Causes of Elevated INR

The American Heart Association notes that antibiotics can elevate INR through multiple mechanisms, including altering gut microbiome, inhibiting CYP2C9, and inhibiting CYP1A2 and CYP3A4 enzymes 2
The American College of Cardiology recommends considering the potential interaction between NSAIDs and warfarin, as NSAIDs can potentiate the risk of bleeding with warfarin through both pharmacological interactions and displacement of warfarin from plasma proteins 3
The American Heart Association suggests that herbal supplements, such as St. John's wort, can decrease INR through enzyme induction, while others like bromelains, danshen, and Ginkgo biloba can increase INR 3
Excessive alcohol consumption inhibits hepatic enzymes and impairs warfarin clearance, according to the American Heart Association 3

Endogenous and Physiological Causes

The American Association for the Study of Liver Diseases notes that liver disease affects INR by reducing production of clotting factors, particularly factors I, II, V, VII, and X 4, 5
In cirrhosis, INR may be elevated due to decreased synthesis of procoagulant factors, though this is often offset by decreased anticoagulant factors like protein C, as reported by the American Association for the Study of Liver Diseases 5

Laboratory factors, such as underfilling blood collection tubes, variations in thromboplastin reagents, and lupus anticoagulants, can falsely elevate INR, according to the American College of Cardiology 6
The American Geriatrics Society recommends managing elevated INR in patients on warfarin therapy based on the degree of elevation and presence of bleeding 1