I often use serum and no preservatives. If kept frozen, no degradation is expected within a short time. I suggest you aliquot the serum in such way you avoid unecessary defrost for analyses.

I used serum too and no preservative added. I will suggest you no to keep your sample for long before your quantification. And do make aliquots as suggested by Jose.

Good luck

Thank you all for your advice, it is very helpful.