What are the recommended blood tests for diagnosing allergies, such as RAST (RadioAllergoSorbent Test) or ELISA (Enzyme-Linked Immunosorbent Assay)?

Blood Tests for Allergy Diagnosis

Allergen-specific serum IgE testing (modern fluorescence-labeled antibody assays like ImmunoCAP) is the recommended blood test for identifying IgE-mediated allergic reactions, though it must always be interpreted alongside clinical history and cannot diagnose allergy in isolation. 1

Primary Blood Test Recommendation

Use allergen-specific serum IgE (sIgE) testing as your primary laboratory test when blood testing is indicated for suspected IgE-mediated allergies. 2, 1 This test measures IgE antibodies directed against specific allergens (foods, pollens, dust mites, animal dander, etc.).

Modern Testing Platforms

• Modern fluorescence-labeled antibody assays (such as ImmunoCAP) are preferred over older radioallergosorbent tests (RAST) for optimal accuracy and reproducibility. 1
• The term "RAST" is now an anachronism, as radioactive-tagged antibodies are seldom used today. 2
• Different laboratory assay systems can provide significantly different measurements for the same serum samples, so predictive values from one system cannot be applied to another. 1

Interpreting Allergen-Specific IgE Results

Negative Results

• A negative sIgE result (threshold <0.35 kUA/L) has high negative predictive value and effectively rules out IgE-mediated allergy. 1

Positive Results

• Higher sIgE levels correlate with increased probability of clinical reactivity, though predictive thresholds vary by allergen. 1
• Positive results require clinical correlation - sensitization (positive test) does not equal clinical allergy, as many patients have positive tests without symptoms. 1

Tests That Should NOT Be Ordered

Total Serum IgE

Do not use total serum IgE for routine allergy diagnosis - it lacks specificity for individual allergens and cannot identify which allergens are causing symptoms. 2, 1

Intradermal Testing (Blood Alternative)

Intradermal testing should not be used for food allergy diagnosis, though it may have limited utility in respiratory allergies when skin prick testing is negative. 2, 1

Atopy Patch Test

The atopy patch test should not be used in routine evaluation of non-contact food allergies. 2, 1

Advanced Blood Testing: Component-Resolved Diagnostics

Component-resolved diagnostics (CRD) measure IgE to specific allergen components and are increasingly used in specialized centers when standard tests are equivocal. 1

Examples of CRD Testing:

• Ara h 2 for peanut allergy 1
• Ana o 3 for cashew allergy 1
• Gal d 1,2,3,5 for egg allergy 1

CRD can reduce the need for oral food challenges by almost two-thirds when used in a 2-step algorithm. 1

Clinical Algorithm for Blood Testing

Step 1: Clinical History First

Start with detailed clinical history focusing on symptoms occurring within minutes to hours after allergen exposure, especially if reproducible on multiple occasions. 2, 1 History takes precedence over diagnostic tests. 2

Step 2: When to Order Blood Tests

Order allergen-specific serum IgE testing when:

• Patients do not respond to empiric treatment 2, 1
• The diagnosis is uncertain 2, 1
• Knowledge of the specific allergen could impact therapy decisions 2, 1
• Skin testing is contraindicated or unavailable 3

Step 3: Interpret in Clinical Context

• Negative sIgE effectively rules out IgE-mediated allergy 1
• Positive sIgE requires correlation with clinical symptoms - confirm that the patient actually has symptoms when exposed to that allergen 1
• If results are discordant with history, consider oral food challenge (the gold standard) 2

Critical Pitfalls to Avoid

Never Diagnose Based on Blood Tests Alone

History and physical examination are essential first steps - blood tests alone cannot diagnose clinical allergy. 2, 1 Multiple studies demonstrate that 50-90% of presumed food allergies are not true allergies when properly tested. 2

Avoid Panel Testing Without Clinical Indication

Order tests directed by clinical history rather than broad panels of allergens. 1 Testing for allergens that don't correlate with the patient's exposure history leads to false-positive results and unnecessary dietary restrictions.

Recognize Sensitization ≠ Clinical Allergy

Many patients have positive sIgE tests without clinical symptoms - this represents sensitization, not clinical allergy. 1 Treatment decisions should be based on clinical reactivity, not test results alone.

Don't Mix Testing Platforms

Avoid comparing results from different laboratory assay systems, as they provide significantly different measurements and have different predictive values. 1

Blood Testing vs. Skin Testing

Skin prick testing is often the initial technique for detecting IgE-mediated allergies and may be performed alongside or before blood testing. 1 However, blood testing is preferred when:

• Patient is taking antihistamines or other medications that suppress skin test responses 2, 3
• Patient has extensive dermatitis or dermatographism 4, 3
• Risk of severe reaction makes skin testing dangerous 2
• Patient preference or age considerations 3

Both skin testing and blood testing require correlation with clinical history and neither can be considered diagnostic in isolation. 2, 1