//www.m.webmd.com/a-to-z-guides/prothrombin-time
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
Taking medicines that can affect the action of blood thinners (such as warfarin) and vitamin K. These include antibiotics, aspirin, cimetidine (Tagamet), barbiturates, birth control pills, hormone therapy (HT), and vitamin K supplements.
Having severe diarrhea or vomiting that causes fluid loss and dehydration.