I am used to work with cortisol EIA kit from AssayDesigns provided by Enzolifesciences without performing any extraction process. However we are used to dilute our samples (fish plasma) at least 10 times, to avoid any matrix effect interferences in the assay. This procedure was first tested in our lab and finally validated regarding the performance characteristics on accuracy and precision (for reference, you can see some publications from my colleague Cyril Delfosse).