Molecular allergy diagnostics using IgE singleplex determinations: methodological and practical considerations for use in clinical routine: Part 18 of the Series Molecular Allergology

Abstract

Allergen molecules (synonyms: single allergens, allergen components) open up new horizons for the targeted allergen-specific diagnostics of immunoglobulin E (IgE) in singleplex determination. The following rationales support the targeted use of allergen molecules and, more importantly, improve test properties: (1) increased test sensitivity ("analytical sensitivity"), particularly when important allergens are under-represented or lacking in the extract; (2) improved test selectivity (analytical specificity), particularly when the selected IgE repertoire against an allergen yields additional information on: (a) potential risk, (b) possible cross-reactivity, or (c) primary (species-specific) sensitization. However, the appropriate indication for the use of single allergens can only be established on a case-by-case basis (depending on the clinical context and previous history) and in an allergen-specific manner (depending on the allergen source and the single allergens available), rather than in a standardized way. Numerous investigations on suspected food allergy, insect venom allergy, or sensitization to respiratory allergens have meanwhile demonstrated the successful use of defined molecules for allergen-specific singleplex IgE diagnosis. Specific IgE to single allergens is limited in its suitability to predict the clinical relevance of sensitivity on an individual basis. In food allergies, one can at best identify the relative risk of a clinical reaction on the basis of an IgE profile, but no absolutely reliable prediction on (future) tolerance can be made. Ultimately, the clinical relevance of all IgE findings depends on the presence of corresponding symptoms and can only be assessed on an individual basis (previous history, symptom log, and provocation testing with the relevant allergen source where appropriate). Thus, also in molecular allergology, the treating physician and not the test result should determine the clinical relevance of diagnostic findings.

Electronic supplementary material: Supplementary material is available for this article at 10.1007/s40629-015-0067-z and is accessible for authorized users.

Keywords: Component-based diagnostics; IgE — Allergen; In-vitro test; Molecular allergy diagnostics.

Fig. 1:

Fc͛RI-bound IgE on effector cells. Light gray area: total bound IgE/cell (number of Fc͛RI occupied by IgE with population-based distribution) on basophilic leukocytes. Dark gray area: specific IgE/cell required for half-maximal cell activation (intrinsic sensitivity of basophils with population-based distribution). The distribution of both variables is approximately normal and can differ significantly; evidently, a fraction (ca. 1%) of bound total IgE is sufficient for half-maximal allergen specific activation. For this reason, the ratio of specific to total IgE is interesting in terms of interpretation. Inset top left: individual mediator release as a function of cell-bound specific IgE; basis for the population-based normal distributions illustrated in the lower part of the figure.

Fig. 2:

Options for the evaluation of logarithmically distributed allergen-specific IgE levels. A quantitative; B semi-quantitative (since entry into force of the German Medical Association guideline, Richtlinie der Bundesärztekammer (RiliBÄK), this term is no longer provided for; specific IgE levels given only in classes are considered as qualitative evaluations); C qualitative. Allergen-specific IgE levels expressed as units of specific IgE, kUA/l (A stands for allergen-specific), using WHO standards for total IgE determination (heterologous calibration). Light gray area: population of serum samples with no allergen-specific IgE (levels fall below the detection limit of 0.1 kUA/l). Dark gray area: population of positive serum samples with logarithmic (hypothetically normal) distribution of allergen-specific IgE levels above the detection limit of 0.1 kUA/l

Fig. 3:

Significance of the total and specific immunoglobulin E (IgE) ratio. Due to the variability of total IgE levels, logarithmically distributed specific IgE (dark gray bars) can also be expressed as a relative quantity of total IgE (light gray bars) [16]. This process „normalizes“ specific IgE to total IgE on a percentage basis (hatched bars). Primarily the borderline cases (see numerical examples) with particularly low (normal distribution curve, far left) or extremely high total IgE (normal distribution curve, far right) make it clear that specific IgE can only be correctly interpreted once total IgE is known. This ratio of specific to total IgE is also found on the surface of effector cells (mast cells, basophil granulocytes), thereby providing the basis for diagnostic ex vivo (basophil activation test, BAT) and in vivo tests (skin prick test, provocation test)

Fig. 4:

Methodological rationales for molecular allergy diagnostics. The variants 1–4 shown in the figure (see also Tab. 2) reflect universal arguments for the methodological use of allergen molecules. They move (virtually) exclusively on the test (sensitization) level, irrespective of the patient’s clinical status. Thus, they improve only the sensitization test per se, without affecting clinical test findings/interpretation, which must always be undertaken by the treating physician (or person requesting the test) based on clinical information provided by the patient (history/provocation) on a case-by-case basis (adapted from [14])

Fig. 5:

Immunoglobulin E (IgE) levels to allergen molecules depending on structural similarity within an allergen family. a Variable, limited cross-reactivity between 2S albumins (stable storage proteins in nuts, pulses, and seeds). b Variable cross-reactivity between Bet v 1-homologous food allergens. c High cross-reactivity due to the strongly preserved and similar structure of profilins (in pollen, latex, and foods)