Blood screening was a procedure that was attempted by Starfleet in the 2370s, to detect Changelings by drawing blood from an individual. When a Changeling's matter is removed from it's body, it reverted to it's gelatinous state, giving the Changeling away.
The procedure was first suggested by Constable Odo in late 2371, when a Changeling hijacked the USS Defiant and attempted to use her to start a conflict between the Federation and the Tzenkethi. Unfortunately the procedure failed, as the Changeling posed as Dr. Julian Bashir, who was performing the blood screenings. (TNG episode: "The Adversary")
By 2372, the procedure was standard across all Federation starships, and all crew and passengers were subjected to blood screenings on a regular basis. In 2372, the Changeling posing as Lieutenant Linda Addison was able to get around the blood screenings by absorbing a sample of Addison's blood that it kept in her quarters aboard the USS Enterprise-E. (TNG eBook: A Sea of Troubles)