Thanks to Fr. Pat and Dirty Rocker for the thoughtful responses! To update, they reran the flow crossmatch test last week with a new sample of my blood, and it was negative. They also reran the old sample, and it came back negative. So they are calling the original flow positive a "fluke", which I take to be a false positive. They will rerun the test one more time two weeks before surgery, but at this point, the procedure looks like a go. I feel excited, relieved, and slightly scared all at the same time.