Resolution of peanut allergy: case-control study

British Medical Journal, April 25, 1998 by Jonathan O'B. Hourihane, Stephen A. Roberts, John O. Warner, Abi Berger

Introduction

The diagnosis of peanut allergy has important consequences for patients and their families. They are told that allergic reactions occur after frequent exposure, that reactions are often severe, and that the allergy persists indefinitely.

The dietary habits of the British population have changed, with vegetarianism becoming more popular and the use of peanut butter apparently increasing as a snack food for children. These changes may be linked to a recently observed decrease in the age of onset of peanut allergy.[2 3]

In longitudinal studies allergies to cows' milk and egg usually resolve early in life; 85% of children with cows' milk allergy first two years of life are tolerant of milk by 3 years of age' and up to 80% of infants with egg allergy are tolerant of egg by 5 years of age.[5 6] There are no similar longitudinal studies of infants with peanut allergy, and the advice that peanut allergy persists is based on a study of older children.[1] The age differences between children with cows' milk or egg allergy and those with peanut allergy may account for the different rates of resolution. Follow up of a population based group of Danish children with cows' milk allergy suggests resolution of the allergy is unusual if it has not occurred by 5 years of age.[7]

In our clinical practice we have observed apparent resolution of peanut allergy in several children affected by peanut allergy at a young age. We report the clinical features of these children and of those of age and sex matched controls who have remained allergic to peanut.

Subjects and methods

We studied children who were referred to the regional paediatric acute allergy and anaphylaxis clinic in Southampton (155 children) or to the paediatric allergy clinic in South Manchester (75 children) for evaluation of suspected peanut allergy between April 1995 and December 1996.

Identification of cases and controls

A child was considered to have been allergic to peanuts if the constellation of typical symptoms had been observed after an unequivocal exposure to peanuts in the 3 years before presentation. Children who had undergone peanut challenge were identified by the relevant author in each hospital. Patients were selected for challenges according to the clinical needs of the patient in each case. Some children were challenged because they had had negative results on skin prick testing with peanut despite a convincing history or because their dietary history suggested that an exposure to peanut had been uneventful. Children with life threatening reactions to peanut were not considered for challenge irrespective of the time since the last exposure. Controls and cases with positive results on skin prick testing were challenged either because the last reaction had been a long time before or because of parental request. Parents often wanted to know whether their child was allergic to peanuts before school entry -- anecdotally, a time of great anxiety for parents of children allergic to certain foods. The challenges were all open food ones' using peanut butter or peanuts according to the age of the subject Every challenge was performed in hospital.[9]

A child was considered to be no longer allergic to peanuts if two criteria were met: (a), they had a clear history of a reaction to peanut and (b), a formal challenge with peanuts or peanut butter gave negative results. We called these children resolvers.

Matching for age and sex was undertaken to control for effects that would be evident when comparing preschool children with peanut allergy and comparatively few other allergies (either to foods or inhalant allergens) with older children sensitised to a wider range of allergens.[2] For each case one control (persister) was identified from children who had a positive skin prick test and a positive challenge with peanut.

Skin prick testing

Skin prick testing was carried out at the initial hospital visit using a 1:20 (wt/vol) solution (Soluprick, ALK, Uppsala, Sweden). A reaction was considered positive if a weal was [is greater than] 3 mm in diameter in the presence of a reaction to 1% histamine of at least 3 mm in diameter.

Measurement of IgE concentration

The concentration of total IgE was measured in serum using an enzyme linked immunosorbent assay system developed by each hospital. The lower limit of detection was 5 KU/ml in each hospital. The concentration of peanut specific IgE was measured using either a commercially available enzyme linked immunosorbent assay kit (Alstat, Wales) in Southampton or the Pharmacia-CAP system (Pharmacia, Uppsala, Sweden) in Manchester. The lower limit of detection of both assays was 0.35 KU/ml.

Data handling

Data were collected from hospital notes by the responsible clinician using a standard data collection form for both the cases and controls. Details of the age of onset, number of exposures, clinical features of reactions, and length of time since last exposure or reaction were noted. The presence of coexisting asthma, eczema, rhinitis, and food allergies was also determined.