Biomarker: F209-IgE Grapefruit

Overview

F209-IgE grapefruit measures immunoglobulin E (IgE) antibodies in blood that recognize grapefruit (Citrus paradisi) allergen proteins. IgE is involved in immediate hypersensitivity reactions, including food-allergic symptoms. A detectable or elevated result indicates sensitization to grapefruit, but does not by itself confirm clinical allergy, predict reaction severity, or establish that grapefruit caused a symptom. The test is clinically useful when interpreted with the patient’s reaction history and, when appropriate, skin testing or a medically supervised oral food challenge.

Clinical Use Cases

Evaluation of suspected immediate reactions after eating grapefruit or grapefruit-containing products.
Assessment of possible citrus-fruit allergy in patients with compatible symptoms.
Investigation of oral allergy syndrome, urticaria, angioedema, respiratory symptoms, gastrointestinal symptoms, or anaphylaxis occurring after citrus exposure.
Identification of sensitization to grapefruit as part of an allergist-directed food-allergy evaluation.
Research assessment of grapefruit-specific IgE and possible cross-reactivity among citrus or other plant foods.

Specimen Types

Serum from venous blood.
Plasma may be acceptable for some assay platforms, according to the performing laboratory’s validated requirements.
Capillary blood-derived serum may be used in some research or validated testing protocols.

Measurement Methods

Fluorescence enzyme immunoassay or other solid-phase allergen-specific IgE immunoassay.
Enzyme-linked immunosorbent assay (ELISA).
Multiplex allergen microarray or immunoassay platforms in research settings.
Skin-prick or prick-to-prick testing with grapefruit may provide complementary evidence of sensitization but does not measure F209-IgE in blood.
Immunoblotting and basophil activation testing are primarily research or specialized methods rather than routine F209-IgE measurement.

Test Preparation and Influencing Factors

Fasting is generally not required for an allergen-specific IgE blood test.
Routine medications usually do not need to be stopped for blood testing, although the ordering clinician should review all medicines and supplements.
Antihistamines can interfere with skin-prick testing but generally do not prevent measurement of circulating specific IgE in blood.
Results should be interpreted in relation to symptoms, timing of exposure, medical history, and other allergy-test findings.
A positive result may represent sensitization without clinical allergy and may occur because of cross-reactivity with related foods or pollen allergens.
The concentration of grapefruit-specific IgE does not reliably indicate reaction severity.
The assay platform, allergen extract, reporting units, and reference categories can affect results; results from different methods should not be assumed to be interchangeable.
Recent severe reactions, eczema, allergic rhinitis, asthma, and other atopic conditions may influence the likelihood of detectable food-specific IgE but do not establish grapefruit allergy on their own.
An oral food challenge, performed under specialist supervision when indicated, is the reference method for confirming or excluding food allergy.