However I often have a problem with the iron levels in my blood. When they take the little drop of blood from the finger and drop it in the copper sulphate solution it floats happily to the top instead of sinking. So then I have to get some blood taken from my non-donating arm and that's tested in another machine. If I actually have a high enough iron level, then the blood is slow in coming. Last time I was due to give blood I couldn't because of an infection, and before that I had low iron for several visits, and I must have missed a couple of others due to holidays.
But today I turned up, my blood drop sank (!) and the blood came out in record time. So Yay!