I'm not really sure of the threshold levels, but it's certainly true that they are higher than those that would cause concern at a routine blood test at the doc's.
My last experience of this was about 11 or 12 months ago at a platelets session. When my blood drop failed to sink convincingly enough at the initial pin-prick test, they tested it with a portable piece of kit, which I believe gave a reading of 132 (mmols?), which was just acceptable. However, a few days after my platelet donation I received a call to say that, on more thorough testing, I registered 129, and I was asked not to donate for 3 months; it's usually monthly for platelets. I'd been every month for nearly 2 years, which can lead to Hb levels falling but, as in my previous post, I think there may have been more to it than that. After 3 months, and some dietary adjustment, it was up again (134?).
It's a few years since I gave 'full blood', so things may have changed but, if you failed the drop test, I seem to remember that you were just asked to miss that one session. Of course, full blood is only taken at 3 or 4 month intervals anyway.