CEA

A protein produced in the developing gut before birth, and again by some tumours and by inflamed bowel.

Its real use is following someone already treated for bowel cancer, where a rising trend matters long before any single value does.

It is not a screening test: smoking alone raises it, and so do inflammatory bowel disease, liver disease and several benign conditions. One value in isolation says very little -- the trend across tests is the whole signal.

Worth discussing a value that rises across successive tests, whatever the absolute number.