CA15 blood test ???

I was wondering if anyone has any information on this blood test.  I was diagnosed in 1995 DCIS 1cm tumour and I had a lumpectomy there was no spread to lymph nodes.  During my follow-up visits to the cancer clinic I would have my mammogram, physical check-up and then I would be sent over to the lab for a CA15 blood test.

After 8 years I was told I was no longer required to come back to my oncologist for follow-up.  So for the last 10 years I have carried on with my regular doctor doing everything my oncologist did including the CA15 blood test.

Every year she tells me that this blood test is NOT recommended for "well" patients and is NOT recommended to be used as a screening tool.  She also said it has a high percentage of false positives and other things can make the test come back positive.......things that are probably nothing but would cause a great deal of stress...

She discourages me to get this test every year.  The reason I continue to get it is that the another GP in our town had BC at the same time as me and was undergoing similar follow-ups with the same oncologist.  After 5 years I heard that this CA15 showed higher levels and after all kinds of testing it turned out it was in her liver.  They did surgery and she is fine today 18 years later. So in my mind I feel like if I don't get this blood test every year they are missing something. Even though my doctor really does not like to even give me the requisition for the lab but I make her.  Then I carry it around for a month or 2 until I get up the nerve to get the test, then I stress out until I hear it is ok.......

When I was in for my yearly check-up last week asked her for the requisition and she really was quite resistant to give it to me.  (I should add I have had this doctor for over 20+ years and she is more than cautious) I mentioned the situation above and she said there were many other circumstances around that diagnosis besides the CA15 blood test.

Does anyone know anything about this test and could please shed some light on what exactly it does or doesn't do....

Thanks everyone:)

Cindee