[sansa blinks. it's strange what people knowledge people do and don't pick up. if the maesters have figured out blood types and intravenous needles it hasn't yet left the citadel, but sansa had managed to absorb the basic principles of blood ministry some time ago.]

Blood transfusions are a very common practice in Blood Ministry. I imagine it concerns itself with the extraction of difficult blood types as well. A little magic from the Sleepers themselves should be enough to keep it in liquid form, at least.