Initiation of Oral Anticoagulation with Coumadin 

The dosing of Coumadin must be individualized according to the patient's response to the drug as indicated by the PT/INR. Use of a large loading dose may increase the incidence of hemorrhagic and other complications, does not offer more rapid protection against thrombus formation, and is not recommended. Low initiation doses are recommended for elderly and/or debilitated patients and patients with potential for increased responsiveness to Coumadin.

STEP 1: Obtain baseline PT/INR.

Begin therapy with Coumadin (Warfarin Sodium) with a dose of 2-5 mg per day with dosage adjustments based on the results of PT/INR determinations.

For Patients on Heparin: Since the anticoagulant effect of Coumadin is delayed, heparin is preferred initially for rapid anticoagulation. Conversion to Coumadin may begin concomitantly with heparin therapy or may be delayed 3-6 days. When Coumadin has produced the desired therapeutic range or prothrombin time, heparin may be discontinued.

STEP 2: Day that the PT/INR is stabilized in the therapeutic range: Check PT/INR daily. Adjust warfarin dose based on the results of PT/INR determinations.

Patients Stabilized in the Therapeutic Range: Intervals between subsequent PT/INR determinations should be based upon the physician's judgement of the patient's reliability and response to Coumadin 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. Most patients are satisfactorily maintained on Coumadin at a dose of 2 to 10 mg daily.

IV. Monitoring-The International Normalized Ration (INR)

Because PT results are very dependent on the thromboplastin reagent used, a system of standardizing the prothrombin time in oral anticoagulant therapy was introduced by the World Health Organization in 1983. It is based upon determination of an INR, which is equivalent to the PT ratio one would obtain if a sensitive reference thromboplastin were used for the PT.

Thromboplastin sensitivity is determined by the manufacturer and is expressed as an International Sensitivity Index (ISI).

The INR can be calculated as: INR = (Observed PT ratio)ISI

The calculation of the INR from the PT ratio is usually performed by the laboratory.

 

Reminder

Be aware of potential drug interactions and other factors that may affect INR (refer to prescribing information for Coumadin).

Patient education about Coumadin is an important part of therapy. Effective therapeutic levels with minimal complications are in part dependent upon cooperative and well-instructed patients who communicate effectively with their physician. Various Coumadin patient educational guides are available to health professionals on request.

  1. Drug Interactions with Coumadin
  • Numberous factors, alone or in combination, including travel, changes in diet, environment, physical state, and medication may influence response of the patient to anticoagulants. It is generally good practice to monitor the patient's response with additional PT/INR determinations in the period immediately after discharge from the hospital, and whenever other medications are intiated, discontinued, or taken irregularly. The following factors are listed for reference; however, other factors may also affect the anticoagulant response.

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