possible for joint fluid to test negative but still have lyme???

My sister has been sick for 15 years.... she got bit by a tick and developed a rash but never thought anything of it....  She currently now has brain lesions, neck problems, weird skin stuff rash things, developed ulcerated colitis and put her in hosp giving her lev and flagyl... after that episode, her knees became very painful and developed fluid on them.... shes been dealing with that for 6 months... (doc said was reaction to lev) She begged her primary to check for lyme, he did with the elisa and it came back negative... he told her they dont do western blot if elisa is negative....  She finally had fluid taken off her knees and checked for lyme, that was also negative but her wbc count is high in the fluid...  She is so depressed, and doesnt know what to do... they are now sending her to rhuemmy at the end of the month... there is a lyme clinic in RI, but need a doctor referal to get in and primary care says she doesnt have it, so they wont refer her....  My question is can you have lyme in joint fluid and test still come out neg????  Thanks....