CEA is a glycoprotein that is expressed by normal cells in the gastrointestinal tract, particularly the colon and rectum. It is often over expressed by adenocarcinomas of the colon with higher values reported in better differentiated cancers. Elevated levels are also seen in smokers and in patients with other malignancies including breast, pancreas, stomach, bladder, thyroid, and lung. Its value as a screening modality has not been validated in randomized trials, and has a colon cancer detection rate of less than 4% in the asymptomatic population. However, the level of CEA in patients diagnosed with colorectal cancer seems to be of important prognostic significance and higher values are generally associated with worse outcomes. The normal level for an individual is 3.0 ng/ml and 5.0 ng/ml in smokers.
Using CEA to follow patients after curative resection has also been shown to be of help in predicting recurrence. The sensitivity and specificity of CEA for detecting liver metastases after resection has been reported as 94% and 96%, respectively. More importantly is the issue of whether detecting these metastases earlier can lead to a greater chance of second curative resection and possibly improve survival. A large meta-analysis showed that of patients who underwent regular intensive postoperative evaluations including CEA, physical exams, liver function tests, endoscopic exams, and radiologic exams, there was a greater likelihood of curative resection and an improvement in survival over less stringently followed patients. Specifically, regular postoperative CEA determination detects metastatic disease earlier and was shown to improve survival.
With regard to the rate of normalization of the CEA level after curative resection, there is little published on this subject. However, the CEA clearance half-life is slightly more than five days. Put another way, the value should decline from the preoperative level by approximately one half every five days. This takes into account a number of factors which can differ from patient to patient such as the ability of the body's organs to clear the CEA and the normal rate of production of CEA in a given persons body. So, about three weeks to a month after all the cancer has been irradicated, the level should be nearly as low as it can get. Even so, the time to normalize can vary considerably.
The effect of chemotherapy on the level of CEA should be to lower or stabilize its level. This assumes that the chemotherapeutic agent in use is active in treating the metastatic disease, which is producing the excess CEA. If the level of CEA continues to rise and progression of disease is documented on exam or imaging studies, consideration could be given to a different therapeutic drug or modality.
The following are some references for more information on CEA values: