Problems complicating the genetic epidemiology of rheumatoid arthritis.

I review problems inherent in considering the genetic epidemiology of rheumatoid arthritis (RA). The major area of concern is in case definition, both in assigning criteria and in establishing time of onset of disease, since presentation fluctuates throughout life. Other problem areas include selection of case and controls and the appropriate approach to analysis. Awareness of the problems in study design and conduct is fundamental to undertaking valid studies in this area.