What carcinoembryonic antigen (CEA) level should trigger further investigation during colorectal cancer follow-up?

Study hypothesis: It is feasible to increase the sensitivity of blood CEA as an indicator of recurrent colorectal cancer while retaining an acceptable level of specificity by specifying a positive result in terms of the change in blood CEA level over time rather than the absolute level of a single measurement.
Pilot study on http://www.isrctn.com/ISRCTN61091474
Main trial on http://www.isrctn.com/ISRCTN41458548
Primary outcome(s): The main outcomes (cancer recurrence, treatment of recurrence with curative intent, and death) are monitored continuously. Blood CEA levels are measured 3 monthly for 2 years and 6 monthly for the next 3 years.