From WebMD: "Diseases that are referred to as sero-negative arthritis include psoriatic arthritis, ankylosing spondylitis, reactive arthritis, the arthritis of inflammatory bowel disease, and sero-negative rheumatoid arthritis. Of these, the first four are closely related to each other and reviewing information about these conditions may be of help to you."
Rheumatoid arthritis can be confirmed by other tests even if the RA factor is negative. I am guessing that your doctor ordered the blood work and xrays to help determine what exactly is going on.
Moderator Crohn's disease, Osteoarthritis and Irritable Bowel Forums
CD, Ankylosing Spondylitis, small fiber peripheral neuropathy, avascular necrosis, peripheral artery disease, pulmonary hypertension, degenerative disc disease, asthma, severe allergy and a host of other medical problems.