A security test in which blood is drawn and watched to see whether it reverts to Changeling protoplasm, widely adopted by Starfleet and the Klingons during the Dominion crisis and repeatedly circumvented.
Blood screening rests on a simple observation: any part separated from a Changeling returns to its gelatinous natural state. Blood taken from an impostor should therefore turn to orange liquid in the vial, while that of a genuine humanoid stays red.
Deep Space Nine introduces the test after The Adversary. Klingons slash their palms for the purpose in The Way of the Warrior, and in Homefront and Paradise Lost Admiral Leyton makes screening compulsory for Starfleet personnel and their families on Earth. Leyton then rigs a test to have Benjamin Sisko arrested as a Changeling.
The method is never dependable. Changelings replace both General Martok and Doctor Bashir, the latter the very person conducting screenings on the station. In the third season of Picard, set decades later, a new strain of Changelings can imitate humanoid blood and internal organs outright.