What can cause a change in INR?

INR monitoring is essential for coumadin (warfarin) patients. They often need their INR checked once a week or so to make sure that their INR is within therapeutic range (most often, 2.0-3.0, but there are certain exceptions).

There are MANY things that can affect INR, and Up To Date has a great article about this. It should be noted that any patient with acute illness (such as a hospitalized patient) can have an INR out of whack.

  • Medication interactions: different medications hit warfarin metabolism at different points:
    • Antibiotics like metronidazole, macrolides, and the fluoroquinolones interfere with intestinal flora metabolism of vitamin K
    • Drugs metabolized by CYP2C9 like fluconazole, amiodarone, fluoxetine, simvastatin and atorvastatin inhibit warfarin metabolism result in decreased warfarin metabolism; inducers like rifampin cause increased warfarin metabolism
    • Acetominophen interferes with vitamin K recycling. The complete mechanism is still not well understood, but the effect is observed to be greater when people take high doses (2-3 g) for at least several days.
    • PPIs like lansoprazole and esomeprazole may affect warfarin metabolism and are associated with increasing the INR
    • NSAIDs and antiplatelet agents cause increased bleeding that is not INR-dependent. Patients on warfarin should use a COX-2 inhibitor when indicated.
  • Diet- leafy greens decrease INR and may result in the need to increase coumadin dose. At a hospital I know in the American South, nurses routinely doubled their patients’ coumadin doses because of the amount of collard greens people ate.
  • Smoking and alcohol are, as always, bad. Ingredients in cigarettes undergo CYP metabolism and may result in higher doses of warfarin required. Because alcohol is metabolized in the liver, which is also responsible for synthesizing coagulation factors, and where warfarin is metabolized, excess alcohol use and liver damage definitely affect the INR.
  • Impaired cardiac, hepatic, or renal function:
    • Cardiac: patients with atrial fibrillation and CHF have a harder time with INR stabilization
    • Hepatic: for the reasons listed under ‘Smoking and alcohol,’
    • Renal: patients with renal impairment may require lower doses of warfarin

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