Just curious what is your downstream application? Is there a possibility of inhibition that you foresee if you were to use commercial kits?
There are preservative tubes that'll allow you to separate plasma from blood while preserving cfDNA and minimizing genomic DNA contamination. See article below. By the way, most of those companies offer samples if you make a point to ask for them :D
Ben Abraham Thank you for your answer. I want to perform qPCR with the plasma. In the end I want to establish a qPCR protocol without performing DNA isolation.