CEA importance is TRENDS. It isn't a good indicator for everyone, but is very good for some people. Anytime your CEA is within normal range, they do not worry about fluctuation. If you are 1 or if you are 3, it is ok. CEA can fluctuate .5 within the same day very easily. Inflammation causes rise in CEA, so if you get the flu, have radiation, surgery or chemo, it can rise. Smokers (current) get inflammation in the lungs from smoking, so they can have a normal CEA of 5. Someone else with a chronic condition causing inflammation like smoking does could have a normal CEA a bit higher also.
The whole importance of CEA is the trend over time. If your CEA is 2, and then it is 4, and then it is 9, you have something to be concerned about and further assessment is needed. More so if it is 2.5, then 10, then 200+. If your CEA is 2.5, and then 1.9, and then 2.7, and then 2.2, and then 3.0, it means nothing - it is all just normal fluctuation within the normal range. For stage IV patients it is used to see if current treatment is working. If someone goes from 475 to 250, to 110, to 15 that is a great sign. If they have been staying lower and then a rise is seen from 15 to 150 to 260, then it probably isn't working anymore. It can be a very helpful tool for those that it is a good indicator for. But some people have multiple tumors and have a CEA of 2.
The bad thing about CEA is so many people get so worried about it fluctuating a little (myself included), when the intent is not to check for tiny fluctuations, but to check for a trend that shows something good or bad is probably going on. It took a while, but I've learned not to freak out about CEA, unless I see a trend over time, just like my oncologist has been trying to get me to realize for 3 years!