A blood screening reveals a Changeling.

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 its body, it reverts to its 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. (DS9 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. The procedure used involved a hypospray configured to draw blood into a vial. Klingon blood screenings simply involved cutting the skin with any blade or sharp implement available. 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 - Slings and Arrows eBooks: A Sea of Troubles, The Oppressor's Wrong)

Following the rescue of the Detapa Council, the crew of Deep Space 9 conducted blood screenings to ascertain if the Klingon claim they had been replaced by Changelings was right. It later emerged that the Klingon task force was commanded by a changeling who had successfully evaded a blood screening. (DS9 episode: "The Way of the Warrior")

Blood screenings became common on Earth when Jaresh-Inyo declared martial law. (DS9 episodes: "Homefront", "Paradise Lost")

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