Can allergic reactions be detected on blood work?

Can Allergic Reactions Be Detected on Blood Work?

Yes, allergic reactions can be detected on blood work through specific IgE antibody testing and serum tryptase levels, but blood tests alone cannot diagnose clinical allergy—they only indicate sensitization and must be interpreted alongside clinical history and symptoms. 1

Blood Tests Available for Detecting Allergic Reactions

Allergen-Specific IgE Testing

• Specific IgE antibody tests are the primary blood test for identifying potential IgE-mediated allergic reactions, using fluorescence-labeled antibody assays (such as ImmunoCAP) that bind target allergens to detect sensitization. 1
• Blood samples can be drawn at the time of the reaction or soon afterward during hospitalization, though negative results should be repeated later as IgE antibodies may be temporarily consumed during the acute reaction. 1
• Available specific IgE tests include: succinylcholine (30-60% sensitivity), various antibiotics (amoxicilloyl, ampicilloyl, penicillin derivatives, cefaclor), latex, chlorhexidine, and bovine gelatin. 1
• Tests for neuromuscular blocking drugs other than succinylcholine are not available in the UK. 1

Serum Tryptase

• Serum tryptase levels correlate with anaphylaxis severity but are elevated less often in food allergy and correlate poorly with plasma histamine levels. 2
• Tryptase should be measured during acute anaphylactic reactions to help confirm the diagnosis. 2

Total Serum IgE

• Total serum IgE should NOT be routinely used to diagnose allergic reactions, as it lacks sufficient sensitivity and specificity compared to oral food challenges and provides no advantage over allergen-specific IgE testing. 1
• Elevated total IgE is frequently found in atopic individuals but can also indicate eczema, chronic skin infections, or underlying inborn errors of immunity rather than specific allergies. 3

Critical Interpretation Guidelines

What Positive Results Mean

• The presence of drug-specific IgE in serum indicates allergic sensitization but is NOT proof that the drug caused the reaction—attribution requires considering all clinical and laboratory information together. 1
• Higher levels of specific IgE antibodies correlate with increased probability of clinical reactivity, but the relationship varies by allergen, patient age, and clinical context. 1, 4
• Sensitization does not equal clinical allergy—many patients with positive IgE tests tolerate the allergen without symptoms. 1

What Negative Results Mean

• Undetectable specific IgE levels occasionally occur in patients with true IgE-mediated allergic reactions, so negative results do not definitively rule out allergy when clinical history is highly suggestive. 1
• If initial tests are negative but clinical suspicion remains high, further evaluation with physician-supervised oral food challenge or repeat testing may be necessary. 1

Limitations of Blood Testing by Allergen Type

When Blood Tests Are Most Useful

• Most reliable for: latex, beta-lactam antibiotics, neuromuscular blocking agents (NMBAs), and certain foods. 1
• Particularly valuable when skin prick testing cannot be performed due to extensive dermatitis, dermatographism, or inability to discontinue antihistamines. 1

When Blood Tests Are NOT Useful

• Blood tests are not useful for: NSAIDs, dextrans, or iodinated radiological contrast media, as anaphylaxis to these agents is typically not IgE-mediated. 1
• Opioid testing is generally not performed because false positives are common due to nonspecific histamine release rather than true IgE-mediated reactions. 1, 5

Common Pitfalls to Avoid

Laboratory Platform Differences

• Results from different laboratory systems (ImmunoCAP, Turbo-MP, Immulite) are NOT comparable—predictive values established for one platform cannot be applied to others. 1
• Each system uses slightly different antigen preparations, producing significantly different IgE measurements for the same serum samples. 1

Diagnostic Errors

• Never diagnose allergy based solely on positive specific IgE without correlating with clinical history—this is the most common diagnostic error. 1
• Do not assume negative IgE excludes allergy if clinical history strongly suggests allergic reaction—proceed to supervised challenge testing. 1
• Avoid testing for total IgE as a screening tool, as it provides no diagnostic advantage and can be misleading. 1

Non-IgE-Mediated Reactions

• Approximately 23.6% of allergic reactions are non-IgE-mediated and will have negative blood tests despite true clinical allergy. 6
• These cases require diagnosis through clinical history, elimination diet, and supervised reintroduction rather than laboratory testing. 6

Timing Considerations

• Blood samples for specific IgE can be drawn during the acute reaction or shortly afterward, but should be repeated 4-6 weeks later if initially negative due to possible antibody consumption during the reaction. 1
• Serum tryptase should be measured during acute anaphylaxis (within hours) for maximum diagnostic utility. 2