Overview
Definition:
Allergy testing in children aims to identify specific allergens responsible for IgE-mediated hypersensitivity reactions, helping to establish a diagnosis and guide management
The two primary methods are skin prick testing (SPT) and in vitro measurement of specific serum immunoglobulin E (sIgE) antibodies.
Epidemiology:
Allergic diseases are common in childhood, with prevalence varying by geographic location and allergen exposure
Asthma, allergic rhinitis, and atopic dermatitis affect a significant proportion of children worldwide
Food allergies are also increasingly recognized as a major concern in pediatric populations.
Clinical Significance:
Accurate allergy diagnosis is crucial for appropriate management, including allergen avoidance and pharmacotherapy
It differentiates true allergies from intolerances, prevents unnecessary dietary restrictions, and can reduce the risk of severe allergic reactions like anaphylaxis
This impacts long-term health and quality of life for affected children.
Diagnostic Approach
History Taking:
Detailed history is paramount
Elicit information on age of onset, specific symptoms (urticaria, angioedema, gastrointestinal distress, respiratory symptoms, anaphylaxis) and their temporal relationship to suspected allergen exposure
Inquire about family history of atopy, previous allergy evaluations, and response to any previous treatments
Note triggers, severity, and duration of reactions
Red flags include severe systemic reactions, multiple food allergies, or poor growth.
Physical Examination:
A thorough physical examination should focus on identifying signs of atopy such as eczema (atopic dermatitis), nasal mucosal swelling and pallor (allergic rhinitis), wheezing or diminished breath sounds (asthma), and urticaria or angioedema
General assessment of growth and nutritional status is also important.
Investigations:
The primary investigations are skin prick testing (SPT) and serum-specific IgE testing
SPT involves introducing small amounts of allergen extracts into the skin and observing for a wheal-and-flare reaction
Serum sIgE measures the amount of specific IgE antibodies against particular allergens in the blood
Other tests like total IgE, eosinophil counts, and mast cell tryptase may be used adjunctively but are not allergen-specific.
Differential Diagnosis:
It is essential to differentiate IgE-mediated allergies from non-IgE mediated adverse food reactions, food intolerances, celiac disease, irritable bowel syndrome, and other gastrointestinal disorders
Respiratory symptoms can also be due to infections or non-allergic airway hypersensitivity
Differentiating these requires careful clinical correlation.
Skin Prick Testing
Procedure:
The skin is pricked with a lancet or needle after a drop of allergen extract is placed on the skin
A positive control (histamine) and negative control (saline) are used
The test site is observed for a wheal and flare reaction after 15-20 minutes
A positive reaction is typically defined as a wheal diameter of 3 mm or more greater than the negative control.
Advantages:
SPT is rapid (results within minutes), cost-effective, and generally considered more sensitive than sIgE testing for many common allergens
It offers a direct in vivo measure of sensitization
It can be performed in children from infancy onwards, with appropriate technique.
Disadvantages:
Requires allergen extracts which may not be available for all allergens
Results can be affected by antihistamine use (which needs to be stopped for 5-7 days), patient skin conditions (e.g., severe eczema), and the skill of the operator
False positives and negatives can occur
There is a small risk of systemic reactions, though rare.
Interpretation:
A positive SPT indicates sensitization to an allergen, not necessarily clinical allergy
Correlation with the patient's history is crucial
Multiple positive tests require careful evaluation for clinical relevance, especially with food allergens.
Serum Ige Testing
Procedure:
A venous blood sample is collected from the child
The serum is then tested for the presence and quantity of specific IgE antibodies against a panel of allergens using various in vitro immunoassay techniques, such as Enzyme-Linked Immunosorbent Assay (ELISA) or Chemiluminescent Immunoassay (CLIA).
Advantages:
Serum sIgE testing is not affected by antihistamine use or skin conditions, making it ideal for children with severe eczema or those unable to tolerate SPT discontinuation of medications
It can be performed in patients of all ages
It is also useful when there is a concern for anaphylaxis or when SPT is technically difficult.
Disadvantages:
Generally less sensitive than SPT for some common allergens
Results can take several days to obtain
Can be more expensive than SPT
A positive sIgE result also indicates sensitization, not necessarily clinical allergy, requiring correlation with symptoms
May have higher rates of false positives for certain allergens.
Interpretation:
Similar to SPT, a positive sIgE result signifies sensitization
The level of sIgE antibodies can correlate with the severity of allergic reactions for some allergens, but this correlation is not absolute
Clinical correlation is paramount for accurate diagnosis.
Comparison And Clinical Use
When To Choose Which:
SPT is generally preferred as the first-line investigation for suspected IgE-mediated allergies when feasible, due to its sensitivity and cost-effectiveness
Serum sIgE is chosen when SPT is contraindicated or technically difficult, or to confirm results in equivocal cases
For food allergies, especially in young children, both methods are used, and component-resolved diagnostics may be employed for complex cases.
Correlation With Clinical Allergy:
Both SPT and sIgE measure sensitization, not necessarily clinical allergy
A positive test must be interpreted in the context of the patient's detailed history and clinical presentation
False positives and negatives can occur with both methods
Oral food challenges remain the gold standard for diagnosing food allergies when clinical history and in vitro/in vivo tests are discordant.
Factors Influencing Choice:
Factors influencing the choice include availability of allergen extracts, patient age, presence of severe eczema or dermatographism, recent antihistamine use, risk of anaphylaxis, and cost-effectiveness
Pediatric allergists will weigh these factors to select the most appropriate diagnostic strategy for each child.
Key Points
Exam Focus:
Understand the principles, advantages, and disadvantages of both SPT and serum sIgE
Know when to contraindicate SPT and when serum sIgE is the preferred modality
Remember that sensitization does not equal clinical allergy
history is king.
Clinical Pearls:
Always correlate test results with the patient's clinical history and symptoms
Consider component-resolved diagnostics for complex food allergies or when differentiating cross-reactivity
Oral food challenges are the definitive diagnostic test when indicated.
Common Mistakes:
Over-reliance on positive SPT or sIgE results without clinical correlation
Discontinuing antihistamines inappropriately before SPT
Misinterpreting cross-reactivity between allergens
Failing to consider non-IgE mediated reactions or intolerances.