Originally: †treated or impregnated with the juice of lemons or other citrus fruit; made slightly acid with lemon juice (obsolete). In later use: combined with citric acid; converted into a citrate (now rare). Now usually: (especially of blood or other fluid) treated with a citrate, typically sodium citrate.
Mid 18th century. Originally from post-classical Latin citratus treated or impregnated with the juice of lemons or other citrus fruit + -ed.