1. STUDY SYNOPSIS Experiments suggest that during treatment with vitamin K antagonists (VKA) the activity of coagulation factors (F) II and X better reflect anticoagulation than does FVII. Based on this a new monitoring prothrombin time base test (Fiix-PT) has been invented which is only sensitive to FII and FX. The Fiix-PT can be converted to INR ("Fiix-INR").We hypothesize that the Fiix-PT may reflect anticoagulation and the antithrombotic effect of VKA as accurately or better than the current PT based tests do (INR based on PT or P&P). The protocol describes a prospective randomized double-blind trial that will be conducted at the Landspitali Anticoagulation Management Center (AMC). The objective of the protocol is to evaluate the efficacy and safety of Fiix-INR as a monitoring test compared to the current PT based assays (INR) used to monitor patients treated with VKA in order to prevent thromboembolism. We will randomize 1200 clients of the AMC into two identically sized monitoring groups, Fiix-INR (test group) and INR (control group). Each individual will be assessed monthly for up to 24 months (an expected average of 18 months) until a total of 600 patients years of observation has been reached in each group. The clinical endpoints to be studied include efficacy (arterial and venous thromboembolic event rate) and safety (bleeding events). Also, surrogate convenience endpoints will be studied such as test frequency and time within target range.