Food Allergy (Pediatric) Diagnostics

Diagnostic Tests  

Tests are selected based on history, which should suggest the possible allergic mechanism involved, and should be focused on the suspected food that provoked the allergic reaction. Laboratory testing is of limited value in the acute care setting since treatment is of utmost importance. Skin prick test (SPT) and serum IgE testing to allergen extracts are the recommended first-line tests in patients suspected of IgE-mediated food allergy. Serum IgE testing to individual allergen molecules is second line.  If the skin prick test is equivocal, serum IgE testing can be performed and vice-versa. This should be considered prior to an oral food challenge in patients with moderate pretest probability or when the manifestations are clinically suggestive of a high pretest probability. This is not recommended for patients with low or very low pretest probability. Skin prick tests or serum IgE testing detect food sensitization but a positive test has low specificity; clinical correlation is necessary. The post-test probability (based on the combination of the patient’s history and test results) will determine if an oral food challenge is needed to confirm the diagnosis of food allergy.  

Basophil Activation Test (BAT)  

A basophil activation test may be performed if the standard allergy test results are equivocal. This is considered in those with an equivocal diagnosis of IgE-mediated allergy to peanut or sesame.  

Skin Prick Test (SPT)

A skin prick test is also referred to as a puncture or epicutaneous test. This helps identify foods that provoke IgE-mediated allergic reactions. A skin prick test alone cannot be considered diagnostic. This is useful in determining the food responsible for IgE-mediated food allergy in patients with confirmed food allergy. Patients should stop taking antihistamines before skin testing to avoid false negative results. Long-acting antihistamines (eg Cetirizine) should be avoided for 10 days while short-acting antihistamines (eg Chlorpheniramine) for 3 days before the skin test. This has a low positive predictive value in the initial diagnosis of food allergy but has a high negative predictive value. A positive test correlates with the presence of allergen-specific IgE bound to the surface of cutaneous mast cells. A positive skin prick test is a wheal with a mean diameter of ≥3 mm than the negative control and a flare of >10 mm. This is different from intradermal testing, which is associated with a greater risk of systemic adverse allergic reactions and is not recommended in the diagnosis of food allergy.  

Allergen-specific Serum IgE Test  

An allergen-specific serum IgE test is a standard method to establish allergen sensitization. This helps identify foods that provoke IgE-mediated allergic reactions. Serum IgE alone cannot be considered diagnostic. This is useful when a skin prick test cannot be done and in the following: When the patient has extensive dermatitis or has severe dermographism; when the patient cannot withdraw antihistamine use; when the patient’s reaction was anaphylactic and there is great risk even for skin testing; and when there is unavailability of extract or reagent for skin testing. This has a high negative predictive value in excluding food allergy. Levels may be used to evaluate whether sensitization to food is increasing, stable or waning over time. This may predict the likelihood of a reaction but not its severity.  

Oral Food Challenge  

An oral food challenge is considered the most definitive test for food allergy. It is recommended as the reference diagnostic test for confirmation or exclusion of food allergy in equivocal cases after a skin prick test, serum IgE test, and/or basophil activation test. This may be open, single-blind, or double-blind placebo-controlled. A double-blind, placebo-controlled food challenge (DBPCFC) is the gold standard test in diagnosing IgE-mediated food allergy. This is performed if the result of an open oral food challenge is indeterminate and in research studies. This may also be done in patients whose symptoms are likely to be non-immune mediated or whose symptoms are non-specific or difficult to evaluate, and in very anxious patients. The disadvantages of oral food challenge include cost, potential for severe allergic reactions and is time-consuming. An open or single-blind food challenge may be used in a clinical setting since DBPCFC can be very expensive and inconvenient. When results are negative, this may be diagnostic in ruling out food allergy. When results are positive, this may be considered diagnostic in patients supported by history and laboratory data. An oral food challenge may be considered when skin prick test and serum IgE test thresholds are <50% negative predictive value to rule out food allergy or when initial test results suggest absence or resolution of allergy. The test should be performed under medical supervision to document the dose that provokes the reaction and to administer treatment should adverse reactions, including anaphylaxis, occur.  

Arachis Hypogaea 2 (Ara h 2) Test  

In patients with suspected IgE-mediated peanut allergy, Ara h 2 test should be considered if only a single diagnostic test is to be used.  A Cor a 14 or Ana o 3 test may be performed in those with suspected hazelnut or cashew nut allergy, respectively. This has a high diagnostic accuracy for peanut-specific IgE but with lower sensitivity compared to skin prick test and serum IgE test.  

Food Patch Test (Food Atopy Patch Test/Food APTs)  

Food patch tests may be used in assessing causative food allergens in patients with pediatric eosinophilic esophagitis but the tests are not standardized and widely validated.  

Food Elimination Diets  

Food elimination diets may be useful in identifying foods causing some non-IgE-mediated food allergies. This is considered diagnostic and therapeutic in patients with adverse reactions to foods, regardless of the mechanism involved. When combined with a convincing history, food elimination diets may be enough to make a diagnosis of food allergy in several food-induced allergic disorders. If instituted for longer periods, ensure that the patient is able to meet nutritional requirements to avoid deficiencies.