A Coumadin does is difficult to dial in because of variables that differ from patient to patient. Liver function and diet play a big role in getting the dose right. A patient can be on more than 2 times the dose of another for the same INR. That's why frequent INR's need to be checked, sometimes as frequently as every 2 weeks. It's also why Xa inhibitors and direct thrombin inhibitors (DTI's) are becoming so popular. They don't need any testing at all, but they're not approved for some things patients need blood thinners for, like mechanical heart valves.