Why wouldn't a ca 125 be zero when there is no cancer? where is the value above 0 coming from?

Basic lab medicine: A small amount of the substance, which is coded by the normal genome, is ordinarily present in the bloodstream. Immunoassays also detect antigens similar to ca125 as weak signals, and a cutoff is established below which a reading is not suspicious for cancer. These facts have a lot to do with why these markers aren't all that helpful in searching for cancers in folks without any signs.