Elevated factor V activity and antigen levels in patients with Covid‐19 are related to disease severity and 30‐day mortality

To the Editor:<br />We were very interested to read the recent manuscript by Stefely and coworkers who reported markedly increased factor V (FV) activity levels in critically ill patients with Covid-19.1 In this report, high FV activity levels were shown to be associated with thrombotic events, whereas declining levels were associated with poor outcome. A paper by Voicu and coworkers confirmed elevated FV activity levels in critically ill Covid-19 patients. We have recently reported on the hemostatic profile of a Covid-19 patient cohort. These patients had much milder disease in comparison to the cohort studied by Stefely. We were interested in investigating if elevated FV activity levels also occur in patients with milder disease. In addition, we wondered if the increase in FV activity reflects an increase in FV protein (antigen) level or whether these unusually elevated FV activity levels could be explained by an increase in specific activity rather than an increase in FV antigen.