From the Guidelines
The typical starting dose of Coumadin (warfarin) is 5 mg daily, with INR checks performed daily until the therapeutic range is reached and sustained for 2 consecutive days, then 2 or 3 times weekly for 1 to 2 weeks, and eventually reduced to intervals as long as 4 weeks once the INR becomes stable. When initiating warfarin therapy, it is essential to monitor the INR response frequently until a stable dose-response relationship is obtained, as the anticoagulant effect is observed within 2 to 7 days after beginning oral warfarin, according to the dose administered 1. Some key considerations for dosing and monitoring include:
- Starting doses of 4 to 5 mg/d should be used in patients sensitive to warfarin, including the elderly, and in those at increased risk of bleeding 1.
- The INR is usually checked daily until the therapeutic range has been reached and sustained for 2 consecutive days, then 2 or 3 times weekly for 1 to 2 weeks, then less often, according to the stability of the results 1.
- Once the INR becomes stable, the frequency of testing can be reduced to intervals as long as 4 weeks, but frequent monitoring should be resumed when dose adjustments are required 1.
- The safety and effectiveness of warfarin therapy depend critically on maintaining the INR within the therapeutic range, as on-treatment analysis of primary prevention trials in atrial fibrillation found that a disproportionate number of thromboembolic and bleeding events occurred when the PT ratio was outside the therapeutic range 1.
From the FDA Drug Label
The dosing of warfarin sodium tablets must be individualized according to patient’s sensitivity to the drug as indicated by the PT/INR It is recommended that warfarin sodium tablets therapy be initiated with a dose of 2 to 5 mg per day with dosage adjustments based on the results of PT/INR determinations. The PT should be determined daily after the administration of the initial dose until PT/INR results stabilize in the therapeutic range. Intervals between subsequent PT/INR determinations should be based upon the physician’s judgment of the patient’s reliability and response to warfarin sodium tablets in order to maintain the individual within the therapeutic range Acceptable intervals for PT/INR determinations are normally within the range of one to four weeks after a stable dosage has been determined
The typical starting dose of Coumadin (warfarin) is 2 to 5 mg per day, with dosage adjustments based on the results of PT/INR determinations. The frequency of INR checks is as follows:
- Daily after the administration of the initial dose until PT/INR results stabilize in the therapeutic range
- Every 1 to 4 weeks after a stable dosage has been determined, based on the physician’s judgment of the patient’s reliability and response to warfarin sodium tablets 2
From the Research
Dose and Frequency of Coumadin
- The typical starting dose of Coumadin (warfarin) is 5 mg, although this can vary depending on the patient's age and other factors 3.
- For elderly patients or those with certain medical conditions, a lower starting dose of 2-4 mg may be recommended 3.
- In some cases, a starting dose of 3 mg has been used, particularly in adult Thai patients, although this may be less effective in achieving therapeutic INR levels 4.
- Another study suggested that a starting dose of 2 mg daily for 2 weeks can be used to predict the maintenance dose of warfarin, with the INR after 2 weeks of therapy predicting 70% of the variability in maintenance dose 5.
Frequency of INR Checks
- The frequency of INR checks can vary depending on the patient's stability on warfarin therapy, with more frequent checks (e.g. 2-4 times per week) needed immediately after initiation of therapy 3.
- As the patient's INR becomes more stable, the interval between INR checks can be lengthened, up to a maximum of 4-6 weeks 3.
- The use of a warfarin dosing calculator can help guide individualized dosing and reduce the need for frequent INR checks, improving the safety and efficacy of warfarin therapy 6.
- The frequency of dose changes can also be used as a surrogate marker to identify patients with suboptimal anticoagulation, with three or more dose changes in the first three months of therapy indicating a higher risk of adverse events 7.