I take xanax and recently had a false negative on a drug screen on a blood test

[RCW63]

I am in remission for cancer and now have chemo induced peripheral neuropathy, which is extremely painful and causes a lot of stress & anxiety. I take Xanax every day and have for over 15 years. I know i take my medicine as prescribed everyday!! My doctor recently asked for a urine sample for drug screening, due to where my cancer was located I find it difficult to pee on command and asked for a blood test.
This was three months ago and she just told me, the blood test was negative for the Xanax. The manufacturer is Dava pharmaceutical which has a lot of bad press online. I don't understand how this is possible. Has anyone ever had a false negative for a drug screen on blood test?

[remar]

Xanax is a short acting medication. I'm not sure how long it stays in your system but if it had been awhile since you took one it may not show up on a blood test.
Pain Drs should realize that these test are at best 95% accurate.
It sounds like you've been a long time patient there so they know you take your meds as prescribed. I would ask for another test and take your Xanax within an hour of having the test done.
My sister and I are both in pain management. She had a bad urine test. We've been going there for awhile so her Dr knew the test had to be wrong. He called my sister, she went in to take another test and everything was fine.
If this happens again ask for another lab to be used. And remember what I said about Xanax. It does not stay in your system for long.