II. Mechanism: Assays

  1. RadioAllergoSorbent Test (RAST)
    1. Older methodology
    2. ImmunoCAP has largely replaced the older RAST methodology
  2. Enzyme-linked Immunosorbent Assay (ELISA, most common)
  3. Fluorescent Enzyme Immunoassay (FIA)
  4. Chemiluminescent Immunoassay

III. Efficacy

  1. Lower Test Sensitivity than allergy Skin Testing
  2. Highly specific for allergens
  3. Standardized testing to 95% Positive Predictive Value for reactivity within the testing population
    1. Contrast with Skin Testing extracts which are not standardized
  4. Positive Result Precautions
    1. Positive tests do not absolutely indicate allergy
    2. Level of specific IgE does not correlate with severity of allergy
  5. Negative Result Precautions
    1. Negative results may indicate lack of prior exposure (and not lack of allergy risk)

IV. Indications

  1. Not recommended for food, venom, or drug allergies unless high risk for Anaphylaxis
  2. Alternative to Skin Testing in certain cases
    1. Age <3 years
    2. Skin Testing contraindicated
      1. High risk for Anaphylaxis
      2. Uncontrolled Asthma or other medical conditions
      3. Essential medications that require continuation (e.g. Beta Blockers, Antihistamines)
      4. Skin conditions that interfere with Skin Testing (e.g. Eczema)
  3. Allergy Testing for common allergens
    1. Pet dander
    2. Dust mites
    3. Pollen
    4. Mold spores

V. Approach: Screen Contents

  1. June Grass and cross reacting grasses
    1. Rye
    2. Sweet Vernal
    3. Timothy
    4. Brome
  2. Oak Tree
  3. Short ragweed
  4. Alternaria mold
  5. Dust mite
  6. Cat

VI. Technique

  1. Allergen coupled to a paper disc
  2. Patient's serum is added to paper disc
  3. Antigen-Antibody forms in the case of allergy
  4. Radioactive anti-IgE added and complexes with above
  5. Gamma counter scores binding

VII. Interpretation: RAST Scoring

  1. Class 0: 200-750 (No significant allergy)
  2. Class 1: 750-1600 (Mild allergy)
  3. Class 2: 1600-3600 (Moderate allergy)
  4. Class 3: 3601-8000
  5. Class 4: 8001-18000
  6. Class 5: 18001-40000 (Most allergic)

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