4.2 Randomization standard (PT-INR) vs. new test (Fiix-INR). The aim is to 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.