Consecutive patients with clinical suspicion of CVT at the emergency department of the University Hospital Bern will be included in this study over a two year period. All included patients will receive standard care applied by the treating physician who will follow international recommendations. D-dimer measurement at entry will be performed using a rapid sensitive assay. FXIII-AP will be analyzed at the Hemostasis Research Laboratory, Department of Hematology, University of Bern based on a highly sensitive and specific ELISA method.The investigators will be blinded for the clinical symptoms and diagnosis of the patient.