So many autoimmune disorders can mimic RA. It is no wonder that it is hard to differentiate them all. RA whether it be seropositive or seronegative can be differentiated based on a rheumatoid factor (RF) test.
https://www.verywellhealth.com/diseases-that-mimic-rheumatoid-arthritis-190335#:~:text=Psoriatic%20Arthritis,-Psoriatic%20arthritis%20(PSA&text=PSA%20can%20closely%20mimic%20RA,antibodies%2C%20making%20these%20indicators%20seronegative.
There are many more forms of "seronegative inflammatory arthritis" which aren't the RA variety. What about all of the other types of inflammatory arthritis instead of PMR?

I was diagnosed with a type of seronegative inflammatory arthritis called reactive arthritis. It derives its name from a reaction to an infection . I was also genetically predisposed.

Reactive arthritis (ReA) also mimics RA except I was RF negative. It wasn't likely to be PMR because I was 32 years old at the time.

I was old enough to have PMR when I reached the age of 52. My rheumatologist was confused and had a hard time deciding what was going on. I was pleading for prednisone but my rheumatologist wanted me off prednisone so she "could see what was going on." That was when PMR was diagnosed.

My first question was what happened to reactive arthritis. I was thinking reactive arthritis had morphed into PMR. My rheumatologist said it wasn't the case of having one thing instead of the other thing. She said I still had reactive arthritis and it was unfortunate but I also had PMR.

One thing about autoimmune disorders that people aren't aware of. You probably won’t hear that having one autoimmune condition increases your risk for getting another. My rheimatologist now says I have a "full range of rheumatology problems."

"For people who have more than one diagnosed autoimmune disease, it’s called polyautoimmunity. The combination of three or more diagnosed autoimmune disorders in one person is called Multiple Autoimmune Syndrome (MAS)."
https://www.benaroyaresearch.org/blog/mystery-multiple-autoimmune-diseases
There should be a law that says you can only have one autoimmune disorder ... preferably none.