Skin testing and patch testing in non-IgE-mediated drug allergy.

Drug skin tests can reproduce delayed hypersensitivity to drugs and entail a moderate reexposure of patients to offending drugs. Drug patch tests (DPTs) and prick tests can be done with any commercialized form of a drug. In non-severe delayed non-IgE-mediated reactions to drugs, intradermal tests (IDT) with delayed readings have a greater value, but their techniques lack standardization. A negative drug skin test does not exclude the responsibility of a drug, and the drug must be rechallenged in non-severe cases. DPTs are useful in maculopapular rashes, flexural exanthemas, and if done in situ, also in fixed drug eruption. Their best indication is in acute generalized exanthematous pustulosis or drug reaction with eosinophilia and systemic symptoms (DRESS). They should be carried out cautiously, following strict guidelines. Prick tests have a low value but they can sometimes be positive on delayed readings. In non-severe delayed reactions to drugs, intradermal tests with delayed readings are the most sensitive skin tests especially for beta-lactam antibiotics, radiocontrast media, heparins but also some biological agents. The value of patch testing varies according to the implicated drug and the non-immediate adverse drug reaction. In DRESS, DPTs have a good value in testing carbamazepine or proton pump inhibitors but remain negative in testing with allopurinol or salazopyrin. In toxic epidermal necrolysis, DPTs are safe but positive in only 9 to 23 % of the reported cases.