Date: 7/27/2009     Author: Atlanta Allergy & Asthma Clinic     Categories: News, Allergy

Hot Topics: Skin Testing vs Invitro Allergy Testing

Skin Testing vs Invitro Allergy Testing

Allergic diseases such as allergic rhinitis, asthma, and food allergy are among the most prevalent chronic diseases treated by physicians in the United States. It is estimated that approximately 40-50 million Americans suffer from allergic diseases and the prevalence is increasing. For patients with symptoms consistent with allergy, it is important to determine the specific allergens responsible for their symptoms in order to accurately diagnose and treat their disease. For those with severe asthma, food allergy and insect sting hypersensitivity, accurate diagnosis and management is essential for potentially life saving therapies.

Testing for allergen specific IgE(sIgE), can be performed by immediate hypersensitivity skin tests, or in vitro commercial immunoassays. Many in vitro IgE assays have been developed. Briefly, these testing methods rely on binding of serum IgE to an antigen bound to a solid phase media. After washing, labeled anti-IgE antibodies bind to the bound IgE and are measured. The amount of conjugated anti-IgE is typically proportional to the amount of the IgE-antigen complex, results are compared to a standard calibration curve. Results are typically reported in kUa/L of s-IgE.

Studies of different assay systems have shown that results are not necessarily comparable. Wood et al compared the accuracy of three commonly used in vitro systems and concluded that there were considerable differences between assays, with some over estimating and others underestimating sIgE levels. Another study by Williams et al found sub-standard performance with multiple cases of poor precision and accuracy among different laboratories, especially for certain antigens. More recently, Wang et al reported significant discrepancies in sIgE values among 3 different in vitro systems, that were both over and under estimating sIgE for both foods (such as milk and peanut) as well as inhalants (cat and dust mite). They concluded that predicted values for clinical reactivity for food specific IgE determined by one assay cannot be applied to results from other assays. Results from patients with extremely high IgE levels (>1000IU) may also give false positive results. Other studies have also compared skin testing to in vitro tests and have found skin tests to be more accurate.

Allergy skin testing is currently performed via a prick test, followed if necessary by intradermal testing. Skin tests are safe, minimally invasive, easily quantifiable and results are available within 15-20 minutes. Costs per antigen are typically less than in vitro tests, results are more sensitive and are easily visualized by the patient. Skin testing may be impractical for patients with severe dermatitis and patients need to avoid antihistamine therapy for some days prior to testing. Other medications such as beta blockers are often potential contraindications.

In conclusion, in vitro tests may present the clinician with diagnostic challenges, due to inter lab variability, non-comparable results between assays, and decreased sensitivity compared to skin testing. Allergy skin testing remains the most accurate and cost effective mode of allergy diagnosis, with immediate results that can then be presented to the patient.

Wood et al Ann All 2007;99:34     Williams et al JACI 2000;105:1221
Wang et al JACI 2008;121:1219    Sharma et al JACI 2008;121:933

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