Allergy Testing

 

IN VIVO TESTING

 

Skin Testing

Definition: method for identifying allergen-specific immunoglobulin E (IgE) via release of histamine prompts development of central wheal and an erythematous flare

How long: assessed 15-20 minutes after allergen is placed

Controls: a positive histamine and negative saline control are placed for comparison

NOTE: over 40 year period, six fatalities from skin testing; however none of the individuals had undergone skin testing alone

 

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 testingˆ 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

 

 

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 patientÕs 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 patientÕs 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.