Page 1327 - Clinical Immunology_ Principles and Practice ( PDFDrive )

1288 Part ElEvEn Diagnostic Immunology

KEY COnCEPtS performance of clinically used total IgE assays showed variable
Immunoglobulin E (IgE) (Reaginic) interference that resulted in a 1.9–51.9% reduction in accuracy,
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depending on the assay. Accurate quantitation of the level of
Antibody Detection uncomplexed or “free” IgE in the serum of treated patients has
• Allergen-specific IgE can be detected by skin test using a puncture been more technically difficult. The performance of the existing
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or intradermal administration of allergen or in the serum by laboratory- free IgE assays has been heavily criticized, leading some investiga-
based immunoassays. tors to a conclusion that there may be no simple and analytically
• In general, the intradermal (ID) skin test is more analytically sensitive accurate clinical method for quantifying the level of free IgE in
than a puncture skin test, which is roughly comparable to the best in the serum of patients who have received omalizumab. This,
vitro methods for IgE antibody detection in serum. however, may change with a new anti-IgE therapeutic that has
• ID skin tests are the diagnostic procedure of choice in the workup of
patients with suspected Hymenoptera venom and drug allergy, while been engineered so it is removed from circulation once it
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both in vitro an in vivo assay methods are complementary for evaluating complexes IgE.
aeroallergen-related disease.
• Serological analyses of IgE antibody–specific for food allergens are Allergen-Specific IgE
often favored over extract-based skin test analyses in part because Laboratories in the United States that perform clinical diagnostic
of more enhanced reagent quality control; however, the double-blind allergy testing must be federally licensed, use a United States
placebo-controlled food challenge (DBPCFC) remains the gold standard FDA-cleared assay method, and perform successfully in an external
for definitive diagnosis of food allergies.
• IgE antibody to allergenic molecules (components) can in some cases diagnostic allergy proficiency survey (e.g., College of American
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(e.g., peanut, hazelnut) provide clarity in terms of the specificity of Pathologists [CAP] SE Survey). These assays, which have been
the patient’s sensitization profile (genuine vs cross-reactivity) and through rigorous validation, have achieved unsurpassed intraassay
relative risk for mild versus serious systemic reactions. precision, interassay reproducibility, and a high degree of
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quantification. Their basic design can be traced to the first IgE
antibody assay, the RAST, reported by Wide et al. in 1967. 12
allergic disease. Analytes commonly measured in these laboratories
include the total serum IgE, IgE antibodies to hundreds of allergen Allergen
specificities, Hymenoptera venom-specific IgG, the IgE antivenom- The most highly variable component of the IgE antibody assay
inhibition test, and mast cell tryptase. IgG antibody measurements is the allergen-containing reagent. Allergens are mixtures of
to allergens other than Hymenoptera venom have not been shown molecules, typically proteins, glycoproteins, lipoproteins, or
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to be clinically useful. Basophil mediator and activation tests, protein-conjugate chemicals or drugs that have been solubilized
although rarely offered as clinical tests because of the requirement from a defined (usually biological) source, a portion of which
for fresh blood, are useful investigational methods that are also can elicit an IgE antibody response in exposed and genetically
reviewed in this section. predisposed individuals. They possess common properties of
stability to processing (e.g., heat) and digestion because of multiple
Total Serum IgE cysteine linkages. They tend to be abundant in nature, form
Of the diagnostic allergy tests that are performed in the clinical aggregates or polymers, commonly interact with lipid structures,
immunology laboratory, total serum IgE is currently the only and serve to defend their biological source. Cross-tabulation of
diagnostic allergy analyte regulated under the US Clinical Labora- the protein family (PFAM) database (n = 16 230 proteins families)
tory Improvement Amendment of 1988 (CLIA-88). Current with the Structural Database of Allergenic Proteins (SDAP)
commercial assays to measure the total level of IgE in serum identified 130 PFAMs in the Allergenic Family database of
employ nonisotopic labels, such as enzymes (horseradish per- allergenic proteins. Thus importantly, allergens comprise a small
oxidase, alkaline phosphatase, β-galactosidase) or fluorophores, fraction of protein families with particular structures and biologi-
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and have been cleared by the US Food and Drug Administration cal functions. The Clinical Laboratory Standards Institute (CLSI)
(FDA). The minimum detectable concentration of the commercial has an established international guidance document that defines
total serum IgE assays is between 0.5 and 1 µg/L. The intermethod the expected performance characteristics of allergenic materials
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agreement of the different commercial IgE assays is excellent used in immunological assays for human IgE antibodies. It
(e.g., intermethod coefficients of variation [CVs] typically provides a compendium of the genus and species of all the allergen
<15%). 13,37 Nonatopic age-adjusted reference intervals for total specificities of clinical interest, subdividing them into extract
serum IgE must be used for normative interpretation. 5 and component allergens and indicating whether they are well-
documented or rare. They are categorized on the basis of their
Total IgE Measurements After Therapeutic sources, into weed pollen, grass pollen, tree pollen, animal dander,
Anti-IgE Administration mold, house dust-mite fecal material, parasites, insect venoms,
Omalizumab (anti-IgE) is currently used as a fourth therapeutic occupational allergens, foods, and drugs. Except for drugs, these
modality to supplement avoidance, pharmacotherapy, and extracts are complex heterogeneous mixtures that contain both
immunotherapy in the management of persistent asthma and nonallergenic and allergenic proteins. Some allergens share
urticaria, and off-label for other IgE-mediated states (e.g., allergic structural similarity or cross-reactive epitopes, and others possess
bronchopulmonary aspergillosis, pretreatment of food allergy unique IgE antibody-binding determinants. Among the different
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patients receiving immunotherapy). Since its conception, clini- species of a genus, such as ragweed (e.g., Canyon, Desert, Giant,
cians have desired to quantify the level of total and “free” Short, Silver, Southern, Western), there is extensive allergenic
(uncomplexed) IgE in anti-IgE treated patients as a rationale cross-reactivity resulting from structural similarity. Extensive
for treatment failures or to justify modification of a patient’s allergenic cross-reactivity has also been documented within other
dosing regimen to maximize treatment success. A systematic pollen groups, such as the grasses (June, Brome, Timothy, Peren-
evaluation of the impact of therapeutic anti-IgE on the nial Rye, Fescue, Orchard, Red Top, Salt, Sweet Vernal, Velvet).