Investigations into matrix components affecting the performance of the official bioassay reference method for quantitation of paralytic shellfish poisoning toxins in oysters

► Underestimation of PST levels by MBA versus AOAC 2005.06. ► Demetallated oyster extracts give better comparison between MBA and AOAC 2005.06. ► Non-oysters spiked with zinc also show underestimation of PST levels by MBA. ► Significant suppression in the MBA due to high concentrations of zinc. ► PSP MBA inappropriate for oyster analysis or other shellfish with high zinc levels.