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SAMJ: South African Medical Journal

On-line version ISSN 2078-5135
Print version ISSN 0256-9574

Abstract

GRAY, C L; LEVIN, M E  and  DU TOIT, G. Which test is best for diagnosing peanut allergy in South African children with atopic dermatitis?. SAMJ, S. Afr. med. j. [online]. 2016, vol.106, n.2, pp.214-220. ISSN 2078-5135.  http://dx.doi.org/10.7196/SAMJ.2016.V106I2.10125.

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.

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