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Warfarin and INR monitoring: Are you on target?
An article for physicians by physicians
Originally published September 2007
IL0730-1-E

Of interest to physicians dealing directly with patients

Although warfarin is a commonly prescribed anticoagulant for the treatment of many clinical conditions, the dosing and monitoring of the medication presents unique challenges for physicians. The following case illustrates some of the problems that may arise during anticoagulant therapy.

Case study

A middle-aged patient with rheumatic mitral valve disease who was receiving warfarin for atrial fibrillation underwent an uneventful mitral valve replacement with a mechanical prosthesis. Postoperatively, the cardiac surgeon prescribed warfarin each day based on that day’s International Normalized Ratio (INR) result. Although the INR increased steadily to 2.4 during the hospital stay, it never reached his desired therapeutic target of 2.5 to 3.5.

The patient was discharged on the fifth postoperative day with a prescription for warfarin 2.5 mg daily. The cardiac surgeon asked the patient to arrange INR monitoring with her internist; there was no communication at the time between the surgeon and the internist. The next INR, done three days later, was 1.9. The internist advised the patient to take warfarin 5 mg that day followed by alternating daily doses of 2.5 and 5 mg. He ordered a repeat INR for four days later, but the patient suffered a left-sided subcortical infarct the morning the INR was scheduled. Her INR was only 1.5 at the time. A mild right hemiparesis, language processing deficits, and bouts of depression persisted. The patient commenced a legal action alleging the physicians’ failure to prescribe an adequate dosage of warfarin postoperatively led to the stroke.

Expert opinion

Peer experts were of the opinion that any delay in attaining therapeutic INR values increased the risk of valve-related thromboembolic events. They expressed the view that the physicians did not meet the standard of care in managing the postoperative anticoagulation treatment. Overall, the experts opined that the physicians should have been more concerned when faced with a subtherapeutic INR in a patient with a freshly implanted mechanical heart valve. They noted that, had more frequent INR tests been performed, there would have been more opportunity to adjust the warfarin dose. Without expert support, a settlement was negotiated and paid by the CMPA.

This case demonstrates just one of the medico-legal issues arising from anticoagulant management that our members encounter. The CMPA will publish in the near future a comprehensive analysis of the medico-legal difficulties associated with antiplatelet agents, anticoagulants and thrombolytics.

Risk management considerations

Based on the expert opinions received in this case, the following have been identified as risk management considerations:

• Have you arranged appropriate follow up and INR monitoring when your patient is discharged from hospital on warfarin?
• Has there been effective communication between physicians during the transfer of care of patients receiving anticoagulants?
• Has there been effective communication with your patient about the anticoagulant therapy and any transfer of care?
• Is there a system in place to review INR results and adjust the warfarin dosage as necessary?
• Are you familiar with and considering current guidelines for the management of patients on warfarin?