Lab

Skin Testing

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Skin Testing, Hypersensitivity Skin Testing

  • Indications
  1. Allergic response to inhalants, food, Insect Bites or medications
  • Mechanism
  1. IgE mediated, immediate-type hypersensitivity
  • Contraindications
  1. Severe Eczematous Dermatitis
  2. Dermatographism
  3. History of Anaphylaxis
  4. Very young
  5. Unable to discontinue Antihistamines
  6. Uncontrolled Asthma (Peak Expiratory Flow < 75%)
  • Preparation
  1. Have Epinephrine and Albuterol available
  2. Discontinue Antihistamines before testing
    1. Second Generation Antihistamines: 2 weeks before test
    2. First Generation Antihistamines: 3 days prior to test
    3. Other Antihistamine-type agents to stop before test
      1. Phenothiazine
      2. Tricyclic Antidepressants
      3. Anticholinergic Medications
      4. H2 Receptor blocking medications (e.g. Ranitidine)
  3. Inhaled Corticosteroids do not affect test
  4. Short-term Systemic Corticosteroids do not affect test
  • Technique
  1. Test 6-10 antigens to determine presence of IgE disease
  2. Antigen exposure techniques
    1. Prick method
    2. Scratch method
    3. Intradermal (injected)
      1. Most accurate
      2. Used in cases requiring higher Test Sensitivity (e.g. Insect sting allergy, Penicillin Allergy)
  3. Use Controls
    1. Positive Control: Histamine
    2. Negative Control: Glycerol-Saline
  4. Read test 15 minutes after application of allergen
    1. Positive if wheal diameter 3 mm > negative control
  5. Use Regional Allergy Screening guidelines
  6. Relies on cross reactivity among antigen groups
    1. Tree
    2. Grass
    3. Weed
    4. Mold
    5. House Dust mite
    6. Cat
  • Interpretation
  1. All Tests negative: No further testing needed
  2. One or more inhalants positive: Complete regional panel
  3. Dust mite only positive: No further testing needed
    1. Avoid Dust mites
    2. Consider Immunotherapy
  4. Cat only positive: No further testing needed
    1. Avoid exposure and reduce dander
    2. Consider Immunotherapy
  5. If Positive Control is Negative
    1. Try RAST testing
    2. Patient may be on Antihistamine or steroid
      1. IF short acting Antihistamine
        1. Stop Antihistamine and retest after 2 days
      2. IF long acting Antihistamine
        1. Stop Antihistamine and retest after 3-4 weeks
  6. If reaction occurs to Negative Control
    1. Dermatographism
  7. Intradermal Allergy Test Indications
    1. Suspected Insect Sting Allergy (e.g. bee, hornet)
    2. Suspected Drug Allergy (e.g. Penicillin, anesthetic)