My mom had colon cancer nearly 12 years ago. Has been cancer free since surgery and chemo. CEA is now 3.7., highest it's been since then. Is this bad?

Not necessarily. when the colon cancer comes back , it usually comes back within the first 5 years from the diagnosis. it is unlikely that the colon cancer comes back after 12 years. in addition, CEA is not very sensitive nor specific for colon cancer. other things like cigarette smoking, other liver problems etc can increase CEA.if she smokes, makes sure to encourage her to quit. she need to follow up with her MD.
I agree with the... ...other 2 colleagues who answered your question: 3.7 is not very high, CEA is neither very specific nor sensitive, CEA can go up due to other causes like smoking, and chances of recurrence after 12 years are very slim. I would just add that having had one colon cancer does not spare you from a second. I hope she is following up with her scheduled colonoscopies.
Not Sure. CEA is best used as a follow-up after colon cancer diagnosis. A rising CEA could mean a recurrence. 3.7 is still not very high. Talk to her surgeon.