Investigations

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).

References:

1. Johnston GA et al. British Association of Dermatologists' guidelines for the management of contact dermatitis 2017. Br J Dermatol. 2017;176(2):317-329
2. Rashid RS, Shim TN. Contact dermatitis. BMJ. 2016;353:i3299.