Coagulation is a biological process during which blood changes from a liquid to a solid, or semi-solid, state. Various coagulation tests can be performed in a clinical lab setting, typically using blood drawn into a "light blue top tube" which contains sodium citrate, a solution that binds calcium in the blood preventing clotting. Once collected, the blood can be subjected to a variety of coagulation tests, including prothrombin time (PT) to determine how quickly a clot will form.