4.1 Participants: All ambulatory patients 18 years or older currently on or starting on warfarin to treat and prevent thromboembolism during the study period at the anticoagulation management center (AMC) at the Landspitali University Hospital in Reykjavik, Iceland with a treatment goal of INR 2-3 will be eligible for participation and randomization irrespective of indication for anticoagulation, age or concurrent medication use. Thus, the study population will have various (or mixed) indications for OA but the majority likely will have atrial fibrillation. Both current patients and new patients will be randomized over a 3-6 month period until adequate number of patients have entered the study. After patients sign informed consent they will be randomized (see METHOD of randomization in chapter 7) to either Fiix-PT (test method, Fiix INR) or PT (PT-INR based on Quick PT method as a control method). We will measure the PT- INR as well as the Fiix - INR in the test group for the sake of safety.