Seropositive RA is a specific but variable condition. Seronegative RA... you are quite right. It's almost easier to group by a rational treatment approach for a chronic inflammatory polyarthritis that looks like RA clinically rather than one diagnostic entity.
Here is kind of a fun prospective study that illustrates your point well: https://pubmed.ncbi.nlm.nih.gov/29998832/ . The investors assessed what patients in their centre were reclassified into 10 years after a diagnosis of seronegative RA.
"13 (13/435 [3%]) could be reclassified as seropositive or erosive RA: 4 turned seropositive (2 for ACPA and 2 for RF [> 2x reference level]) and 9 developed erosions typical for RA. " ... "Reclassification revealed 68 (16%) cases of polymyalgia rheumatica, 46 (11%) psoriatic arthritis, 45 (10%) osteoarthritis, 38 (8.7%) spondyloarthritis, 15 (3.4%) plausible reactive arthritis, 10 (2.3%) gout, 17 (3.9%) pseudogout, 6 (1.4%) paraneoplastic arthritis, 6 (1.4%) juvenile arthritis, 2 (0.5%) haemochromatosis, 3 (0.7%) ankylosing spondylitis, 2 (0.5%) giant cell arteritis, and 8 miscellaneous diagnoses. The other 140 patients (32%) could not be reclassified in any clear-cut diagnosis "
Caveat: A big limitation is we don't know how many patients were diagnosed with seropositive RA in that time frame: I'd wager it's over an order of magnitude higher. These were probably less clear cut cases.
Here is kind of a fun prospective study that illustrates your point well: https://pubmed.ncbi.nlm.nih.gov/29998832/ . The investors assessed what patients in their centre were reclassified into 10 years after a diagnosis of seronegative RA.
"13 (13/435 [3%]) could be reclassified as seropositive or erosive RA: 4 turned seropositive (2 for ACPA and 2 for RF [> 2x reference level]) and 9 developed erosions typical for RA. " ... "Reclassification revealed 68 (16%) cases of polymyalgia rheumatica, 46 (11%) psoriatic arthritis, 45 (10%) osteoarthritis, 38 (8.7%) spondyloarthritis, 15 (3.4%) plausible reactive arthritis, 10 (2.3%) gout, 17 (3.9%) pseudogout, 6 (1.4%) paraneoplastic arthritis, 6 (1.4%) juvenile arthritis, 2 (0.5%) haemochromatosis, 3 (0.7%) ankylosing spondylitis, 2 (0.5%) giant cell arteritis, and 8 miscellaneous diagnoses. The other 140 patients (32%) could not be reclassified in any clear-cut diagnosis "
Caveat: A big limitation is we don't know how many patients were diagnosed with seropositive RA in that time frame: I'd wager it's over an order of magnitude higher. These were probably less clear cut cases.