Tumor marker tests are subject to wide variation, and are only meaningful when taken in context of multiple diagnostic tests. When viewed by themselves, they are almost meaningless. Different cancers will express certain antigens and proteins when they are active and growing. For instance cholangiocarcinoma often expresses the carbohydrate 19-9 antigen. 40 is considered a normal level. Some cholangiocarcinomas express this antigen at high levels, some express it just above normal, and some variants will not express it at all even when it is growing. The CA19-9 must be viewed in sequence over a period of months, and then compared with CT scans and liver function tests to give an overall picture.
In my case, my CA19-9 was around 65 when I was first diagnosed. Since I have been on chemotherapy over the last 6 months, it has fallen to 40. This has been supported by multiple CT scans which show no growth over the same 6 months, and liver function tests which are perfectly normal.
Unfortunately, one will see certain cases where the CA19-9 normal or only minimally elevated, yet scans show consistent disease progression and deteriorating liver function tests. So, CA19-9 needs to be evaluated only in the context of the overall big picture.
This is also the case with other cancers. For instance, regular hepatocellular carcinoma (HCC) expresses alphafetoprotein, or AFP. AFP of 9 or less is normal. AFP of 400 or more may indicated a rapidly growing cancer. However, there are some cases where the cancer is growing, but inexplicably is not expressing elevated levels of AFP.
Likewise for prostate cancer, which expresses PSA, or prostate-specific antigen.
I hope this helps to make some sense out of a confusing subject. I have spent much time researching this to try to understand this elusive lab test. Please don't take my explanation as professional. Ask your onco doc, and don't let him/her go until it is explained to your satisfaction!
Violarob in Texas