To give blood you have to have 13.5 somethings per something in your iron count. I have 13.3. Put another way, when they take the drop of blood they drop it into a tube full of blue liquid and it has to sink within 15 seconds. Mine floats.
I'm assured I'm not anaemic but still ... Must be just one of those things. My diet hasn't changed and I haven't been ill. Maybe I've always been just at the upper end of the 15 seconds. So, plenty of lean red meat, pulses and brown bread required. And less tea, which blocks iron take-up, apparently.
Nah, stuff that.
UPDATE: Best Beloved comments that this is obviously what lay behind all those floating witch trials. They weren't rejecting the water of their baptism, they were just severely anaemic.