Warfarin Sensitivity Management

Patient Factors

Patients over 75 years require approximately 1mg/day less warfarin than younger individuals to maintain comparable INR levels, according to the American College of Cardiology 1
Concomitant medications that inhibit CYP2C9 can significantly increase warfarin sensitivity, as noted by the American College of Cardiology 2
Nutritional status, particularly vitamin K deficiency, can increase sensitivity to warfarin, as reported by the American College of Cardiology 3
Acute medical illnesses, especially those affecting liver function or causing fever, may temporarily increase warfarin sensitivity, according to the American College of Cardiology 1
Reduced intake of vitamin K-containing foods can enhance warfarin's anticoagulant effect, as noted by the American College of Cardiology 2

For patients showing extreme sensitivity to warfarin, initial doses should be lower (0.5-2mg daily) rather than the standard 5mg daily, as recommended by the American College of Cardiology 2
More frequent INR monitoring is essential in these patients, initially daily until stabilized, according to the American College of Cardiology 2
When INR becomes excessively elevated (>5.0) due to warfarin sensitivity, temporary discontinuation of warfarin is recommended until INR falls to safer levels, as suggested by the American College of Cardiology 4

Overlooking drug interactions that may further increase warfarin sensitivity in already sensitive individuals can lead to dangerous INR elevations, as noted by the American College of Cardiology 1

The American College of Cardiology recommends that when INR becomes excessively elevated (>9), vitamin K₁ (3-5mg orally) should be given, anticipating that the INR will fall within 24-48 hours 5
The American College of Cardiology suggests that for life-threatening elevations or serious bleeding risk, prothrombin complex concentrate supplemented with 10mg of vitamin K₁ by slow intravenous infusion is recommended 5
The American College of Cardiology states that for patients with INR >9 without significant bleeding, oral vitamin K₁ is the treatment of choice unless very rapid reversal is required 6

Medications like acetaminophen can significantly elevate INR after only a few days of therapy, even at moderate doses (>2g/day) 7

The American Geriatrics Society recommends considering drug interactions, dietary changes, acute illness, and medication non-adherence as common causes of significant INR increase, particularly in patients taking warfarin 8

Accidental double-dosing or taking extra doses of warfarin can lead to INR elevation, and patients should be educated on proper dosing instructions, particularly elderly patients 8
Pharmacy dispensing errors providing higher strength tablets can also cause INR elevation, highlighting the need for careful monitoring 8
Patient confusion about dosing instructions is a critical pitfall to avoid, especially in elderly patients 8

The American Geriatrics Society suggests that antibiotic prescriptions require INR monitoring within 3-7 days of initiation, as this is a common cause of sudden INR elevation in stable patients 8
Recent medication changes of any kind warrant increased INR monitoring frequency, according to the American Geriatrics Society 8

For INR 5.0-5.6 without bleeding, withholding 1-2 doses of warfarin and resuming at a lower dose when INR approaches therapeutic range is recommended, as per the American Geriatrics Society 8
Rechecking INR within 24-48 hours to ensure a downward trend is crucial, and identifying and addressing the underlying cause can prevent recurrence, according to the American Geriatrics Society 8