Allergy Investigations

Investigations for food allergy are neither necessary nor interpretable in the acute setting

Serum tryptase

• Enzyme present in mast cell granules, released during mast cell degranulation.
• Role in Anaphylaxis:
  • Serum levels increase during anaphylaxis.
  • Peak concentration: 1-2 hours post-onset.
  • Return to baseline: Within 6 hours.
• Diagnostic Use:
  • Helps distinguish anaphylaxis from other causes of symptoms.
  • Acutely Elevated Tryptase: Defined as > 1.2 x baseline tryptase (µg/L) + 2 µg/L.

Baseline Tryptase

• Marker for Mast Cell Number: Elevated in mastocytosis.
• Associated Risks:
  • Severe anaphylaxis in response to various drugs and insect stings.
  • Risk of osteoporosis.
• Monitoring:
  • Periodic checks if elevated.
  • Bone marrow examination if baseline level > 20 µg/L.
• Implications for Allergic Individuals:
  • Susceptible to severe reactions to hymenoptera stings.
  • Potential for severe reactions to certain food and drug allergies.

Key Points

• Tryptase Peak: 1-2 hours after anaphylaxis onset, baseline within 6 hours.
• BLoods to be taken: take at 1, 6 and 24 hours
  • normal: <1ng/mL
  • non-specific and anaphylactoid reactions: 1-15ng/mL
  • true anaphylaxis levels: >15ng/mL
• Elevated Tryptase Calculation: > 1.2 x baseline + 2 µg/L.
• Baseline Tryptase Monitoring:
  • Elevated in mastocytosis.
  • Periodic checks and bone marrow examination if > 20 µg/L.
• Increased Risk: Severe reactions to stings, food, and drug allergies in individuals with elevated baseline tryptase.

Total IgE

• Poor sensitivity; low values do not exclude atopy.
• Patients with allergic conditions, such as asthma, allergic rhinitis or atopic dermatitis, often have higher serum levels of IgE than the general population. 
• Elevated total IgE may indicate that the patient is atopic, it provides no information relating to a specific condition or to what allergens the patient may be sensitive. 
• There is also a significant overlap in total IgE levels between those with and without allergic disease. For these reasons, the utility of total IgE measurement is limited.
• However, there are specific clinical situations in which total IgE is helpful. These include
  • allergic bronchopulmonary aspergillosis
  • parasitic infections
  • monitoring of immunosuppressant or biological therapies for eczema and a few rare immune disorders and malignancies. 
  • assess the likely benefit of anti-IgE therapy in patients with moderate-to-severe asthma

Eosinophil Count

• Eosinophils are specialised white blood cells that are designed to kill worms and parasites. 
• They can also cause tissue inflammation in allergy. 
• High eosinophil count does not prove that symptoms are due to allergy, and a normal eosinophil count does not exclude allergy. 
• Therefore measuring eosinophil counts has a limited role to play in allergy testing.
• Causes of Eosinophilia
  • Atopic Conditions:
    • Atopic dermatitis
    • Asthma
    • Various forms of rhinitis
    • Allergic drug reactions
  • Parasitic Infestation:
    • Strongyloides
  • Eosinophilic Gastrointestinal Disorders:
    • Eosinophilic esophagitis, gastritis, and colitis
  • Adrenal Insufficiency:
    • Due to critical illness
  • Multiorgan Autoimmune/Idiopathic Diseases:
    • Eosinophilic granulomatosis with polyangiitis
  • Myeloid Neoplasms:
    • With eosinophilia
  • Secondary Eosinophilia:
    • Due to lymphoid or epithelial neoplasms
  • Congenital Syndromes:
    • Hypereosinophilic syndrome
    • Omenn syndrome
    • Familial eosinophilia

Non-allergic angioedema Type I & II (C1 esterase inhibitor) hereditary angioedema

• This should be considered in anyone with a family history of angioedema without urticaria
• Relevant investigations include
  • C1 esterase level and function,
  • C3 and C4
• Abnormal levels occur in persons with deficiency which has autosomal dominant inheritance. 
• Persons with this disorder usually experience symptoms from puberty onwards and may have life-threatening episodes which are not responsive to adrenaline. 
• Many patients also experience recurrent abdominal pain due to visceral angioedema. 
• Functional deficiency may occur on a genetic basis or, in older persons, it can result from an interfering autoantibody associated with lymphoma. 
• Abnormal functional assay results should always be repeated.
• These assays are performed on serum and reimbursed by Medicare. 
• Sequencing for the detection of mutations causing Type I and II hereditary angioedema is performed by the laboratory and the fee is $418, non-rebatable.

Diagnosis of specific allergan to food allergy requires convincing history and is supported by:

• Skin Prick Testing (SPT)
• radio-allergosorbent test (RAST) Specific IgE tests 
• Oral Challenge 

Skin Prick Testing (SPT)

• Purpose: Detects allergen-specific IgE for foods, aeroallergens, some venoms, antibiotics, and latex.
• Measures allergen-specific IgE antibodies in the blood.
• Not used for non-IgE-mediated allergic reactions or other types of hypersensitivity (e.g., Type II, III, IV hypersensitivity reactions).
• Procedure:
  • Method: Pricking skin with allergen extract solution.
  • Results: Available within 20 minutes, discussed with doctor.
  • Sites: Forearm or back.
  • Symptoms: Small itchy lump (wheal) if allergic.
  • Precautions: Avoid antihistamines, creams, moisturisers before testing.
  • Resultant wheal and flare reaction peaks at 15–20 minutes, then read.
• Interpretation:
  • Positive SPT indicates sensitisation, not necessarily clinically relevant allergy.
  • Must be interpreted with clinical history, signs, and allergen exposure.
  • False positives and negatives can occur.
  • Not reliable in diffuse dermatological conditions or dermographism.
  • Medications (antihistamines, antidepressants) can interfere.
  • Fresh foods may be necessary for certain allergens (e.g., fruits).
  • Negative SPT almost eliminates diagnosis of IgE mediated food allergy (negative predictive accuracy >95%).
  • Positive SPT with clear history can confirm diagnosis; if history is uncertain, formal oral food challenge may be needed.
  • Positive predictive accuracy <50% for suspected food allergy.
• Risks:
  • Systemic reactions, including anaphylaxis (risk 0.02%).
• Interfering Drugs:
  • Antihistamines
  • Tricyclic antidepressants
  • H2 antagonists
  • Neuroleptics

Intradermal Testing

• Method:
  • Intradermal injection of dilute allergen solutions.
• Use:
  • More sensitive than skin prick testing.
  • Used for drug and hymenoptera venom allergy.
• Considerations:
  • Higher risk of false positives and systemic allergic reactions.
  • Best performed in specialist facilities with anaphylaxis management resources.

Radio-Allergosorbent Test (RAST)

• Purpose: Detects free antigen-specific IgE in serum.
• Sensitivity and Specificity: Generally less sensitive and specific than SPT.
• Not used for non-IgE-mediated allergic reactions or other types of hypersensitivity (e.g., Type II, III, IV hypersensitivity reactions).
• Reporting: Semi-quantitative score or quantitative CAP RAST measurement.
• Usage:
  • Useful when SPT is contraindicated.
  • Not affected by prior antihistamine or other drug use.
  • Can be performed on patients with widespread skin disease.
  • Difficult to interpret with high total IgE levels (>1000 kU/L), common in severe eczema.
• Testing:
  • Available for a wide range of allergens.
  • Medicare rebate in Australia restricts testing to four allergens at a time.
  • Allergen mixtures (e.g., food mix) not recommended.
• Clinical Application:
  • Published data correlates serum specific IgE levels with probability of reaction for some foods (egg, milk, soy, peanut).
  • Positive RAST must be interpreted in clinical context. – Indicates sensitization but does not always correlate with clinical allergy.
  • Negative results = rule out IgE-mediated allergies.
  • GPs use RAST results for allergen avoidance while awaiting specialist review.
• False Positive:
  • Cross-Reactivity:
    • Similar protein structures between different allergens can cause cross-reactivity, leading to false positives.
    • Common in pollens, certain fruits, vegetables, and latex.
  • High Total IgE Levels:
    • Patients with very high levels of total IgE (>1000 kU/L), such as those with severe eczema, may have nonspecific binding, resulting in false positives.
  • Contamination:
    • Laboratory contamination of allergen extracts can cause false positives.
• False Negative Results
  • Low Sensitivity:
    • Early Phase of Allergy: During the early phase of sensitization, IgE levels may be too low to detect.
    • Low Affinity IgE: IgE antibodies with low affinity for the allergen may not be detected.
  • Allergen Extract Quality:
    • Inadequate Allergen Representation: The allergen extract used in the test may not include all relevant allergenic components.
    • Degraded Allergen Extracts: Poor storage or handling can degrade the allergen extracts, reducing their effectiveness in detecting specific IgE.
  • Technical Issues:
    • Assay Limitations: Variations in assay performance or technical errors during the test can lead to false negatives.
    • Detection Thresholds: Some assays may have higher detection thresholds, missing low levels of specific IgE.
  • Biological Variability:
    • Allergen-Specific Factors: Some allergens may not trigger a strong IgE response, even in sensitized individuals.
    • Individual Immune Response: Variability in individual immune responses can affect the production of detectable IgE levels.
  • Medication Interference:
    • Immunosuppressants: Medications that suppress the immune system (e.g., corticosteroids) can lower IgE production and lead to false negatives.
  • Chronic Conditions:
    • Severe Eczema: In patients with high total IgE levels due to conditions like severe eczema, specific IgE may be masked by the high total IgE, leading to false negatives.

Patch Testing

• Useful for contact allergic dermatitis (e.g., nickel, cosmetic preservatives, plants).
• Hypoallergenic tape with allergen paste applied to skin, usually the back.
• Tapes left in place for 48 hours and kept dry.
• Test site read at different intervals.
• Eczema-like rash indicates sensitivity.

Oral Food Challenge

• Gold Standard for diagnosing food allergy / Diagnoses food-related non-IgE mediated allergies.
• Purpose:
  • Confirm diagnosis where unclear.
  • Determine resolution of previously documented food allergy.
• Procedure:
  • Performed by specialists.
  • Elimination Diet:
    • Suspected allergenic foods are removed from the diet for a period (usually 2-6 weeks).
    • Symptoms are monitored for improvement.
    • Foods are then gradually reintroduced one at a time to observe for reactions.
  • Oral Food Challenge:
    • Involves giving the patient increasing amounts of the suspected allergen and observing for delayed reactions.
    • Stopped at first objective reaction.
    • Various vehicles (solid and liquid) used to disguise foods.
    • Tolerance of a serving size portion indicates lack of reactivity.
  • Can also be used for drug and latex challenges.

Comparison of skin prick testing and RAST

Skin prick testing	RAST
Requires adequate training to perform reliably	Widely available – can be ordered by the GP
Requires adequate area of normal skin	Can be performed where there is extensive skin disease or in patients with dermatographism
Certain drugs (eg. antihistamines, tricyclic antidepressants) interfere with results	Can be performed if the patient is unable to cease taking medications
Must be delayed for 4–6 weeks after an anaphylactic reaction (results may not be interpretable if there has been massive mast cell degranulation)	Can be performed in the setting of recent anaphylaxis
Minor discomfort – itch	Venesection may be painful or anxiety provoking
Wide range of allergens can be tested (including fresh foods)	Some food allergens, drugs and rarer pollens may not be available
Large number of allergens can be tested at any one time	Number of allergens tested is limited by Medicare rebate (maximum of four allergens can be tested at any one time)
Results available immediately	Results may not be available for several weeks
Elevated IgE levels do not affect results	False positives may occur in patients with high total IgE levels