clinic, because of prior 'selection' based upon history and examination. The 'placebo effect' of many foods is strong and has been demonstrated in many studies comparing the results from open (unblinded) tests with those from blinded ones.20 The placebo effect is applicable to both patient and investigator alike, underlining the importance of blinding both investigator and patient.
The DBPCFC is largely restricted to practising allergists, and not widely available in the United Kingdom. Paediatricians in the UK more commonly use open challenges. This is appropriate in infants where there is less of a problem of psychological overlay. There is still the issue of the psychology of the parents and practitioner. A negative open challenge is very useful in ruling out the role of food in causing or exacerbating the symptoms.
There is also a therapeutic as well as a diagnostic role for food challenges. This is best illustrated in children. Infants with food allergies, in particular allergies to egg and milk, have a high likelihood of becoming tolerant to the food, 'growing out' of their allergy. Interval re-challenges are essential, to 'test' their continuing sensitivity to the particular food.
The designing of a food challenge involves several steps that are detailed below.21
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