The cancer antigen 19-9 tumor marker level is a protein found on the surface of some cancer cells, notes Lab Tests Online. Cancer cells shed the protein, allowing a physician to test a patient's level with a blood sample. Patients with advanced pancreatic cancer, colorectal cancer, lung cancer and cancer of the gallbladder commonly have elevated CA 19-9 levels, although it also exists in small amounts in healthy individuals. Physicians follow the course of a patient's cancer with these levels.
CA 19-9 is not a tool physicians use to diagnose cancer because it is not sensitive or specific enough, states Lab Tests Online. Instead, physicians use CA 19-9 to differentiate between pancreatic cancer and other conditions, to look for a recurrence of pancreatic cancer and to monitor how a patient responds to treatment for pancreatic cancer. Patients with pancreatic cancer have elevated CA 19-9 levels more often than patients with other forms of cancer, and doctors only use it as a tumor marker for cancer if the specific form of cancer produces it.
CA 19-9 levels also elevate with noncancerous conditions, including bile duct obstruction, reports Lab Tests Online. In bile duct obstruction, levels fall after a physician removes the blockage.