Drug patch testing in systemic cutaneous drug allergy.

Patch testing with the suspected compound has been reported to be helpful in determining the cause of a cutaneous adverse drug reaction (CADR) and in studying the pathophysiological mechanisms involved. The main advantages of drug patch tests are that they can be done with no hospital surveillance because they induce only rarely adverse reactions and that any commercialized form of a drug can be used. In contrast, intradermal tests can be performed only with injectable forms or with a pure and sterile form of the drug. It is advised to perform drug patch tests during the 6 months following the CADR as we do not know whether positive results will persist. Due to the possibility that a low concentration might yield false negative results, drug patch tests have to be performed with rather high concentrations of the commercialized form of the drug, mostly diluted at 30% in petrolatum and/or in water. For some drugs and severe CADR, it is necessary to tests with lower concentrations or in other vehicles. Drug patch tests are positive in ca. 32-50% of patients who have developed a CADR. The clinical relevance of drug patch tests depends on the clinical features of the CADR (valuable in testing generalized eczema, systemic contact dermatitis, maculopapular rash, acute generalized exanthematous pustulosis, fixed drug eruption) and on the involved drug. As false positive results can be observed, it is always necessary to consider the relevance of any positive drug patch test. Their specificity and their negative predictive value have not been yet determined.