Could the cea level show if there are still microscopic cells left behind after the tumor is out?

CEA: Cea is a helpful index after treatment of GI tumor, but i doubt it is so sensitive to show few residual cells.

Yes: Cea, a protein released into the blood, may be produced by certain cancers, but not all and even not all of a specific type, e.g. Not all colon cancers will produce abnormally elevated CEA in the serum, but some do. If measured before surgery, and elevated, then the CEA may a useful lab test to follow going forward, as a sustained elevation may indicate subtle cancer persistence or recurrence.