5.4.3 Blinding. The study will be double-blinded, i.e. all assessors of clinical effect and dosing staff as well as patients will be blinded to the monitoring test-method. Blinded test results (INR) will be administered by the coagulation laboratory staff which cannot be blinded since they do the laboratory testing. The coagulation laboratory will measure INR (PT) on one STA-R instrument and Fiix-INR (Fiix-PT) on another STA-R instrument. All results, however will be reported out as "INR". The patients will not be informed on which test is used to monitor their anticoagulation and only a value ("INR") will be reported to patients. The test results will also be reported electronically as "INR" to the dosing staff (nurses, biomedical scientists, physicians) and study nurse who will not have knowledge on which test is used to monitor each patient. The data will be managed by the study nurse who will not have access to monitoring test allocation. All formal assessors of clinical events and secondary endpoints will also be blinded to test allocation. The dosing staff will dose patients based on INR, i.e. using the DAWN anticoagulation management software (Onundarson, Einarsdottir et al. 2008).