Purpura Thrombotica or Thrombotic purpura (TTP) is a rare blood disorder affecting blood coagulation. Small blood clots (thrombi) develop in the vessels throughout the body and this can lead to organ damage and a reduction in blood flow as a result of the thrombosis. If left untreated the condition is most likely fatal but since the mid-20th century treatment in the form of plasma exchange has been available. The condition affects the skin causing 'purpura' a rash of purple spots on the skin caused by bleeding from the small blood vessels. Chromolithograph, 1878-1888. Illustration by E. Burgess after R. W. Sherwin, Mabel Green and Jonathan Hutchinson.