00:38 Jul 23, 2011 

"Free" IL-6 is not bound to protein. "Total" IL-6 includes both bound and unbound IL-6.

Free and Total is used in many biological assays for various hormones.

The actual protein to which it is bound depends on the context. In the example below, the protein is an anti-IL-6 immunoglobulin.
http://onlinelibrary.wiley.com/doi/10.1002/eji.1830250207/ab...

19:51 Jul 22, 2011 

I leave authoritative comment to someone with medical knowledge, but normally free would mean that it is free in the blood serum and therefore free to bind to sites or react with other agents, while the total would include quantities that are bound to other sites.
"The serum concentration of interleukin 6 (IL-6), a cytokine that plays a central role in inflammation, increases with age."

19:21 Jul 22, 2011 

...it would refer to the level of IL6 in the serum: the level of it that is 'free', and the total level (i.e. including that which is not free)

Of course, to understand it properly, you need to know what IL6 refers to; personally, not being a medical expert, I haven't the foggiest idea!