Interpreting skin prick tests in the evaluation of food allergy in children

Skin prick tests (SPTs) are utilized routinely in the evaluation of food allergy and several authors have discussed their utility. Efforts to standardize SPT reagents and procedures have been made, but the accuracies of different recording techniques have not been clearly defined. The aim of this study was to compare different SPT recording methods with the outcome of oral food challenge and determine whether they offer any advantage over the criteria proposed by Bock and May (1).

Children suspected of IgE-mediated symptoms to any of five common food allergens (egg, milk, peanut, soy and wheat) were skin tested by the prick technique utilizing commercial extracts. The wheal reactions were recorded by two different methods: first by measuring the largest diameter of the wheal and the diameter orthogonal to it (mean wheal diameter), and second by recording the surface area of the wheal with a hand-held scanner. Wheal sizes above the 95% confidence interval of tolerant individuals were considered positive. The results of double-blind, placebo-controlled food challenges were considered the "gold standard" for diagnosis. Cut-off values were compared for positive responses in our study population (mean diameter/surface area of wheal): 4 mm/16 mm2 for egg, 5 mm/29 mm2 for milk, 6 mm/40 mm2 for peanut, 3 mm/9 mm2 for soy, and 3 mm/7 mm2 for wheat.

Significant differences in wheal sizes were seen between individuals who were allergic or tolerant to egg (P < 0.001), milk (P < 0.001), wheat (P < 0.005), and peanut (P < 0.05). Reactivity to soy could not be predicted based on SPT results (P = n.s.). The sensitivities and the specificities of the two recording methods were similar. The predictive values were not significantly different from that of the commonly utilized method of grading SPTs (i.e. positive = mean diameter 3 mm or greater than the negative control).

Skin prick tests are a useful procedure for evaluating clinical reactivity to egg, milk, peanut and wheat, but not to soy. While the size of the SPT wheals may be interpreted utilizing mean diameter or surface area cut-offs, the predictive values of these measurement methods were no better than the commonly utilized grading method where a positive skin test was recorded as a mean wheal diameter 3 mm greater than the negative control.