SERPINE1 and Stroke: There are several possible explanations for the lack of correlation of SERPINE1 antigen levels with stroke: (1) SERPINE1 has a short half-life of 90–120 min at 37 °C after release into the bloodstream; (2) the level of the released protein fluctuates with diurnal variation; and (3) SERPINE1 exists with tissue plasminogen activator in the blood in active, latent, and complex forms, and the values can vary depending on the form of the antigen being measured62–64.