Your question about the number of patients with RA who are seronegative piqued my interest, @jba. I did a quick Google search and found this interesting information from research that Mayo Clinic did (go figure).

The Number of People with Rheumatoid Arthritis Is Staying Steady, But More People Have the Seronegative Kind https://creakyjoints.org/diagnosis/more-rheumatoid-arthritis-patients-seronegative/
"The percent of people who have rheumatoid arthritis (RA) hasn’t changed much over the past few decades. From 1985 to 1994, about 40 people per 100,000 had it; between 2005 and 2014, that number was 41 out of 100,000. What has changed, however, is the proportion of patients who have the seronegative form of the disease.

This information comes thanks to a new analysis that was conducted by Mayo Clinic researchers and published in the journal Annals of the Rheumatic Diseases. Between 1985 and 1994, only 12 out of 100,000 people with RA were seronegative. Between 2005 and 2014, that number jumped to 20 out of 100,000."