Which test is best for diagnosing peanut allergy in South African children with atopic dermatitis?

BACKGROUND Diagnosing peanut allergy based on sensitisation alone leads to an unacceptable rate of overdiagnosis. OBJECTIVE To define parameters that may help differentiate peanut allergy from asymptomatic sensitisation in a cohort of South African (SA) children with atopic dermatitis (AD). It is the first study in SA to utilise oral food challenge tests and analyse peanut component patterns. METHODS This was a prospective, observational study at a paediatric university hospital in Cape Town, SA. Children with AD, aged 6 months - 10 years, were recruited randomly. They were assessed for sensitisation and allergy to peanut by questionnaire, skin-prick tests (SPTs), immuno solid-phase allergen chip (ISAC) tests, ImmunoCAP component tests to Ara h 1, 2, 3, 8 and 9, and incremental food challenges. RESULTS One hundred participants (59 Xhosa (black Africans) and 41 of mixed race, median age 42 months) were enrolled. Overall, 44% of patients were peanut sensitised and 25% had a true peanut allergy. SPTs and ImmunoCAP Ara h 2 produced the highest areas under the receiver operating characteristic curve for predicting peanut allergy in peanut-sensitised patients. The ISAC test was less sensitive, more specific and produced significantly lower median values than ImmunoCAP tests. Ara h 2 was the most useful component in differentiating allergy from tolerance in both ethnic groups, being positive in 92% of allergic and 40% of sensitised but tolerant children (p<0.001). There was little additional contribution from Ara h 1 and 3. Ara h 8 and 9 were associated with tolerance. Commonly used 95% positive predictive values (PPVs) for SPTs, peanut-specific IgE and Ara h 2 levels fared suboptimally in our population. Maximum PPVs for this study population were found at SPT 11 mm, peanut IgE 15 kU/L and ImmunoCAP Ara h 2 of 8 kU/L, but these adjusted levels still had suboptimal PPVs in Xhosa subjects. Severe peanut allergy was associated with increased median peanut IgE and Ara h 2. CONCLUSIONS The component Ara h 2 was useful for differentiating allergy from tolerance in both ethnic groups in this SA cohort. Ninety-five percent PPVs for peanut allergy tests may need to be revised, especially in Xhosa patients. An SPT result ≥11 mm as well as Ara h 2 ≥8 kU/L had the best predictive value for peanut allergy.