What blood test is used to diagnose allergies?

Blood Test for Allergy Diagnosis

The allergen-specific IgE (sIgE) blood test, measured using modern fluorescence-labeled antibody assays such as ImmunoCAP, is the primary laboratory test for diagnosing IgE-mediated allergies. 1, 2

What the Test Measures

• Allergen-specific IgE testing detects sensitization to particular allergens by measuring IgE antibodies in the blood, though a positive result indicates sensitization only—not necessarily clinical allergy. 3, 1
• Modern fluorescence-labeled antibody assays like ImmunoCAP are preferred over outdated RAST tests for identifying IgE-mediated sensitization. 1, 2
• Over 400 characterized allergens are available for testing, including foods, inhalants, insect venoms, and medications. 4

How to Interpret Results

• A negative result (<0.35 kU/L) effectively rules out IgE-mediated allergy with >95% negative predictive value. 1, 2
• A positive result (≥0.35 kU/L) requires correlation with clinical history, as sensitization does not equal clinical allergy—50-90% of presumed food allergies based on history alone are not actual allergies when formally tested. 3, 1, 2
• Higher sIgE levels correlate with increased probability of clinical reactivity, though predictive values vary between studies and patient populations. 3, 2
• Undetectable sIgE levels occasionally occur in patients with true IgE-mediated allergy, so if clinical history is highly suggestive, further evaluation with physician-supervised oral food challenge is necessary. 3

When to Use This Test

• Order sIgE testing when skin prick testing cannot be performed due to extensive dermatitis, dermatographism, or inability to discontinue antihistamines. 1, 2
• Use for suspected IgE-mediated reactions to foods (milk, egg, peanut, tree nuts, wheat, soy, fish, shellfish), inhalants (pollens, molds, dust mites, animal dander), insect venoms, or medications. 1, 2, 5
• For inhalant allergens, ImmunoCAP demonstrates 85-95% specificity and sensitivity, making it highly reliable. 4
• For food allergens, specificity decreases compared to inhalants, and definitive diagnosis often requires oral food challenge when results are equivocal. 4

Critical Pitfalls to Avoid

• Never diagnose allergy based solely on positive sIgE without clinical correlation—sensitization is not synonymous with clinical allergy. 3, 1, 2
• Do not assume undetectable sIgE excludes allergy if clinical history is highly suggestive—proceed to oral food challenge in these cases. 3, 1
• Never compare results across different laboratory platforms (ImmunoCAP vs. Turbo-MP vs. Immunlite)—predictive values are platform-specific and cannot be applied to other test methods. 3, 1, 2
• Results vary significantly between different assay methods, with absolute agreement in only 55-65% of cases and differences in one or more IgE classes in 35-45% of cases. 4

Test Performance by Allergen Type

• For nut allergy, ImmunoCAP has the highest detection rate (71%) compared to skin prick tests (56%) or multiplex testing (65%). 5
• For wheat-induced anaphylaxis, ImmunoCAP identifies 100% of cases, superior to multiplex testing (67%). 5
• For oral allergy syndrome, multiplex allergen chip testing (ISAC) has higher detection rate (88%) than ImmunoCAP (69%). 5
• For latex allergy, skin prick testing remains the preferred diagnostic method. 5

Non-IgE-Mediated Allergies

• Approximately 23.6% of children with food allergy have non-IgE-mediated disease, defined as positive oral challenge with sIgE <0.35 kU/L and negative skin prick test. 1
• Diagnosis of non-IgE-mediated allergy relies on clinical history, elimination diet, and supervised reintroduction rather than laboratory testing, as sIgE testing will be negative by definition. 1

Additional Considerations

• Different laboratory assay systems provide significantly different measurements for the same serum samples, so establish which platform your laboratory uses and interpret accordingly. 2, 6
• Age and seasonal variations affect total IgE levels, which can be used as a screening tool but normal values do not exclude atopic disease. 4
• Most atopic patients with total IgE >1000 IU/ml have positive specific IgE against some allergen. 4