Sometimes the doctor's take the results of xrays, MRI's and bloodwork and use it all together to tell what type of arthritis you have. For example if you show up with lower spine degeneration and sacroilliac joint degeneration and a high bloodcount for Rheumatoid Factor, They will diagnose you with spondylarthropathy caused by Rheumatioid Arthritis. They are also able to diagnose Ankylosing Spondylitis the same way. They also do xrays of the hands and are able to tell you if you have Rheumatoid Arthritis just by looking at the pattern of deterioration in the hands.

Hope this helps,
Billye