Skin testing in delayed reactions to drugs.

Drug skin tests (eg, patch tests, prick tests with delayed readings, intradermal tests [IDT], especially with delayed readings) are used to investigate cutaneous adverse drug reactions (CADR) in delayed hypersensitivity reactions caused by a particular drug. Their value depends on the clinical features of the CADR and on the drug tested. In maculopapular rash (MPR), drug skin tests are of value, beginning with patch tests, and followed: 1) if negative by prick tests (with delayed readings at 24 hours); and, 2) if the injectable form of the drug is available, with IDT with immediate and delayed readings. This article discusses details of the use of patch tests as they apply to patients with various drug reactions. Drug skin tests are useful to study cross-reactivity between suspected drugs. False positive results can occur. The negative predictive value of drug skin tests is approximately 90%.