The Thing takes place shortly after John Carpenter's movie ends. You're dropped first into the US base in Antartica, then relocated to the Norwegian base, and it isn't long before you discover how horribly things went wrong. You're isolated at the bottom of the world with people who may or may not be who you think they are, it's dark and inhospitable, and you never really know what's around the next corner. You know that some thing could be lurking anywhere, and when you find one, it ain't pretty.
Due to the scripted nature of some "thing-outs", someone can test clean and then explode into a monster a few moments later. It's a bug, sure, but it can also lead to a different conclusion; The Thing is learning to dodge the blood test.