A negative Bart test means little unfortunately, as it's very unreliable. If you have the symptoms - like severe Neuro symptoms - then you have the infection.
I like Dr. Schaller's writings on Bartonella, and it's what helped my doctor determine what my treatment should be and what to watch for.
Here is a list of Bart symptoms according to Schaller (too long to post here):
www.personalconsult.com/posts/bartonella-checklist.html