@waflanders, typically CA 19-9 is not used alone. Further investigation with imaging studies, like CT scans are needed to confirm the diagnosis or recurrence.

From American Cancer Society https://www.cancer.org/cancer/gallbladder-cancer/detection-diagnosis-staging/diagnosis.html
"People with gallbladder cancer may have high blood levels of the markers called CEA and CA 19-9. Usually the blood levels of these markers are high only when the cancer is in an advanced stage. These markers are not specific for gallbladder cancer – that is, other cancers or even some other health conditions also can make them go up.
These tests can sometimes be useful after a person is diagnosed with gallbladder cancer. If the levels of these markers are found to be high, they can be followed over time to help tell how well treatment is working."

Monitoring CA 19-9 is a less invasive test and can be the proverbial canary in the coal mine. How are you feeling about the current follow-up plan?