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 calcium[needed to initiate clotting] and the Fluoride blocks enzyme activity stopping the glucose or lactate from being acted on by the enzymes present in the sample. This makes the sample fine for measuring Glucose but crap for measuring any Enzyme or of course Calcium. Many things will go off in stored serum, if there is no preservative,but there is an art in choosing the right anticoagulant and concentration in the final sample.
Tom
