You just asked the golden $billion dollar question and boy do I wish I had that answer for you and all of us others who are still diagnostically playing limbo. This question has been raised multiple times on multiple threads and the best summary of an answer I can give you is this:
Many AI diseases share common symptoms and traits that overlap each other. Additionally, several diseases, such as Lupus, are know to mimic other diseases in their appearance both physicially and from a laboratory perspective. Diagnosing Lupus can be, for a lot of people, a long frustrating journey filled with ups and downs and times where your doctors say you do have it but then turn around and say you don't. Sometimes your blood work meets the criteria for diagnosis and then the next time it might not. In your case, having fibro isn't making a Lupus diagnosis easier as your doctor is still weeding and sorting out the Fibro symptoms and reactions from this other AI disease(s) and it's (their) symptoms and reactions. From what you are saying, you have a very proactive doctor. He didn't completely rule out Lupus b/c you acknowledged that he put you on the pred to see if you have the typical Lupus reaction toward it. He sounds like my rheumy who is focusing on another (currently) primary AI disease and recognizing that there is something else that seems to be going on and has even acknowledged Lupus as a possible fit. I know it's hard - but stick with him on this and see where he goes. As slow as it may seem - he is moving forward and doing so cautiously which, in my opinion, is a good thing so he can properly treat you. (((hugs)))
PS- What part of NY are you in? I think I asked you on another thread but brain fog prevents me from remembering if that is true or not.