Warfarin Dose Calculator

Warfarin dose adjustment is done on the weekly total rather than the daily tablet because the drug has a half-life of roughly 36 to 42 hours and the clotting factors it suppresses turn over slowly, so the INR reflects an average of intake over the preceding several days. Working in weekly milligrams also allows fractional average doses that no single tablet can deliver, for example alternating 5 mg and 7.5 mg days to reach 43.75 mg per week. The adjustment percentages applied here follow conventional anticoagulation-clinic practice: increase 10 to 20 percent when the INR is below 1.5, 5 to 15 percent when it is below target, hold and reduce 5 to 15 percent when modestly high, and hold with a 10 to 20 percent reduction when the INR is between the upper limit and 5.0. Because factor II has a half-life near 60 hours, an INR drawn sooner than about five days after a change does not yet reflect it. Target ranges are indication-specific: 2.0 to 3.0 for atrial fibrillation, venous thromboembolism, and bioprosthetic valves, and 2.5 to 3.5 for most mechanical mitral prostheses. Quality is judged by time in therapeutic range, with above 65 to 70 percent regarded as well-controlled anticoagulation. Above an INR of 9, or with any serious bleeding, dosing arithmetic stops being the relevant question and reversal with vitamin K, four-factor prothrombin complex concentrate, or plasma takes over. The algorithm here also cannot see the variables that most often move an INR: a course of fluconazole or metronidazole, a change in dietary vitamin K, alcohol, thyroid status, decompensated heart failure, or CYP2C9 and VKORC1 genotype, which together explain a large share of interpatient dose variability.

The drug traces to a cattle disease. In 1933 a Wisconsin farmer named Ed Carlson drove dead cattle and a milk can of unclotted blood to the laboratory of Karl Paul Link at the University of Wisconsin; the animals had eaten spoiled sweet clover silage. Link and his students spent years isolating the responsible agent, and in 1940 Harold Campbell crystallised dicoumarol, the coumarin derivative formed during spoilage. A more potent synthetic analogue was patented in 1948 and named warfarin after the Wisconsin Alumni Research Foundation, which held the patent, initially as a rodenticide. Human use followed after cases showed the effect was survivable and reversible with vitamin K; it was approved for medical use in 1954, and President Dwight Eisenhower received it after his 1955 heart attack. Dosing remained crude while laboratories reported prothrombin time in seconds or as a percentage of a local control, because thromboplastin reagents differed enormously in sensitivity and results were not comparable between hospitals. The World Health Organization addressed this in 1983 by adopting the International Normalized Ratio, which rescales the measured prothrombin ratio using each reagent International Sensitivity Index and made shared target ranges possible. Mechanistic understanding caught up in 2004, when the VKORC1 gene encoding vitamin K epoxide reductase was identified, explaining both warfarin action and much of the inherited variation in dose requirement. Since 2010 direct oral anticoagulants have taken most non-valvular indications, leaving warfarin dominant for mechanical valves, severe renal impairment, and antiphospholipid syndrome.