Evaluation of Allergic Potential of Rubber Products: Comparison of Sample Preparation Methods for the Testing of Polymeric Medical Devices

A round-robin study was organized in the international standardization working group, ISO/TC194/WG8, with the purpose to compare the efficacy of different sample preparation procedures for determining the sensitization potency of polymeric medical devices. Three custom-made rubber samples were supplied to the participating laboratories and tested according to predetermined extraction and test protocols: A, formulated with a high level of mercaptobenzothiazole (MBT); B, formulated with a low level of MBT; and C, formulated with zinc dibutyldithiocarbamate. Extraction protocols included the so-called ISO method that applies physiological saline and/or vegetable oil extraction media without concentration (designated here “simple extraction”) and the so-called Japanese method that applies organic solvent extract (in this study chloroform–acetone mixture) after concentration to the test systems (designated here “exhaustive extraction”). Tests included human patch test, guinea pig maximization test (GPMT), adjuvant and patch test (APT), and modified local lymph node assay (LLNA). Patch testing in humans indicated that samples A and B were able to elicit delayed-type contact allergy in subjects previously sensitized to MBT chemicals, and that the order of eliciting potential was A=B≫C. According to the exhaustive extraction protocols, samples A and B were both identified as sensitizers, while sample C was identified as a weak or nonsensitizer by all test methodologies (GPMT, APT, and LLNA). In contrast, the simple extraction protocol using vegetable oil gave sometimes weakly positive but sometimes negative or ambiguous results for samples A and B, while the simple extraction protocol using physiological saline did not show any positive result for any samples by GPMT. In conclusion, the animal test results by the exhaustive (so-called Japanese) extraction method corresponded to the patch test results in human volunteers with previously documented MBT contact allergy; and the exhaustive method seems to be more sensitive than the simple (so-called ISO) extraction method for identifying sensitizing hazard of lipophilic chemicals contained in polymeric materials.