F10 and atrial fibrillation: There are three major findings from this study: (1) one in three patients with stroke and factor Xa inhibitors for atrial fibrillation used an inappropriate dose regimen of the anticoagulant at the time of stroke onset; (2) underdosing lead to significantly lower plasma levels on admission and (3) underdosed patients showed significantly higher NIHSS scores on admission and a worse functional outcome after 3 months.