Challenges progress through increasing doses of the active foodstuff. In practice, investigators start at half the minimum amount that the patient has reported as causing symptoms and then double the amount at each step. The time period between each challenge should be greater than the time interval within which symptoms occur. The patient should be carefully observed throughout the challenge procedure, or if the challenge is occurring at home they should keep a symptom diary. Some advise a separate placebo challenge period with similarly increasing amounts of placebo matching the active challenge. The order of the placebo and active challenge should be randomly decided. The investigator may need to carry out several challenges, in particular if the patient suffers mainly from subjective symptoms, with no more objective signs.
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