Why do my joints, hands, knees keeps retaining fluid? I have RA.

To answer your last question first, the only difference between seronegative and seropositive RA is the absence of rheumatoid factor in the blood of those who are seronegative, which is only about 20% of cases, and the presence of this factor in the other 80% of RA cases. Mild or severe disease can occur in both situations, but as a group those with high amounts of rheumatoid factor tend to have somewhat worse disease.
The recurrence of fluid in your knees may be the result of several different problems. In general, if the RA inflammation is under control, there should not be a recurrence of fluid in the knees, which is known as effusions. One possibility is that the inflammation is not under good control, and your medications might need adjustments. Another possibility is that you have a lot of damage to the cartilage in your knees and you have superimposed degenerative arthritis. This could require a knee replacement. X-rays of the knees would help assess the situation as well as an analysis of the fluid in the knees. Less likely would be mechanical problems resulting from ligament or cartilage damage. An MRI would be of help here. So again, there are several different possible causes.
Joseph Biundo MD