The CA 72-4 test is a blood test used to monitor stomach cancer. CA 72-4 is a type of protein produced by stomach cancer cells. This test is often used to monitor the progression of stomach cancer after treatment or to detect early recurrence. High levels of CA 72-4 may be a sign of stomach cancer or other stomach diseases. However, this test is not used to diagnose cancer because CA 72-4 levels can also be elevated in some other benign conditions.