D2.414 - Diagnostic Distribution of Patch, Delayed-Reading Intradermal, and Drug Provocation Tests in Delayed-Type Cutaneous Drug Hypersensitivity

In the diagnosis of Type IV drug-induced cutaneous reactions, patch tests (PT), delayed-reading intradermal tests (IDT), and drug provocation tests (OPT) are essential for clinical decision-making. This study aims to evaluate the diagnostic outcomes of drug hypersensitivity investigations in patients referred to our center.

Patients with a history of Type IV drug hypersensitivity reactions presenting between January 2016 and December 2025 were included. Demographic data and results of PT, delayed-reading IDT, and OPT (performed ≥6 weeks post-reaction) were retrospectively analyzed. All tests were conducted by an experienced allergist using guideline-recommended concentrations.

The study population (n=72) was 77.8% female, with a mean age of 42.7 (±13) years. Clinical phenotypes were maculopapular exanthema (MPE, 50.0%), bullous reactions (11.1%), and fixed drug eruptions (FDE, 9.7%), followed by other less common reaction types. No significant associations were found between patch test positivity and clinical phenotype, number of culprit drugs, latency periods, or laboratory parameters (p > 0.05). PT was performed with a single culprit drug in 38.9%, multiple culprits in 44.4%, and only alternative drugs in 16.7% of patients. Overall PT positivity was 22.2%, observed in 28.1% of multiple-culprit and 25% of single-culprit tests, while no positivity occurred with alternative drugs (p = 0.042). Positivity rates varied by drug group: 66.7% for "other" drugs (vitamins, antifungals, etc.), 25.0% for beta-lactams, and 15.4% for NSAIDs. No positivity was observed for DMARDs or non–beta-lactam antibiotics (p = 0.025). OPT was performed in 33 patients. Among 16 PT–positive patients, 5 underwent OPT with alternative drugs, all of which were negative. In PT–negative patients, culprit-drug OPT was positive in 1/10 (10%), while alternative-drug OPTs were negative in 18. Delayed-reading IDT performed on 17 patients (13 PT-negative and 4 PT-positive), yielding two positive results. 

Significantly higher PT positivity in the ‘other drugs’ group suggests that drug-specific factors influence results more than clinical history. The consistent lack of positivity to alternative agents suggests that proceeding directly to provocation testing with alternative drugs—bypassing skin tests—may be a safe and efficient diagnostic approach.