@carneydh ,

Note: I have ZERO medical training.

35 is near the ULN (Upper Limit of Normal) for CA19-9; 371 is nearly a 1000% increase and 10x the ULN. I would ask for another scan ASAP and strongly consider resuming chemo ASAP if they can't find another definitive cause for the high reading. If it comes down with chemo, then you've *probably* got cancer somewhere.

(The above assumes you responded well to chemo before; CA19-9 came down with previous treatments and you didn't have any adverse effects. If it didn't come down with the chemo you had (e.g., Folfirinox), then see about switching it now to GAC, or vice-versa.)

Without direct evidence (i.e., scan or biopsy) of cancer it's difficult to justify chemo; you could try alternative tests (ctDNA like Signatera, Guardant, and Galleri), but they are not definitive, and subject to false negatives that would mask disease rather than reveal it.

My personal experience was having CA19-9 go down to 12 one month after a "perfect" Whipple, followed by an intermediate reading of 35 two months after Whipple and 77 two months after that with a 1.3 cm recurrence detected on MRI.

EUS biopsy, Signatera, and Galleri were all negative (didn't get a PET scan or resume chemo then, sadly), so docs recommended new tests and scans 6 weeks later. By then, CA19-9 was 277, Signatera was positive, the mass was 2 cm and suspected metastatic. Three weeks after that, CA19-9 was almost 700 and metastasis was confirmed.

I have a lot of faith in the sensitivity and correlation of CA19-9 to disease/treatment in recurrent patients if you know your baseline, track the numbers diligently, do all tests at the same lab, account for possible noise sources (other inflammatory disease, vitamins & supplements, etc).

Don't delay; PDAC can get out of hand very quickly!