FSTL3 and COVID-19: Additionally, PAI-1 was studied because it is involved in the coagulation pathway and has been implicated in ARDS (12), so it seemed reasonable to use it as an additional queried biomarker of interest in COVID-19 due to its multiple roles in inflammation, coagulation, as well as an activin-response protein, to determine if there was anything unique about the activin/FLRG pathway.