Final answer:
Blood is drawn for a CEA level to monitor for the recurrence of cancer post-treatment, as changes in CEA levels can be an indicator of the cancer's status.
Step-by-step explanation:
The nurse should inform the client that the reason blood is being drawn for a carcinoembryonic antigen (CEA) level is to monitor for cancer recurrence. CEA is a type of tumor marker that can be found at higher levels in individuals with certain types of cancer, including colorectal cancer. It is used after treatment, such as surgery or chemotherapy, to monitor the effectiveness of the treatment and to check for signs of the cancer returning. A sequence of rising CEA levels may indicate a need for further diagnostic tests, whereas steady low CEA levels can provide reassurance to the patient.