Q:

12/20/2012
Our lab director asks if we can substitute EIA for I-CAP (IBT-Viracor). She notes that "Mayo (Clinic) uses EIA". The question is whether this method is as good as I-CAP for food allergy. Are the cut points published for I-CAP transferable to EIA tests for food allergy?

A:

Thank you for your inquiry.

Copied below you will find the description of the I-CAP from the website of IBT-Viracor labs and a link to that site. Beneath that you will see the same description for the enzyme-linked immunoassay system (EIA) performed at the Mayo Clinic. In addition, I have copied below a link to the Mayo Clinic Laboratory website that describes this test - and below that is an additional link to the actual report that the Mayo medical lab uses, giving the grading system of the test. Since you already are using the I-CAP test, you should be familiar with their grading system for comparison.

As you can see, according to the IBT-Viracor website, they use the Phadia ImmunoCAP system which they refer to as the I-CAP. You can also see that the Mayo Clinic system uses a similar system for specific IgE (FEIA), also developed by Phadia.

Based on this information, both tests are valid assays. Clearly, all in vitro tests for specific IgE do not have the same level specificity and sensitivity (see the link copied below to the American College Pathologists discussion of this issue, as well as the abstract copied below from The Journal of Allergy and Clinical Immunology ). But since the two systems in question are both Immunocap assays they are likely to have relatively good equivalence.

In summary, both of the tests you referred to are valid tests and can be used comfortably to assess allergen-specific IgE.

Thank you again for your inquiry and we hope this response is helpful to you.

The test is designed as a sandwich immunoassay (a method using two antibodies, which bind to different sites on the antigen or ligand). The basis of the innovative ImmunoCAP technology is the 3 dimensional cellulose sponge matrix. It consists of a cellulose derivative enclosed in a capsule. This unique reaction environment allows for the binding of clinically relevant allergens, even those present at very low levels. The hydrophilic, highly branched polymer provides an ideal microenvironment for allergens, binding them irreversibly while maintaining their native structure. This extremely high overall binding capacity is achieved through a high binding capacity per mg cellulose in combination with an optimal amount of cellulose in each solid phase. This ensures binding of all relevant antibodies, regardless of antibody affinity, while still giving low non-specific binding. This provides high sensitivity so that very low concentrations of allergen-specific IgE antibodies can be detected

• Calibration

The calibrators are directly traceable to the WHO International Reference Preparation for human IgE 75/502. Measured response values for allergen specific IgE antibodies are evaluated against a Total IgE calibration curve and expressed as concentration of Allergen specific Units (kUA/l).

Currently, there is no established international reference preparation for allergen specific IgE antibodies. For total IgE protein a mass Unit (U) is defined by the World Health Organization (WHO) International Reference Preparation 75/502. 1 IU has been shown to be equal to 2.42 ng IgE protein. ImmunoCAP Specific IgE detects IgE antibodies in serum in the range of 0.1-100 kUA/l.

U = international unit for IgE as defined by WHO International Reference 75/502

UA = allergen specific unit

All Information provided by Phadia. ImmunoCAP is a registered trademark of Phadia

Method Description Describes how the test is performed and provides a method-specific reference.

Specific IgE from the patient's serum reacts with the allergen of interest, which is covalently coupled to an ImmunoCAP. After washing away nonspecific IgE, enzyme-labeled anti-IgE antibody is added to form a complex. After incubation, unbound anti-IgE is washed away and the bound complex is then incubated with a developing agent. After stopping the reaction, the fluorescence of the eluate is measured. Fluorescence is proportional to the amount of specific IgE present in the patient's sample (ie, the higher the fluorescence value, the more IgE antibody is present). (Package insert: ImmunoCAP System Specific IgE FEIA, Uppsala, Sweden Rev 02/2005)

Allergy testing: from skin to tube to chip Source: College of American Pathologists

The Journal of Allergy and Clinical Immunology
Volume 121, Issue 5 , Pages 1219-1224, May 2008Background
In vitro testing is commonly used to diagnose and manage allergies. Clinical reactivity has been correlated with food-specific IgE levels by using the ImmunoCAP (Phadia, Uppsala, Sweden).
Objective: To determine whether IgE levels derived from different assays are equivalent to those measured by ImmunoCAP.
Methods: Fifty patients from the Mount Sinai Pediatric Allergy practice were prospectively enrolled. For each deidentified sample, specific IgE levels were measured to egg, milk, peanut, cat, birch, and Dermatophagoides farinae at different laboratories, each using a different assay system (Phadia ImmunoCAP, Agilent Turbo-MP, and Siemens Immulite 2000). Results were analyzed to determine whether IgE measurements were equivalent. Food allergen–specific IgE levels were correlated with clinical data and around empirically determined thresholds that predict probability of clinical disease in 50% or 95% of subjects.
Results: Variable degrees of agreement existed among the 3 assays. Immulite 2000 overestimated all specific IgE levels compared with ImmunoCAP. Turbo-MP overestimated for egg but underestimated for birch and D farinae. Differences for milk, peanut, and cat were observed, without a trend toward overestimation or underestimation. Furthermore, several values for the food allergens were discrepant around the 50% and 95% positive predictive values for clinical reactivity.
Conclusion: Discrepancies in specific IgE values from 3 different assays can potentially lead to altered management and treatment. The predictive values for clinical reactivity associated with food-specific IgE levels determined by ImmunoCAP should not be applied to results from other assays.

Sincerely,
Phil Lieberman, M.D.

AAAAI - American Academy of Allergy Asthma & Immunology