Are patients on blood thinners more susceptible to low or drops in hemoglobin levels? What causes that drop? My Dad is currently going through a blood transfusion, in another country, where I can't just call the doctor and ask what is going on. I'm trying to research answers to keep my brain busy. He went in for knee replacement surgery. He has a metal heart valve, so is on Warfarin, and has been stable on that for years. But apparently his hemoglobin dropped to 77 (from 150, three days ago pre-knee surgery). I'm trying to understand a little more so I know what risks for him might be.