It is not recommended to add serum (e.g. FBS) to the final XF assay medium because it affects medium buffering capacity and may bind non-specifically to drugs/compounds to be tested. Further, it is not recommended to use serum in the sensor cartridge injection ports due to potential port leaks and/or injection failure.
If FBS is absolutely required, a low concentration of 1-2% may be used, or empirically determined via a serum titration experiment. Note that presence of serum in XF assay media may alter the efficacy of FCCP or drugs/compounds being tested, and thus the optimal concentration of FCCP or drugs/compounds being tested may increase in the presence of serum.