Aspirin: How Does It Thin Blood?

Dear Dr. Dave and Dr. Dee,

My doctor made me postpone surgery because I was taking aspirin. She wouldn't do the surgery until 10 days after I stopped taking aspirin. She said it would take that long to get it out of my system. That seems rather odd to me.

Signed,

What's the big deal about aspirin?

Dear What's the big deal about aspirin,

Aspirin prolongs the time that it takes for your blood to stop oozing (bleeding time), and therefore can predispose to surgical bleeding complications. Aspirin does this by decreasing the ability of billions of tiny cells in the bloodstream to stick to each other. These cells are called platelets.

Normally when there is an injury, platelets will adhere to the site of injury and stick to each other forming a plug or platelet aggregate, after which blood clotting is initiated. Blood will still clot when you take aspirin, but it will tend to ooze longer before it clots. That's a less than optimal situation when you're having surgery.

Aspirin's antiplatelet effect lasts for the lifetime of the platelet. A typical platelet has a lifespan of 7-10 days, at which point the body removes it from circulation and replaces it with a new one. Your surgeon had to postpone your surgery for 10 days after you stopped taking the aspirin because that's how long it takes your body to replace all the old platelets with new ones.