Skin Testing

Aka: Skin Testing, Hypersensitivity Skin Testing, Intradermal Test, Skin Prick Test

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II. Indications

  1. Allergic response to inhalants, food, Insect Bites or medications
    1. High probability of Food Allergy based on detailed history

III. Mechanism

  1. IgE mediated, immediate-type Hypersensitivity
  2. Reaction results in a wheal at the exposure site, whose size correlates with the likelihood of allergy

IV. Contraindications (Use serum Ige testing instead)

  1. Severe Eczematous Dermatitis
  2. Dermatographism
  3. History of Anaphylaxis to Antigen to be tested
  4. Very young
  5. Unable to discontinue Antihistamines
  6. Uncontrolled Asthma (Peak Expiratory Flow < 75%)

V. Preparation

  1. Have Epinephrine and Albuterol available in case of Anaphylaxis
  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. Avoid High potency Topical Corticosteroids
    1. Avoid at the test site for 3 weeks prior to testing
  4. Medications that do NOT affect testing
    1. Leukotriene Receptor Antagonists
    2. Inhaled Corticosteroids
    3. Short-term Systemic Corticosteroids

VI. 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

VII. 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)

VIII. Efficacy

  1. High Test Sensitivity and Test Specificity
  2. Expect smaller positive reactions in age <2 years or age >70 years
  3. There is a lack of standardization of extracts
    1. Results may vary between testing facilities

IX. References

  1. (1999) NPI Allergy Testing Conference, Las Vegas
  2. Chang (2018) Am Fam Physician 98(1): 34-9 [PubMed]
  3. James (2002) Am Fam Physician 66(4):621-6 [PubMed]

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