Patch Testing

Aka: Patch Testing, Patch Test

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II. Indications

  1. Allergic Contact Dermatitis (Type 4 Hypersensitivity)

III. Mechanism

  1. Contact Dermatitis is a delayed T cell-mediated response (Type 4 delayed Hypersensitivity)
  2. Most common contact allergans
    1. Rhus Dermatitis (e.g. Poison Ivy)
    2. Nickel Allergy
    3. Cosmetic allergy
    4. Rubber product allergy (Latex Allergy)

IV. Efficacy: Evaluating Allergic Contact Dermatitis

  1. Test Sensitivity: 70%
  2. Test Specificity: 70%

V. Technique

  1. Place drop of each diluted agent in Patch Test chamber
  2. Apply patch to non-irritated, clean skin of back
  3. Outline each patch outline with pen
  4. Label agent locations on skin and in medical record
  5. Remove patch in 48 hours
  6. Measure skin reaction (see below)

VI. Approach: Interpretation of skin test

  1. Erythema: 1+ (Mild)
    1. Accuracy 20%
  2. Edema or Vesicles involve <50% of patch area: 2+
  3. Edema or Vesicles involve >50% of patch area: 3+
    1. Accuracy 90%

VII. Causes: False Positive (consider if multiple positives)

  1. Hyperirritability of skin
  2. Contaminant irritant or allergen on tested skin
  3. Reaction to tape holding patch in place
  4. Irritant Contact Dermatitis
    1. Reaction resolves more quickly than in allergy
    2. Burning Sensation contrasted with itching in allergy
    3. Sharply demarcated reaction

VIII. References

  1. Chang (2018) Am Fam Physician 98(1): 34-9 [PubMed]
  2. Mydlarski in Middleton (1998) Allergy, Mosby, p. 1141-2 [PubMed]
  3. Riedl (2003) Am Fam Physician 68:1781-90 [PubMed]

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