There are fist short of a zillion commercially available kits for measuring CORT on plasma/serum. All are reasonably priced and give good results. Which you choose will depend upon the equipment that you have on hand (e.g., gamma counter, scintillation counter or microplate reader.

Thank you so much.

If you find the CORT-kit too much expensive, I propose the corticosterone binding assay using simple human plasma.

http://www.sciencedirect.com/science/article/pii/0039128X71900559