The only reason to use serum instead of plasma is that some of the chemicals used as anticoagulant might interfere with whatever you are measuring. For example measurement of glucose or lactate is carried out on samples using Fluoride Oxalate as anticoagulant. The Oxalate locks the free...
If you really must have serum collect the blood into glass tubes , plastic tubes are very slow to clot, it needs a water wettable surface to disrupt the platelets and initiate the clotting process. you then need to let it stand for some time to let the fibrin retract.
what do you want to...
Two points , do you really need serum. It is usually easier to obtain plasma , by centrifuging blood which has been collected into a tube with anticoagulent. LiHeparin or oxalate.
If you really need serum you must allow the blood to clot and the fibrin to retract, and then centrifuge.
It is...