Allergy Testing

Specimen Collection

Serum Testing

Serum IgE testing requires a blood sample, and the clinician must specify each allergen-specific antibody to be measured. Tests may be grouped into panels containing related allergens. These panels are often expensive, and clinicians should consider the number and clinical relevance of the panels ordered. Samples are analyzed using an enzyme-linked immunosorbent assay. During this assay, the allergen is bound to a testing surface, the serum sample is added, and any corresponding allergen-specific IgE antibodies bind to the allergen. An enzyme-labeled reagent and substrate are then added, and an automated instrument measures the resulting signal. Results may vary among approved testing instruments, and clinicians should consider these differences when interpreting findings. Radioallergosorbent testing is no longer used.[5] For a suspected anaphylactic reaction to a medication, clinicians should obtain 2 blood samples for measurement of mast cell tryptase at specified intervals after the reaction.[4] Mast cell tryptase is one of the enzymes released by mast cells when they degranulate in reaction to an allergen. Tryptase levels peak at approximately 3 hours, and serial samples are necessary to detect the increase and subsequent decline.[6] Other forms of allergy testing do not require collection of a blood specimen.

Procedures

Skin Prick Testing 

Allergen solutions at concentrations ranging from 1:10 to 1:20 g/L, along with positive and negative controls, are applied to the skin at least 2 cm apart. A specialized device is then used to puncture the skin and introduce the allergen beneath the outer epidermal layer. The positive control is typically histamine dihydrochloride at a concentration of 10 g/L, and the negative control is typically glycerinated saline. Control results are assessed after 10 minutes, and allergen results are assessed after 15 to 20 minutes.[2] In a sensitized individual, IgE-bound cutaneous mast cells degranulate in response to the allergen and release inflammatory mediators, producing a wheal at the testing site.[7]

Intradermal Testing

Each allergen is injected intradermally to produce a wheal, similar to the technique used for a tuberculin test. Allergen dilutions typically range from 1:500 to 1:1000.[1] Positive and negative controls are also included. If skin prick testing is performed concurrently, the same controls may be used. A positive result is defined as a wheal measuring at least 5 mm or a reaction larger than the negative control. Intradermal testing may produce a positive result after negative skin prick test results; however, the intradermal findings may not be clinically significant.

Patch Testing

A patch containing the allergen is applied to the skin, usually on the back, for 48 hours. Results are assessed 15 to 60 minutes after the patch is removed.[1]

Indications

Indications for allergy testing include persistent asthma, suspected medication allergy, suspected insect allergy, suspected food allergy, seasonal rhinitis, rhinoconjunctivitis, and rhinitis associated with otitis media.[7] Serum IgE testing is helpful for patients who cannot stop taking their antihistamines, cannot tolerate skin prick testing, or have other skin diseases that can interfere with skin testing. Skin prick tests are usually used for food allergies, although IgE serum tests are being done more frequently by the primary care clinician in place of immediate referral to an allergist.

Serum IgE and skin prick tests are frequently used together for food allergy testing to confirm a true food allergy before eliminating it from the patient’s diet.[8] Intradermal testing is usually more sensitive than skin prick testing and can be used after negative skin prick test results when the clinical history remains suggestive. However, intradermal testing carries a higher risk of anaphylaxis because it uses a higher concentration of the allergen than skin prick testing.[9]

If an IgE test is performed and has unexpected results when considering the history, skin prick testing should be done to further solidify the diagnosis of an allergy.[1] Serum testing measures circulating free IgE and does not account for IgE bound to mast cells from existing sensitization.[5] IgE serum testing is not reliable for suspected drug allergies.[4] Patch testing can be used for drug allergies with skin manifestations.[10] This method is commonly used to diagnose or confirm allergic contact dermatitis because these delayed reactions are mediated by T cells.

Normal and Critical Findings

Skin Prick and Intradermal Testing

The positive control normally produces a 3 mm wheal. If the positive control does not produce the expected response, the test results may be invalid and require repeat testing. An allergen response measuring at least 3 mm larger than the negative control is generally considered positive.[1] Skin prick testing has approximately 90% sensitivity for food allergies, although specificity is approximately 50%. Therefore, skin prick testing should not be used as a screening tool for food allergies because false-positive results may lead to unnecessary dietary restrictions.[2] Milk and egg are among the most commonly identified food allergens during testing.[11]

Skin prick testing results can be combined with serum IgE results to improve diagnostic accuracy, particularly when the results are discordant. Results from skin prick and intradermal testing alone cannot establish a diagnosis and must correlate with the patient’s clinical history. Some patients may have elevated IgE levels or positive skin prick or intradermal test results without developing symptoms during natural exposure. Conversely, patients may have negative skin prick test results despite a history of reacting to the allergen. Serum IgE testing may produce false-positive results, and intradermal testing may yield positive findings at concentrations higher than those encountered during natural exposure. When skin and serum testing do not establish a clear diagnosis, a supervised allergen challenge may be appropriate.

Serum IgE Testing

Elevated IgE levels do not necessarily correlate with reaction severity. Serum test results must be interpreted in the context of the patient’s history and clinical examination findings because IgE levels alone cannot establish a diagnosis. False-positive results may occur, and testing may fail to identify an allergy if the suspected allergen was not included in the panel.[5] Elevated levels may also correlate with more chronic allergic diseases than acute reactions.[12] Conversely, patients with low serum IgE levels may remain at risk for anaphylaxis because serum testing does not measure IgE bound to mast cells or tissues. Advantages of serum IgE testing include objective measurement, reduced operator dependence, and the ability to compare and review results over time.[2]

Patch Testing

A trained clinician should interpret patch test results. Wheat is the food allergen most commonly identified through patch testing.[11]

Interfering Factors

Skin Prick Testing

Patients should discontinue antihistamines 1 week before testing, histamine 2 receptor blockers 48 hours before testing, tricyclic antidepressants up to 2 weeks before testing, and omalizumab 6 months before testing because these medications may suppress or alter the response. Patients who experienced anaphylaxis within the preceding 30 days should not undergo skin prick testing because test results may be false-negative.[1] Test results can be complicated by eczema and other skin conditions, which can give false positives.[4] Furthermore, the allergen specimens can have different concentrations based on geographic areas and can have varying degrees of testing reactions.[7] Wheal size may also vary according to the skin prick testing device used.[9]  

Serum IgE Testing

High levels of antibodies do not always correlate with the severity of the allergies or reactions but do correlate with a higher chance of a reaction.[8] Other conditions may also cause elevated IgE levels. Antihistamines do not interfere with serum IgE test results, and a recent allergic reaction does not alter IgE levels.[2]

Skin Patch Testing

Distinguishing a true allergic reaction from an irritant response may be difficult.[1] Since the allergic reaction mechanism is mediated by T cells instead of mast cells, antihistamine treatment does not interfere with the results. This test also has some subjectivity, since a trained technician reads it.

Complications

Anaphylaxis is a potential complication of any allergy testing that involves introducing an allergen to the patient. This happens when the reaction is severe, and the mast cells cause more than a local reaction. With that in mind, however, intradermal testing has a higher risk of systemic reactions than skin prick testing because it uses higher concentrations of the allergens.[1]

Patient Safety and Education

Skin Prick Testing

Patients with uncontrolled asthma, reduced lung function, atopic dermatitis, acute or chronic urticaria, dermographism, active angina, cardiac arrhythmias, older adults, pregnant women, and those with comorbidities are more likely to have adverse effects or difficulty with the treatment of any adverse effects. However, these patients are not at a higher risk of anaphylaxis.[1] This population may fare better with serum IgE testing instead.

Clinical Significance

Allergy testing can help prevent serious medical emergencies by identifying triggers for allergic reactions and asthma exacerbations. Accurate identification of allergens, particularly in patients with confounding medical conditions, may reduce complications and the need for office or hospital visits. This information can help patients manage asthma and allergies more effectively at home. Improved symptom control may reduce medication use and healthcare visits while improving overall health and well-being. Results from serum IgE and intradermal testing help allergists select appropriate initial allergen dilutions for injection immunotherapy.[5]