Allergy Testing

Indications

The first step to effective allergy testing is a thorough history. With a good history, the clinician can determine whether it is appropriate to test a single allergen, a broad panel of common allergen, or anything in between. Allergy testing can be clinically useful for investigating:

  1. Latex allergy
  2. Food allergy
  3. Medication allergy
  4. Venom allergy 
  5. Environmental allergies (causing: asthma, conjunctivitis and rhinitis) including:
  • Tree, grass, and weed pollen (targeted to geography)
  • Molds
  • Dust mites and cockroach
  • Animal dander

 

In Vivo Testing

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Skin Testing

Allergen is introduced to the surface of the skin and scratched or poked with a lancet. Positive (histamine) and negative (saline) controls are used. The most common locations are the forearm and back. Results are read in 15-30 minutes. The test is positive if a particular antigen causes a wheal. 

Skin poke testing has a very small risk of serious complications. One large study found systemic allergic responses in 0.02% of patients that underwent skin poke testing. There are no reports of deaths resulting from skin testing, though a there are a small number of case reports of anaphylaxis and death following intradermal allergen testing.

Medications and Skin Testing

What medications suppress skin test?

  1. First generation Anti-H1 Histamines (ex: Diphenhydramine/Benadryl)
  2. Second generation Anti-H1 Histamines (ex: Loratadine/Claritin)
  3. Tricyclic antidepressants
  4. High dose, long-term oral steroids
  5. Potent topical steroids

What medications do not suppress skin test?

  1. Asthma medications
  2. Short burst of oral steroids

 

Types of Skin Testing

Percutaneous Testing

How administered: introduced through prick or puncture method on volar surface of arm or upper back, allergen extracts diluted to 1:10 to 1:100
How long: evaluated at 15-20 minutes
Positive test: wheal with at least 3mm diameter of induration with surrounding erythema compared to negative (diluent) control
Who: no age limit, but rarely done if <6mo old
Advantages: immediate results, rarely induce irritant reactions, correlate better with clinical history
Disadvantages: dependent on consistent technique, some decrease in extract potency over time, NOTE: systemic reactions have been observed

Intradermal Testing

How administered: involves injecting specific allergen into dermal layer using 26- to 30-gauge needle, make small bleb (like tuberculin test), diluted to 1:1,000, but introduces more allergen
How long: evaluated at 15-20 minutes
Positive test: wheal with at least 5mm in diameter of induration with surrounding erythema compared to negative (diluent) control
Who: people with significant allergic history, but negative or equivocal percutaneous test
Advantages: more sensitive, fewer false-negative reactions
Disadvantage: low but is real risk of anaphylaxis, so need personnel and equipment do manage

CAUTION: When interpreting food skin testing only a fraction of positive children will react when food challenged
NOTE: Most common food allergies in children: milk, wheat, soy, egg, peanut, tree nuts, fish

Video of skin prick testing for patients: http://www.mayoclinic.org/tests-procedures/allergy-tests/multimedia/allergy-tests/vid-20084715

 

In Vitro Testing

Indications for testing: severe cutaneous disease, cannot discontinue medications, have history of severe anaphylaxis

Disadvantages: decreased specificity for highly atopic patients with elevated total IgE, increased cost, delays in results, laboratory reliability, assessment of threshold values are difficult to interpret for clinical significance
NOTE: not as sensitive as skin testing in defining clinically pertinent allergens

Types of In Vitro Testing

Radioallergosorbent test (RAST)

How administered: specific antigen attached to solid-phase disk and incubated in patient's serum (which contains IgE), then radioactive anti-IgE antibody added and amount of radioactivity is measured
Results: semiquantitative and reported in scales ranging from 0-4 or 0-6
Multiple types: measures IgE antibody or IgG or IgG4, but no clinical significance for IgG and IgG4

Enzyme linked immunosorbent assay (ELISA)

How administered: specific antigen attached to solid-phase component and incubated in patient's serum (which contains IgE), then second antibody added that has enzymatic activity that produces colored reaction product that is detected

 

References

  1. Lasley M et al. Testing for Allergy. Pediatrics in Review. 2000; 21(2): 39-43.

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