Immediate Hypersensitivity Reactions Workup

Updated: Feb 09, 2015
  • Author: Becky Buelow, MD, MS; Chief Editor: Michael A Kaliner, MD  more...
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Laboratory Studies

Some laboratory tests may be helpful in determining whether a reaction is truly allergic in nature.

Obtaining a serum tryptase level soon after the onset of symptoms can be helpful in differentiating anaphylaxis from other forms of shock and from other symptom complexes that may be confused with anaphylaxis. The tryptase level can be elevated, which is indicative of mast cell degranulation. False-negative results can occur, especially when food is the cause of anaphylaxis. Ideally, the tryptase level should be drawn within 4 hours after the event.

An elevated eosinophil count may be observed in patients with atopic disease.

IgE levels may be elevated in patients who are atopic, but the level does not necessarily correlate with clinical symptoms.

In vitro assays that measure serum antigen-specific IgE (i.e., radioallergosorbent test [RAST], ImmunoCAP, Immunolite) can be useful in identifying which allergens are causing symptoms for the patient. More sensitive tests have been available in recent years and have a greater positive predictive value for foods. These tests can sometimes detect clinically irrelevant allergens, however, creating false-positive results to some foods. Molecular diagnostic tests are now available that can detect allergenicity to specific food protein molecules known to trigger IgE-mediated reactions. [34]


Other Tests

Skin tests

Skin tests can be performed in the outpatient setting in the allergist's office and are very useful in the evaluation and management of allergic rhinitis, allergic asthma, food allergy, venom allergy and Penicillin drug allergy.

Skin prick tests involve pricking the skin where diagnostic allergen has been placed. A positive reaction consists of a wheal and flare that occurs within 15-20 minutes. Use of proper controls is a key component to interpretation of the tests but is often not included with kits marketed to nonspecialists.

Intradermal (ID) tests involve injecting environmental allergen into the superficial dermis. ID tests have many more false positive reactions, and the clinical significance of a positive ID test is questionable. ID tests are used for drug allergy (Penicillin and local anesthetic skin testing) and venom allergy testing. ID tests are never used for food allergy testing.

Food skin tests have a higher false-positive rate than skin tests for aeroallergens, but negative food skin test results can be helpful in excluding IgE-mediated allergies, including food-related exercise-induced anaphylaxis, especially if a fresh food is used as the antigen. No standardized food testing extracts are available.

For the most part, standardized diagnostic allergens are not available for drugs. Penicillin is the only drug for which a standardized diagnostic allergen exists, but even this test is only available for the major determinant, one of many possible allergens in the penicillins. Nonstandardized skin tests can be performed for the minor determinants in penicillin or for other drugs (ie, by pricking the skin where drug solution has been placed). Protocols are available for testing to certain medicines, such as penicillin and local anesthetics.

Skin tests are useful in identifying hypersensitivity to venom. Testing should only be performed in patients who have diffuse cutaneous symptoms and are ≥16 years old or in patients who have systemic symptoms. Unfortunately false-negatives do occur, but if testing is positive then venom immunotherapy can be life-saving.

Spirometry/pulmonary function tests

Spirometry or pulmonary function tests offer an objective means of assessing asthma.

Peak-flow meters can also be used for this and can be used by patients at home to monitor their status. It is important to remember readings from peak-flow meters are effort dependent. Personal spirometers that measure FEV1 are now also available for home use.

Inhalation challenge with histamine, methacholine, mannitol, and specific allergen can be used to confirm airway hypersensitivity or allergen sensitivity.

Measurement of exhaled nitric oxide can be used to evaluate inflammation in the airways seen with asthma and to follow efficacy of or adherence to anti-inflammatory medications (eg, inhaled corticosteroids).

Nasal smear tests

A nasal smear can be performed to look for eosinophils. However, regular use of a nasal corticosteroid can lower the eosinophil count.

Elevated eosinophil levels can be consistent with allergic rhinitis as well as nonallergic rhinitis with eosinophilia syndrome (NARES).

Induced sputum: Sputum induced from the airways can be evaluated for eosinophils, which is a measure of inflammation seen in asthma.