stylistic:tone, structural:summarization
Unknown blood type

Because blood type O negative is compatible with anyone, it is often overused and in short supply. According to the Association for the Advancement of Blood and Biotherapies, the use of this blood should be restricted to persons with O negative blood, as nothing else is compatible with them, and women who might be pregnant and for whom it would be impossible to do blood group testing before giving them emergency treatment. Whenever possible, the AABB recommends that O negative blood be conserved by using blood type testing to identify a less scarce alternative.