Worldwide prevalence of juvenile arthritis why does it vary so much?

OBJECTIVE: To review epidemiological studies of childhood arthritis from 1966, and to identify possible reasons for the wide-ranging results for both prevalence and incidence of juvenile arthritis (JA). JA is the term used here collectively for juvenile rheumatoid arthritis, juvenile chronic arthritis, or juvenile idiopathic arthritis as defined in the respective published studies.
METHODS: A review of 34 epidemiological studies of JA since 1966 was undertaken.
RESULTS: Prevalence of JA is reported as 0.07 to 4.01 per 1000 children. Annual incidence is reported as 0.008 to 0.226 per 1000 children. The major factors contributing to differences in estimates include (1) factors due to diagnostic difficulties, to the development of new diagnostic criteria, and to the differing definitions of clinical cases; (2) differences in case ascertainment (community based versus clinical case studies, qualification and experience of study clinicians, definition of study population); (3) factors occurring with the passage of time, i.e., standard of living, health care resources, and increasing knowledge; and (4) small studies and hence more chance fluctuation. The major variation in reported prevalence was due to the difference between true community based studies involving children from within classrooms or homes (and not necessarily previously diagnosed with JA) compared with clinical case studies of children who (by definition) had been previously diagnosed. The highest prevalence was reported for true community based studies.
CONCLUSION: Many factors contribute to the discrepancies between reported prevalence and incidence for JA. Studies based truly in the community reported the highest prevalence, as previously undiagnosed cases were included. Future studies involving standardized criteria and standardized case ascertainment done by fully trained clinicians should show greater consistency of results.