Blood screening

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 (NX-74205) 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. 

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 (NCC-1701-E).