The use of serologic tumor markers in gastrointestinal malignancies.

The tumor markers CEA, CA 19-9, and AFP have a distinct and useful role to play in the management of gastrointestinal malignancies. These are clearly not ideal tumor markers. They lack day-to-day reproducibility and sufficient sensitivity and specificity to be useful as general screening tools. Taken as isolated values, they are lacking in precision. However, used in the appropriate context, with an understanding of their associated biology, these tumor markers help to manage patients and might reduce the economic burden of decision making. As we develop a better understanding of the process of neoplastic transformation and progression, new tumor markers will become available. Markers may, in the future, be used to predict responses to particular therapies or overall prognosis. Some will be molecular probes, such as the K-ras or FAP gene mutations, and others may relate to alterations in regulated processes, such as increased expression of a key enzyme in DNA synthesis, thymidylate synthase.