BACKGROUND: Drug patch tests (PTs) can reproduce delayed hypersensitivity to 
drugs and entail a moderate re-exposure of patients to offending drugs.
OBJECTIVES: To determine the value of PTs for identifying the responsible drug 
in severe cutaneous adverse drug reactions (SCARs) such as acute generalized 
exanthematous pustulosis (AGEP), drug reaction with eosinophilia and systemic 
symptoms (DRESS) and Stevens-Johnson syndrome/toxic epidermal necrolysis 
(SJS/TEN).
METHODS: In a multicentre study, PTs were conducted on patients referred for 
DRESS, AGEP or SJS/TEN within 1 year of their SCAR. All drugs administered in 
the 2 months prior to and the week following the onset of the SCAR were tested.
RESULTS: Among the 134 patients included (48 male, 86 female; mean age 51·7 
years), positive drug PTs were obtained for 24 different drugs. These included 
positive tests for 64% (46/72) of patients with DRESS, 58% (26/45) of those with 
AGEP and 24% (4/17) of those with SJS/TEN, with only one relapse of AGEP. The 
value of PTs depended on the type of drug and the type of SCAR (e.g. 
carbamazepine was positive in 11/13 DRESS cases but none of the five SJS/TEN 
cases). PTs were frequently positive for beta lactams (22 cases), pristinamycin 
(11 cases) and in DRESS with pump proton inhibitors (five cases), but were 
usually negative for allopurinol and salazopyrin. Of 18 patients with DRESS, 
eight had virus reactivation and positive PTs. In DRESS, multiple drug 
reactivity was frequent (18% of cases), with patients remaining sensitized many 
years later.
CONCLUSIONS: PTs are useful and safe for identifying agents inducing SCAR.