For anyone who cares, I looked into it and it seems that it refers to the pro-coagulant activity factor VIII rather than the factor itself.
I came across a pretty good explanation in a Hematology text that is basically:
Factor VIII:c - refers to the pro-coagulant activity of factor VIII. This is measured by PT.
Factor VIII:Ag - refers to antigens of factor VIII that are detected by immunoassay.
My understanding is that these are terms derived from when people didn't really know a lot about factor VIII and even thought vWF was part of factor VIII, rather than its own protein.
Anyway, thought I'd just post in case anyone has the same question since VIII:c is mentioned in RR path and many people use it as a reference...
