Hi Scooter,
I can imagine you are very worried about your sister and the swings in her CA 19-9 numbers. Allow me to tag fellow members @palvi @rtw007 @buckslayer @allboyzoo @beachdog @baileycody, who can share their experiences.

Scooter, when did you sister start chemo? How many treatments has she had?

Everyone's ca19-9 reading is unique to the individual. I understand that there are people who have completely "normal range" CA19-9 despite active pancan. My wife's CA19-9 was over 3000 when the recurrence (ovarian tumor) was spotted in 2/21 CT having shot up from under 200 in 1/21. Immediately after surgery with clean margins, her CA19-9 rose to over 3300. The oncologist wasn't concerned since she was scheduled for chemo as soon as she was recovered from the surgery. According to a friend, a retired pancan researcher, the CA19-9 can increase in response to dead cells. After 8 GAC treatments, her CA19-9 is below 200. She's completed 10 treatments now with 2 more scheduled. Won't have another CA19-9 until after treatment 11.

In my wife's case, CA19-9 seems to track well with the presence/absence of active cancer cells but it is not foolproof. Ongoing scans and paying attention to symptoms are key to detection. BTW, we've had almost all of my wife's CT and MRI scans re-read for second opinion. Our insurance covers second opinions.