It’s a DNA TEST matched to dna of your tumor

Just to elaborate a bit, Signatera tests for tumor DNA in your blood. So it can be used frequently for surveillance.

First, the company sequences the DNA from your tumor surgical specimen, looking for mutations, and then it designs a test just for your tumor that detects those mutations.

My impression from reading the pancreas cancer board is that for them, normal tumor markers start to rise significantly before the Signatera comes up positive. I had a different blood tumor DNA test after my cancer recurrence was clearly visible by CT scan, with several sizable tumors, and it didn't detect anything.

So testing for tumor DNA in your blood is a nice idea, but it seems to have a problem with low sensitivity/false negatives in many cases.