3 Warfarin Therapy

How warfarin dosing is guided

Warfarin is an oral vitamin K antagonist. Its dose is individualized using the international normalized ratio (INR), a standardized measure of blood-clotting time. Too little anticoagulation can leave a patient at risk for clots, while too much raises bleeding risk.

The prescriber or anticoagulation service—not the patient—should direct dose changes. Patients should take warfarin as prescribed and keep scheduled INR tests.

Interpreting INR results

Warfarin dosing is adjusted to keep the INR within the patient’s prescribed target range. A common target for atrial fibrillation and venous thromboembolism is $2.0\text{–}3.0$, but the target depends on the indication; mechanical heart valves may require a different target.

An INR below the prescribed range may mean anticoagulation is inadequate and clot risk is higher. For example, an INR of $1.6$ is below a target range of $2.0\text{–}3.0$. Report the result to the anticoagulation service for instructions; do not take an extra dose on your own.

An INR of $4.8$ is above the usual target and indicates increased bleeding risk. Without bleeding, a clinician may temporarily hold or adjust warfarin and arrange follow-up INR testing. Vitamin K is not routinely given for every elevated INR without bleeding. Management depends on the INR, bleeding risk, and clinical circumstances.

Missed doses and monitoring

Do not double a dose to make up for a missed dose remembered the following day. Follow the prescriber’s missed-dose instructions or contact the anticoagulation service.

Keep scheduled INR tests, take warfarin as prescribed, and tell clinicians and dentists that you take it.

Food and medication interactions

Many medicines and supplements can alter INR or bleeding risk. Contact the prescriber or pharmacist before starting a new medicine, such as an antibiotic, or a supplement, such as a vitamin K supplement. Starting, stopping, or changing interacting medicines may require closer INR monitoring.

Vitamin K–rich foods do not usually need to be avoided. Keeping intake reasonably consistent is generally more important than eliminating these foods. Discuss planned diet changes with the care team, since changing diet may require closer INR monitoring.

Bleeding precautions and urgent symptoms

Seek emergency care for major or uncontrolled bleeding, vomiting blood, black or bloody stools, a severe or unusual headache, or a head injury or fall—especially if the head was struck. Report unusual bruising, blood in the urine, nosebleeds, or other abnormal bleeding promptly.

Use sensible precautions such as a soft toothbrush and an electric razor. Check with a clinician before taking over-the-counter pain medicines such as ibuprofen or naproxen.

Reversal for life-threatening bleeding

Life-threatening bleeding is an emergency. Warfarin is stopped, the bleeding source is treated, and clinicians generally use four-factor prothrombin complex concentrate (4F-PCC) together with intravenous vitamin K for rapid reversal. Plasma may be used when 4F-PCC is unavailable. Reversal decisions and treatment must be managed by emergency clinicians.