Paralytic shellfish toxins - call for uniform reporting units

Paralytic shellfish toxins (PSTs) are a group of over 50 closely related naturally occurring tetrahydropurine toxins with saxitoxin (STX) as one of the most common and potent. Saxitoxin was first isolated and characterised from the butter clam Saxidomus giganteus as the dihydrochloride salt (Schantz et al., 1975). It is traded as a stand-alone reference material, or included in diagnostic test kits, mainly as the dihydrochloride salt or diacetate salt. This is primarily due to the saxitoxin hydrate (free base) form having poor stability. However, the certificate of analysis for various reference materials may state the concentration as either the salt or free base equivalent, and care must be taken to ensure the value is traceable and used correctly in subsequent calculations.