3PC-043 How reliable is patch testing? Applicability in real-life practice for the diagnosis of allergic reactions

<h3>Background and Importance</h3> Patients are often labelled as allergic to antimicrobials without an accurate diagnosis. Such over-labelling can lead to both individual health and global public health consequences. To achieve an accurate diagnosis, various in vitro and in vivo tests are conducted. In this context, hospital pharmacy services frequently contribute to the development of diagnostic tests, including prick tests, intradermal tests (IDT), patch tests (PT) and drug provocation tests (DPT). <h3>Aim and Objectives</h3> Compare the PT results with the final diagnosis established by the allergist through clinical assessment and additional tests. <h3>Material and Methods</h3> Observational and retrospective study at a tertiary level hospital including patients who were dispensed at least one antimicrobial patch test kit from the pharmacy service between October 2020 and September 2024. Each kit contains a single antimicrobial agent, delivered in multiple doses or vehicles. The data were collected from the electronic health record (EHR) and pharmacy dispensing programme. <h3>Results</h3> A total of 49 antimicrobial patch tests kits were conducted on 32 patients referred to the allergology department for suspected adverse drug reactions. The primary reason for consultation was exanthematous reactions (62.5%) with more severe reactions such as DRESS syndrome and toxic epidermal necrolysis also noted. Of the antimicrobials tested, 91.8% were antibiotics, notably the beta-lactam group (22.2%), clindamycin (17.8%) and sulfamethoxazole/trimethoprim (13.3%). Of the 49 drugs tested, eight (16.3%) were positive in the DPT, which is the gold standard method in a drug allergy assessment. Additionally, two drugs demonstrated a positive outcome in skin tests, increasing the positivity rate to 20.41%. Nevertheless, there are 14 additional drugs that represent unconfirmed allergies through either skin tests or DPT. A positive patch test reaction occurred once, representing 12.5% of confirmed reactions via DPT and 4.5% when considering those patients exhibiting unconfirmed allergies. The positive predictive value (PPV) of the PTs is 100% and the negative predictive value (NPV) is 40%. <h3>Conclusion and Relevance</h3> PT are an appropriate diagnostic tool for type IV hypersensitivity-related reactions, given their 100% PPV. However, the limited value of negative results must be considered. Investigating methods to enhance the reliability of negative results would be beneficial, minimising reliance on DPT and optimising patient safety. <h3>References and/or Acknowledgements</h3> <h3>Conflict of Interest</h3> No conflict of interest