I was diagnosed with RA and none of my blood shows I have it, based on my symptoms it was determined I have it and treatment was started. It's called Seronegative RA and isn't rare at all. It is possible as the disease progresses a person can become Seropositive and blood work will then reflect the disease, but that could never happen as well. If you are diagnosed by a doctor and are having doubts, get a second opinion and if that comes back the same... start treatment.