Background The drug patch test (DPT) is useful as a tool for diagnosing delayed hypersensitivity skin reactions to medications. However, there is no consensus on concentration and vehicle for testing, which justifies the need to standardise the conducting method.
Purpose To describe a method of preparation of DPTs from active ingredients (AI) commercialised as drugs as well as pure substances, to unify the available information and to add our experience, so providing a methodology for those AI not described in the current literature.
Material and methods Retrospective analysis of DPTs performed in the hospital pharmacy department of a 300 bed hospital over a period of 50 months. For those AI in which information was available at the time of the study, the patch was prepared according to the concentration and vehicle described in the literature. In those cases where there was no agreement about the vehicle to choose, it was selected according to the solubility of the AI in water. For those AI not described in the literature, the development of the test depended on the concentration to be tested, the formulation of the drug and the choice of vehicle.
Results 122 AI and 178 types of DPTs were tested, with a total of 377 DPTs prepared. For 55.8% of the tested AI, there was no clear information on concentration and vehicle at the time of its preparation; currently, this information does not exist in 36.9% of tests requested. A total of 72.1% of DPTs were prepared in petrolatum (AI insoluble/poorly soluble in water). For 27.3% of the AI for which there was information about procedure of preparation, there was controversy about whether to use the commercialised drug or pure allergen. The mean concentration of AI in the starting drug was 39% (median 25%). 29% of drugs contained ≤10% AI (≥50% AI: 35% of the drugs). The mean concentration of AI in DPT was 59% (median 1.8%). A total of 50.1% of DPTs tested had an AI concentration ≤2%.
Conclusion This study presents action lines to improve the use of the patch test, highlighting the importance of conducting multicentre studies that standardise the procedures.
No conflict of interest.