help for blood collection and centrifugation for ELISA - (Nov/12/2012 )

Hello!
Is it still okay to centrifuge and collect plasma or serum from frozen rat whole blood samples?
I have frozen rat whole blood samples immediately after collection and stored in -80. I was not able to centrifuge them to collect serum and plasma for ELISA (BDNF or corticosterone) immediately. Is there any protocol for this?
Thanks !!!

The blood cells will have lysed so you won't be able to efficiently remove them.