Abstract Background: The objectives of this study were to demonstrate the usefulness of Receiver Operating Characteristic (ROC) analysis in the definition of a positive threshold in the comparison of two in vitro tests for detecting IgE antibodies and to determine the clinical relevance of specific IgE tests according to the sensitivity and specificity determined by ROC analysis. Methods: Specific IgE levels were compared with the use of both MATRIX (Abbott Laboratories, Chicago, Ill.) and PHADEZYM (Pharmacia, Uppsala, Sweden) tests in 109 patients: 86 allergic patients and 23 control subjects. All allergic patients had a clinical history of allergy to mite, cat, grass pollen, or birch pollen and positive skin prick test results to the suspected allergen. Using various levels of specific IgE, we determined the points of the ROC curves with the best ratio of true positives to false positives for four aeroallergens (i.e., mite, cat, grass pollen, and birch pollen). Results: Except for mite allergen there was no correspondence between the positive threshold values determined by ROC analysis with MATRIX and PHADEZYM tests. In terms of class, when ROC analysis was used, the threshold was class III with the MATRIX test and class II with PHADEZYM test for cat, grass pollen, and birch pollen and class III for mite. According to the threshold value, sensitivity and specificity for MATRIX and PHADEZYM tests were similar except for cat, for which the PHADEZYM test had a lower sensitivity (0.71) than the MATRIX test (0.90). With mite allergen the predictive value of only one positive MATRIX test result was 0.96. The specificities of MATRIX and PHADEZYM tests were 100% in both nonatopic and atopic control groups. Conclusions: We conclude that ROC analysis is a reliable method for comparing IgE in vitro tests and that it emphasizes the lack of correspondence between arbitrary units given for each in vitro test. ROC analysis appears to be a good method for helping to standardize IgE in vitro tests.