Procalcitonin was reviewed on Taming the SRU in the context of other biomarkers (ESR, CRP) last year with a grand rounds discussion of its utility in the setting of a febrile 7 week old. As procalcitonin has continued to gain traction in the world of pediatric EM – receiving evaluation as part of proposed protocols for management of febrile infants (1) and for its utility as an aid to diagnosis of Serious Bacterial Infection (2,3) – we turned our attention this year to procalcitonin’s use in adults. Procalcitonin testing has been studied and available for decades (as St. Emlyn’s noted in an update earlier this year), but has not really established itself in widespread use in adults (as EMDocs noted in a sepsis update in 2014). In this blog post, we take the approach of going back to what is known (and is NOT known) about the biochemical basics of this molecule to give context to the sometimes confusing smorgasbord of proposed applications for procalcitonin testing that exists in the literature.