What samples should be sent for a patient presenting with allergies and rashes?

Diagnostic Samples for Allergies and Rashes

For patients presenting with allergies and rashes, skin testing is the preferred diagnostic method, supplemented by specific IgE blood testing when skin testing is contraindicated or inconclusive. 1

Skin Testing

Primary Testing Method

• Skin testing is the preferred diagnostic tool with higher sensitivity (70-75%) than in vitro testing for detecting allergen-specific IgE antibodies 1
• Percutaneous (prick/puncture) testing should be performed first as it consistently produces reproducible results 1
• Intradermal skin testing may be required for some allergen extracts when prick tests are negative but suspicion remains high 1

When to Use Skin Testing

• For patients with suspected inhalant allergies (pollens, molds, animal dander, dust mites, cockroach) 1
• For evaluation of Hymenoptera venom allergies 1
• For assessment of specific allergen sensitivity prior to immunotherapy 1

Considerations for Skin Testing

• Selection of allergens should be based on patient's history, environmental exposures, and regional aerobiology 1
• Quality control measures are vital for accurate and reproducible results 1
• Testing should be performed by individuals experienced in handling allergenic products 1

Specific IgE Blood Testing

Indications for Blood Testing

• When skin testing is contraindicated (widespread skin disease, severe eczema, dermatographism) 1
• When patients are on medications that suppress skin test reactions 1
• For uncooperative patients or those who cannot tolerate skin testing 1
• When the diagnosis remains uncertain despite skin testing 1

Interpretation Considerations

• Sensitivity is approximately 70-75% compared to skin testing 1
• Results may be affected by cross-reactive proteins, specific IgG antibodies, and high total IgE 1
• Positive results must be correlated with clinical symptoms for relevance 1

Special Considerations for Rash Evaluation

For Contact Dermatitis

• Patch testing is recommended to identify responsible allergens 2
• Samples should be applied to the skin and left in place for 48-72 hours 2

For Urticaria/Angioedema

• For acute urticaria: test for potential triggers including medications, foods, and infections 3
• For chronic urticaria: consider testing for animal dander, ASA sensitivity, and food additives 3

Common Pitfalls and Caveats

• False-positive skin test results may occur due to cross-contamination of allergen extracts (e.g., dog dander extracts contaminated with house dust mite allergens) 4
• Skin test positivity alone does not define clinical sensitivity - must be correlated with symptoms 1
• Avoid testing with non-standardized allergen extracts which may produce inconsistent results 1
• Testing should be performed before starting antihistamines or other medications that could suppress reactions 5

Algorithm for Allergy/Rash Diagnostic Testing

1. Initial Assessment:

   • Perform skin prick testing with clinically relevant allergens based on history 1
   • Document clear rhinorrhea, nasal congestion, pale nasal mucosa for allergic rhinitis 1

2. If skin prick tests are negative but suspicion remains high:

   • Proceed to intradermal testing for increased sensitivity 1
   • Consider specific IgE blood testing as complementary method 1

3. If skin testing is contraindicated:

   • Order specific IgE blood tests for suspected allergens 1
   • Interpret results in context of clinical presentation 1

4. For rash evaluation:

   • For contact dermatitis: perform patch testing 2
   • For urticaria: test for specific triggers based on chronicity 3

5. For non-responsive cases or uncertain diagnosis:

   • Consider controlled allergen challenges in specialized settings 1
   • Evaluate for associated conditions (asthma, atopic dermatitis, etc.) 1