Tubes

There are a variety of tubes that are used in the lab by the phlebotomist. Each tube is used for a variety of tests. The reason 1 tube cannot be used for all tests is because there are additives in each and depending on the test needed, some additives wil give incorrect results and by extension, incorrect diagnosis.
There is also an order of draw. As some tubes allow blood to clot and others dont, It  is important to know which tube blood should be taken in for what test and how long the sample can be stored before it no longer is usable.

As one can see from this picture, test tubes are color coded to make it easire for phlebotomists to differenciate between them and know which to use for the tests requested by the doctor.