Allergy Testing
David Stukus, MD
Indications for Allergy Testing
•Persistent symptoms, despite regular use of medications, such as antihistamines and/or nasal steroid sprays
•Exposure to potential allergens with ongoing symptoms (eg, cat or dog inside the home)
•Consideration of allergen immunotherapy for severe refractory symptoms
•Children with persistent asthma who
—Require more than 1 controller medication
—Have ongoing symptoms despite adherence to therapy
—Require help to determine a prognosis
•Need to establish the proper diagnosis and best treatment options
Appropriate Age for Allergy Testing
•There is no age limitation in the application of allergy testing or interpretation of the results. If a child is old enough to develop immunoglobulin E (IgE)–mediated inhalant allergies, then he or she is old enough to undergo allergy testing to detect specific IgE.
•Younger children often have negative allergy test results because their symptoms are not caused by IgE-mediated allergies.
•Perennial allergic rhinitis (caused by exposure to dogs, cats, dust mites, cockroaches, or rodents) typically does not develop until at least 12 months of age and can begin anytime thereafter.
•Seasonal allergic rhinitis (caused by exposure to trees, grasses, weeds, ragweed, or mold spores) develops as follows:
—Typically not until at least 2–3 years of age
—Unlikely in children <18 months of age
Skin Prick Testing
•Skin prick testing involves the introduction of a small amount of allergen percutaneously, through the use of a prick device.
•Test samples should be placed on the upper back or volar aspect of the forearm.
•If the patient has specific IgE attached to localized mast cells, the skin prick test for an allergen will result in a localized wheal (bump) and flare (erythema) response.
• Test results should be interpreted 15 minutes after application.
•The mean diameter of the wheal is measured to determine sensitization to that particular allergen. A positive (histamine) and negative (saline) control sample should always be placed and used for comparison and interpretation of specific allergen wheal diameters.
•A wheal ≥3 mm larger than the negative control sample is consistent with the presence of allergic sensitization.
•The size of the skin prick test result does not correlate with severity
of symptoms but does correlate with likelihood of the test result being clinically relevant.
•Medications can interfere with test results (Table 7-1); antihistamines should be discontinued at least 3–5 days prior to testing.
| Table 7-1. Differences Among Serum-Specific Immunogloblulin E Diagnostic Tests | ||
| Skin Prick Test | Serum-Specific Immunoglobulin E | |
| Sensitivity | High | High |
| Specificity | Low | Low |
| Medications that may interfere with results | •Antihistamines •Tricyclic antidepressants •H2 histamine blockers •Long-term systemic corticosteroids (>2 wks) | None |
| Adverse effect | •Localized pruritus, discomfort •Very low risk for anaphylaxis | •No risk for allergic reaction •Localized trauma from venipuncture |
| Timing of results | 15–20 min | Hours to days, depending on the laboratory |
Serum-Specific IgE Testing
•Immunoassay is used to measure levels of specific IgE toward an allergen through routine venipuncture.
•Results are reported as a range from 0.1 to 100 kU/L.
•Similar to skin prick testing, the level of specific IgE does not correlate with severity of symptoms but does correlate with the likelihood that allergy is present.
•Many laboratories also report arbitrary classes assigned to certain ranges; these have no clinical relevance and should not be used in the interpretation of results.
•Serum IgE results are not affected by any medications; patients do not need to discontinue antihistamines prior to testing (Table 7-1).
Total IgE Levels
•Total IgE levels are not diagnostic for any condition and should not be obtained routinely.
•Atopic conditions, such as atopic dermatitis, allergic rhinitis, and asthma, are generally associated with higher total IgE levels.
•There are 2 scenarios in which a total IgE level is indicated and useful:
—To determine if allergic bronchopulmonary mycosis (aspergillosis) is present, in which total IgE levels often double or increase substantially during an acute flare
—To determine dosing of omalizumab (anti-IgE monoclonal antibody) for patients with moderate to severe asthma who are candidates for this therapy
Interpretation of IgE Results
•Both skin prick and serum IgE tests are associated with high negative predictive values but poor positive predictive values. Neither test can be used as a screening tool or as a predictor of future allergy.
•The size of the test result does not correlate with severity of symptoms, only the likelihood that allergy is present.
•For inhalant allergens, predictive values are not well established for any indoor or outdoor inhalant allergens. Interpretation relies on clinical history, pretest probability, and exposure history.
•For food allergens, predictive values have only been established for the
8 most common food allergens: milk, egg, wheat, soy, peanut, tree nuts, fish, and shellfish. Determination of food allergy relies on clinical history, pretest probability, and size of the test result.
•Foods should never be removed from the diet solely on the basis of IgE test results. Owing to high rates of falsely increased and clinically insignificant results, a thorough history should be obtained to elicit reproducible, immediate-onset symptoms consistent with IgE-mediated food allergy prior to ordering any food allergen IgE testing or interpreting the results.
Indications for Repeat Testing
•Repeat testing is rarely necessary any sooner than 12 months after the last test, unless new symptoms develop.
should be conducted.
•Food allergy testing should be repeated annually for anyone with physician-diagnosed food allergy who is strictly avoiding a certain food (or foods). Many children with food allergies will develop tolerance with age.
•Testing should not be repeated for these reasons:
—To gauge effectiveness of immunotherapy or to determine when to discontinue immunotherapy. This is best determined through assessment of symptoms or predetermined time frames for maintenance immunotherapy (3–5 years).
—To determine effectiveness of medical management. This is best approached through assessment of symptoms.
When to Refer
•Asthma or allergy symptoms persisting despite adherence to baseline daily therapy (antihistamines with or without nasal steroid sprays, inhaled steroids)
•Skin prick testing required
•Immunotherapy or use of monoclonal antibodies being considered in the management of severe refractory asthma
•Education or information needed for allergy diagnosis, allergen avoidance, prognosis, and additional treatment considerations
•Patient requires National Heart, Lung, and Blood Institute asthma guideline step 4 care or higher
•Assistance needed in the interpretation of previously obtained IgE test results
Resources for Families
•Allergy Testing (American Academy of Allergy, Asthma, and Immunology). www.aaaai.org/conditions-and-treatments/library/allergy-library/all-about-allergy-testing
•What You Need to Know About Food Allergy Testing (Kids With Food Allergies). www.kidswithfoodallergies.org/page/food-allergy-test-diagnosis-skin-prick-blood.aspx
Clinical Pearls
•Skin prick and serum IgE tests alone are not diagnostic for any allergies.
•The size of an allergy test result can help determine the likelihood of allergy being present, as determined by the clinical history.
•Allergy testing for inhalant allergens can help identify the cause of nasal and ocular symptoms, appropriate treatment strategies, avoidance measures, and prognosis for children with asthma.
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