Acronym for “polyethylene glycol,” a potentiating agent used in blood bank tube testing to enhance the interaction between red blood cells and antibodies. PEG is a long, water-soluble chain that competes with water for space around the red cell surface. The exclusion of water molecules allows red cells to get closer together and concentrates antibodies around the red cell surface. Both of those changes make it more more likely that an antibody will be able to bind to its corresponding red cell antigen. PEG works really well, and many immunohematology reference laboratories prefer it to other forms of testing. It can be overly sensitive to the detection of warm autoantibodies, however. Finally, one bit of trivia: PEG testing does not include an interpretation at 37C, since centrifuging red cells with PEG surrounding them leads to false positive reactions. Instead, after the incubation, the red cells are washed (to remove both PEG and unbound antibodies) and the AHG phase begins.