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Correlation of serum allergy (IgE) tests performed by different assay systemsJulie Wang, MD, James H. Godbold, PhD, Hugh A. Sampson, MD There are three pieces of the puzzle that physicians put together when trying to get a picture of whether their patient is allergic. The clinical history, physical exam, and test results are often all they have to make critical diagnostic and treatment decisions. Definitive profiles are not always achieved through history or physical exam alone, making the diagnostic tests physicians select all the more important. When Skin Prick Test results are not available, clinicians often defer to serum allergy (IgE) tests. In the case of food allergies, specific IgE levels known as published decision points have been established as predictors of clinical reactivity. These reactivity thresholds are based on levels detected using the ImmunoCAP®. assay and are frequently relied upon to determine how food hypersensitivities will be managed. These decision points are not absolute, however, and should be used in conjunction with patient history to establish the likelihood for reactivity. For example, when an allergen specific IgE level is above a threshold of a 95% positive prediction for an allergic reaction, a presumptive diagnosis is made and a food challenge is avoided altogether. If the IgE level falls below a threshold of a 50% positive predictive value for significant reactivity, a food challenge in the allergist’s office is considered. Physicians also use these serum assays to follow individual IgE levels over time, watching for a decrease that would indicate that the patient is developing tolerance to certain foods, like milk and egg. But, not all serum allergy (IgE) tests are created equal. In the May 2008 issue of the Journal of Allergy and Clinical Immunology, Wang et al published a study designed to determine the equivalency levels of IgE antibody levels determined by 3 assays available at commercial laboratories. The authors compared IgE levels of ImmunoCAP® with those measured by Immulite® and Turbo-MP® and found significant variability in the results of the 3 assays. These differences are concerning because they have the potential to impact management of patients with milk, egg and peanut allergies. Predictive IgE values of clinical reactivity for food allergens determined by the ImmunoCAP® should not be applied to results obtained by other commercial assays. Additionally, any ongoing monitoring of IgE levels in individual patients should be performed with the consistent use of the same assay. |
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