An astute physician should not need to patch test. Right?

Many clinicians believe that a thorough history and physical exam are sufficient for an accurate diagnosis of ACD. They believe that patch testing is unnecessary because they can tell whether a reaction is ICD or ACD simply by evaluating the dermatitis. Results of several studies, however, show that clinicians are often wrong when guessing whether contact dermatitis is irritant or allergic. In fact, experienced dermatologists may only suspect the true allergen in 50% of cases. Patch testing is the only way to differentiate between the two conditions because clinically and histologically ICD and ACD cannot be reliably differentiated.

Podmore P, Burrows D, Bingham EA: Prediction of patch test results, Contact Dermatitis 11:283–284, 1984.