I am 70 taking coumadin 5 m"g daily 9 months after massive pe. could preventive antibiotics augmentin (amoxicillin and clavulanate) 875mg twice a day for one week raise inr?

Yes: Due to the way any antibiotics can change the gut flora process vitamin K which will affect the ability of coumadin to change the INR. Let the doctor prescribing your coumadin know that you are about to start Augmentin (amoxicillin and clavulanate). They should have a protocol in place to adjust monitoring schedule.

Coumadin/Augmentin: Using coumadin (warfarin) together with amoxicillin (in augmentin) may increase the risk of bleeding, especially if you are elderly or have kidney or liver impairment. You may need more frequent monitoring of your prothrombin time or INR by your doctor to use both medications. The question is why are you taking prophylactic antibiotics. They have very limited indications.