Carcino Embryonic Antigen (CEA) blood test is carried out to check for the return of colon and other cancers in those already diagnosed. It is not generally recommended as a screening test for cancer in the general population, but may be useful as part of a general investigation into suspected cancer in some individuals. It can be used to monitor the effectiveness of treatment and also the recurrence of disease. This test is usually tested before, during and after starting treatment.