CEACAM5 and cancer: CEA measurement is especially useful to monitor cancer treatment, to identify recurrences after surgical resection, to localize cancer spread, for staging or for prognosis.30 After surgical resection of cancerous tissues, the concentration of CEA usually decreases to a very low level and a series of increase in CEA level can be an indicator of the recurrence or metastasis of cancers.31 Therefore, it is of critical importance to detect ultra‐low level of CEA for the screening and monitoring of cancers.