We know the components of the blood are-plasma and blood corpuscles.
Plasma forms 55% of the total blood and blood corpuscles forms 45% of the total blood.
Blood corpuscles are RBC's, WBC's and Platelets.
Plasma forms liquid portion of the blood and contain 90% water, 7% plasma proteins and 3% other substances.
Plasma proteins are-SERUM ALBUMIN, SERUM GLOBULIN, PROTHROMBIN AND FIBRINOGEN.
In that proteins FIBRINOGEN functions as a clotting agent/substance. hence blood clots after sometime at the place of injury.
when we take blood sample from human body then due to presence of FIBRINOGEN it will clot immediately and we are unable to test that blood.
SERUM is the fibriongen free plasma. hence in today life when the blood is collected for testing then the clotting factor i.e. fibrinogen is removed from it and the blood dose not clot and we can test it without any difficulty.
In short blood clots and serum dose not when it is taken out of the body. 
