M. Lu1, D. Davies1, J. Davies1, S. Chan1, B. Barnacle2, A. Lee2, T. Kovitwanichkanont2, 3
Background: Strip patch testing is 30 times more sensitive compared to traditional patch testing as it reduces the stratum corneum thickness by 50% thereby allowing a 30% reduction of standard test substance concentration. Positive patch test results accomplished at lower concentrations improve the validity of allergenicity of test substances. Our objective was to formulate lower concentration test substances and to compare sensitivity of strip versus traditional patch testing at lower concentrations.
Case Description: A 33 year-old Asian woman was admitted with Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) syndrome after two days of tuberculosis (TB) therapy consisting of rifampicin, isoniazid, ethambutol, pyrazinamide and pyridoxine. Patch testing was used to identify causative medication(s) to guide second-line treatment options.
A previous case study tested all five TB medications using white soft paraffin (WSP); however, negative reactions were observed at lower concentrations. For our case, ethambutol 1%, pyrazinamide 1% and pyridoxine 10% was formulated in WSP as they are poorly water-soluble; however, isoniazid is highly water-soluble therefore a 1% aqueous solution was formulated for better skin penetration. The formulated test substances and intravenous rifampicin solution were used for both strip and traditional patch testing. Using the standard reading criteria, positive reactions occurred with all five TB medications via strip patch testing whereas traditional patch testing only showed positive reactions for rifampicin, isoniazid and ethambutol.
Conclusion: Our department successfully formulated lower concentrations of test substances for strip patch testing which was found to be more sensitive than traditional patch testing.