Extension of food allergen specific IgE ranges from the ImmunoCAP to the IMMULITE systems

Background: Quantitative measures of food specific IgE antibody using the Phadia ImmunoCAP method have documented value in the diagnosis of food allergy. Objective: To define factors that could be used to relate IMMULITE-measured IgE antibody levels for chicken egg white, cow's milk, and peanut into ImmunoCAP-comparable quantities that could then be correlated with published levels that have been generated with the ImmunoCAP system. Methods: Serum samples from 328 patients (median age, 5.4 years; age range, 118 years; 32% female) who were known to be IgE positive (>0.1 kU/L) to chicken egg white (n = 120), cow's milk (n = 135), and/or peanut (n = 304) were analyzed in both the ImmunoCAP and IMMULITE autoanalyzers. Results: IgE antibody levels from both assays for each of the 3 food specificities were highly correlated: r ²= 0.95 for egg white, r² = 0.93 for milk, and r ² = 0.95 for peanut (P <.001). Empirically determined IMMULITE/ImmunoCAP ratios (mean ± 1 SD) were 4.85 ± 1.79 kU/L (egg), 2.33 ± 1.0 kU/L (milk), and 1.86 ± 0.98 kU/L (peanut). For milk and peanut, the IgE antibody levels for individuals who either passed or failed a food challenge were not significantly different between the assay methods. Because of the small sample size of egg white challenged patients, no statistical analysis was performed. Conclusion: These data indicate that specific IgE levels to egg white, milk, and peanut measured by the IMMULITE and ImmunoCAP systems are highly correlated and that differences between the systems are circumscribed and modest (IMMULITE was a mean of 2- to 5-fold higher than ImmunoCAP).