When is allergy testing indicated?

When is Allergy Testing Indicated?

Allergy testing should be performed when patients fail empiric treatment, when the diagnosis is uncertain, or when identifying the specific allergen will change management decisions—not as a routine first-line diagnostic tool. 1

Clinical Diagnosis Comes First

The diagnosis of allergic conditions is primarily clinical and does not require testing in most cases. 1, 2 You should make the diagnosis based on:

• History of symptoms: nasal congestion, runny nose, itchy nose, sneezing, or red/watery eyes 1
• Physical examination findings: clear rhinorrhea, nasal congestion, pale discoloration of nasal mucosa 1
• Temporal patterns: seasonal variation, exposure-related symptoms 3

Many patients can be successfully managed with empiric treatment (environmental controls, allergen avoidance, or pharmacotherapy) without confirmatory testing. 1

Specific Indications for Allergy Testing

Respiratory Allergies (Allergic Rhinitis, Asthma)

Perform specific IgE testing (skin or blood) when: 1

• Inadequate response to empiric treatment after 2-4 weeks of moderate-dose intranasal steroids and antihistamines 1
• Uncertain diagnosis when symptoms could represent multiple conditions 1
• Need to identify specific allergens to guide targeted environmental controls or immunotherapy 1, 3
• Considering immunotherapy for patients with symptoms uncontrolled by pharmacotherapy 3
• Chronic or recurrent sinusitis to identify underlying allergic contributors 4

Drug Allergies

Penicillin Allergy

Do NOT test patients with histories inconsistent with allergy (headache, family history, diarrhea alone). 1 Simply remove the label after counseling, or offer a single-dose amoxicillin challenge if the patient is anxious. 1

Perform penicillin skin testing for: 1

• History of anaphylaxis to penicillin 1
• Recent reaction suspected to be IgE-mediated 1
• Adults with distant reactions (>5 years) who are at higher risk or anxious about direct challenge 1

Direct amoxicillin challenge without skin testing is appropriate for: 1

• Pediatric patients with benign cutaneous reactions (maculopapular rash, urticaria) without systemic symptoms 1
• Adults with distant (>5 years) benign cutaneous reactions, though this has less supporting evidence than in children 1

Food Allergies

• Eliminate the suspected food first as initial treatment 5
• If elimination is ineffective, perform IgE-specific antibody or skin testing to exclude allergens 5
• Oral food challenge should be performed to confirm the diagnosis 5

Insect Venom Allergies

• Patients with anaphylactic reactions to insect stings should undergo IgE-specific antibody or skin testing 5

Contact Dermatitis

• Patch testing (not IgE testing) identifies allergens responsible for contact dermatitis 5

Choosing Between Skin Testing and Blood Testing

Skin testing is preferred as first-line because it is: 1, 3, 2, 4

• More sensitive than serum IgE testing 1, 3
• Less expensive 1
• Allows direct observation of the body's reaction 1
• Provides immediate results (15-20 minutes) 6, 7

Use serum-specific IgE testing when: 1

• Skin testing is contraindicated (severe dermatographism, extensive eczema, inability to discontinue antihistamines) 1
• Patient cannot tolerate skin testing 1
• Reliable laboratory services are available (important consideration in resource-limited settings) 2

Critical Interpretation Principle

A positive test indicates sensitization, NOT necessarily clinical allergy. 1, 3, 4, 6, 7 Up to 54% of the general population tests positive on skin prick tests without having clinical symptoms. 3

You must correlate test results with the clinical history to determine clinically relevant allergens. 1, 3, 4, 6, 7 Testing without clinical correlation leads to overdiagnosis and unnecessary avoidance measures.

What NOT to Test

• Total IgE: has limited diagnostic value for allergic rhinitis 1
• IgG antibodies to allergens: not beneficial for identifying allergen triggers in IgE-mediated disease 1
• Routine imaging: CT imaging is not recommended as first-line for allergic rhinitis 1
• Non-allergic adverse drug reactions: skin testing detects IgE antibodies and is not useful for non-IgE-mediated reactions 1

Common Pitfalls to Avoid

• Testing before trying empiric treatment: Most patients respond to initial therapy without needing testing 1
• Testing patients with non-allergic symptoms: Headache, isolated GI symptoms, or family history alone do not warrant testing 1
• Ordering tests without clinical correlation: Positive tests without matching symptoms lead to false diagnoses 1, 3, 4
• Using blood tests when skin testing is available: Skin testing is more sensitive and cost-effective 1, 3