Tests
1st tests to order
in vitro IgE-specific immunoassay
Test
Normative results available for CAP fluorescent enzyme immunoassay (CAP-FEIA) system; 95% positive predictive values in patients with a history of a reaction.[56]
Higher concentrations of food-specific IgE correlate to increased likelihood of a reaction on ingestion.[57][58][59]
It is important to recognize that IgE values below the predictive values are still relevant. With an IgE level of 2 kUA/L for peanut, milk, and egg the patient still has a 50% chance of having food allergy.
Food challenge may not be necessary if CAP IgE values exceed predictive levels.[57][58][59] Values obtained from other testing systems are not interchangeable.[60]
Do not perform an indiscriminate battery of lgE tests, including food IgE testing, in patients without a relevant medical history consistent with potential IgE-mediated food allergy, because false or clinically irrelevant positive allergy tests for foods are frequent.[36][37]
Result
egg: ≥7 kUA/L (≥2 kUA/L if ≤2 years old); milk: ≥15 kUA/L (≥5 kUA/L if ≤2 years old); peanut: 14 kUA/L; tree nuts: approximately 15 kUA/L; fish: 20 kUA/L
skin prick testing
Test
Highly reproducible and less costly to perform than in vitro tests.
Sensitivity >90%, specificity approximately 50%.[3]
The larger the wheal, the greater the likelihood of clinical allergy, with a wheal diameter >8 to 10 mm indicating a greater likelihood of having a clinical reaction.[61]
Negative predictive accuracy is >95% for most foods (wheal diameter <3mm greater than the negative control) and is helpful for excluding IgE-mediated allergic reactivity.[59]
90% to 95% positive predictive accuracy for most foods in most patients. Accuracy may be <90% in young infants.[2][58]
Minimal patient discomfort.
Results within 15 minutes.
Safely performed in patients of any age.
Do not perform an indiscriminate battery of lgE tests, including food IgE testing, in patients without a relevant medical history consistent with potential IgE-mediated food allergy, because false or clinically irrelevant positive allergy tests for foods are frequent.[36][37]
Result
wheal diameter 3 mm greater than control
Investigations to avoid
IgG testing
Rationale
IgG testing is unproven in this setting and can lead to inappropriate diagnosis and treatment. Appropriate diagnosis of food allergy requires specific IgE testing in a patient with a relevant medical history.[36]
Tests to consider
food challenges
Test
Food challenge performed by giving increasing amounts of suspected allergen over time.
Setting equipped with the necessary medications, equipment, and staff to treat anaphylaxis is mandatory.
Patient is challenged with an initial dose for the test food that is unlikely to produce a reaction, then progressing to a dose that should trigger a reaction.
Double-blind placebo-controlled food challenges are considered the key test in diagnosing food allergy.[50] Open challenges are prone to bias.
Challenges are graded; there should be an equivalent number of placebo and food steps.
If patient passes challenge, then an open feeding is performed. If open feeding is tolerated, then food allergy is excluded.
Result
allergic reaction
component-resolved diagnostics
Test
Purified or recombinant allergens are used to identify specific IgE sensitization to proteins within an individual food allergen.
Some studies have shown an increased ability to predict the likelihood of having a severe allergic reaction to foods like peanut or hazelnut; however, geographic pollen sensitization patterns may affect results, and additional studies are needed to justify the use of component-resolved diagnostics for foods other than peanut and hazelnut.[52][53]
The role of component testing continues to evolve.[54]
Result
positive
Emerging tests
atopy patch testing
Test
Identifies allergens that cause reactions through delayed contact hypersensitivity where T cells play a major role.[55]
Allergenic extract is occluded against intact skin for 48 hours.
Standardization of extracts and interpretation method needed before this can be incorporated into clinical practice.[55]
For investigational use only. Patch testing is well validated for contact dermatitis but not food allergy in general.
Result
erythema and induration
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