@whilted , I don't mean to scare you either, but instead reiterate the need for vigilance and self-advocacy.

You might ask the doc if you can do the CA19-9 tests a few more times (weekly, every 10 days, or at least bi-weekly) before the next visit. The value can fluctuate for numerous reasons, but if you have several tests in that 1-month window, you'll have more data points for your trend line, and a better idea of what her normal is. Two readings one month apart is not bad, but with "noise" in the measurements and only two data points you don't really know which number(s) to trust.

The test is not expensive, or at least shouldn't be. I used to get mine tested every two weeks with a self-pay price of about $25. Just make sure all your tests are done at the same lab, because results from different labs are not interchangeable. Also note that vitamin B7 supplements (Biotin) in high doses can suppress the CA19-9 level on some tests, so avoid those at least a few days before each test.

Meanwhile... I assume your mom has already had genetic tests like Invitae and Guardant. The Galleri test from Grail might also be a good data point to get, if you can afford the self-pay cost (approx $1000). Get those tests done asap so you have results ready when/if treatment decisions have to be made, not 3 weeks later. If they wind up eventually doing any kind of biopsy, ask them to get enough tissue for full next-generation sequencing and a Signatera test that can be used in follow-up disease tracking if necessary.

For me, slowly rising CA19-9 was the first indication my cancer had returned after surgery at a top hospital, beating out the MRI (top hospital), Signatera, Galleri, and even an endoscopic biopsy.

Since she had Folfirinox before, I assume she still has her chemo port. I would be ready to resume chemo on short notice, but you might also want to see if you can get a surgical opinion from Dr. Christopher Wolfgang at NY Langone (formerly at Johns Hopkins) who has a reputation for taking on some challenging surgeries others refuse to do. Get that appointment in the pipeline as well, because it might take a while to see him, and if he's willing to do it before chemo starts, you can avoid a possible 30-day chemo washout period.