Blood sampling vacuum tubes. Blood is analysed for levels of components such as cholesterol and glucose, for pathogens like bacteria and viruses, and to determine a person's blood group. These tubes with coloured caps are part of the Vacuette blood collection system from Greiner Bio One, which draws venous blood into a vacuum inside the tube. The colour-coding indicates the use of the tubes. Two of the seven colour codes are shown here: purple (haematology), grey (glucose determination).