CA 19-9 is a protein found on the surface of some cancer cells. When it detaches from the surface, CA 19-9 can be detected in the blood. CA 19-9 levels are an indicator used mainly to monitor pancreatic cancer. Test results are expressed in kilounits of CA 19-9 per litre of blood (kU/L). Results may vary from one laboratory to another, depending on the technique used. It is therefore recommended to always use the same laboratory to monitor CA 19-9 levels.
High CA 19-9 levels are strongly associated with advanced pancreatic cancer (high levels in 70–95% of individuals with advanced pancreatic cancer). Varying levels over time make it possible to monitor treatment efficacy or detect a recurrence. Levels can also be high in other cancers (bladder, gallbladder, colorectal, gastric, ovarian, lung or liver) and in certain benign (non-cancerous) conditions such as liver stones, pancreatitis, cystic fibrosis, liver disease and thyroid disease. Since this test is not very sensitive and can also be positive in several other cancers and benign conditions, CA 19-9 testing is not recommended for pancreatic cancer screening, only for pancreatic cancer monitoring.