If you don't have access to CTDNA or Signatera (ask your oncologist), then just trust the standard of care in your area, and in your case the CT or PET. Again, The concept of assay variability is the issue here.... because a CEA value of 1 2 3 or 5 is really the same number, it's only one that number climbs to 10 or 20 is it meaningful at all. I believe a good oncologist would not act on a CEA value alone.