While there are "normal" ranges published for CA19-9 assays (which vary between manufacturers), my understanding is that these are based on some sort of 95% confidence interval for whatever group of normal non-cancerous people they used to establish the test basis. Individuals can have their baseline "normal" be different than a large population. Also since CA19-9 values can be affected by conditions other than cancer tumors, the data needs careful review prior to jumping to any conclusion . There is a recent article on use of CA19-9 as a leading indicator to CT scans for conditions where adenocarcinoma is known - "CA19-9 as a Dynamic Biomarker for Continuous Monitoring of Therapeutic Efficacy in Pancreatic Adenocarcinoma" https://www.mdpi.com/2072-6694/17/24/3902. In my wife's case her initial tumor showed almost no CA19-9 activity at all. The CA19-9 data have become useful only in the case of the metstatic recurrence. The attached picture shows the jump when metastases occurred in 2025 - subsequent chemo reduced the CA19-9 & the tumors. However we are now at 14 weeks of continuing small monotonically increasing CA19-9 values (red circled area) which seem to correlate with very early and marginal selected tumor nodule growth (and only at the limits of the CT resolution). We expect the next CT scan is 3 weeks to more clearly show tumor growth (and we'll adjust treatment). I think the punchline is to know each person has a somewhat unique CA19-9 profile at the low baseline values and to watch the trend lines as more informative at low values than an absolute number. Caveat: "Your mileage may vary."