investigations

Last reviewed 09/2019

Differentiating allergic contact from irritant and endogenous dermatitis on clinical features alone is unreliable (especially in hand and facial dermatitis). Hence any patient with a chronic or persistent dermatitis, or atopic/endogenous dermatitis that was previously well controlled with topical therapy and then becomes difficult or impossible to control with the same topical treatment should be referred to a dermatologist for patch testing.

  • patch tests are the gold standard for diagnosing allergic contact dermatitis
  • it has a sensitivity and specificity of between 70% and 80%.4
  • a series of individual allergens are applied to the patients skin at standardised concentration under occlusion
  • the back is most commonly used, limbs, in particular the outer upper arms, are also used
  • optimum timing of patch test readings is on day 2, followed by day 4
    • some allergens often do not yield positive reactions until after day 4, hence a third reading at day 7 will pick late reactions to certain substances
  • readings are graded by the intensity of the reactions under the applied patch tests (1,2)

If photoallergic contact dermatitis is suspected, photopatch testing may be carried out (1).

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