Epidemiology of Sjögren's syndrome, especially its primary form.

During the last decades different sets of criteria have been used for defining primary Sjögren's syndrome. Using these criteria prevalence estimates have varied between 0.04%-4.8%. This hundred-fold distribution in estimated frequency is partly due to differences in age groups studied, classification criteria used and methods used for objective evaluation of lachrymal and salivary gland hypofunction. Another problem which makes comparison between studies difficult is that workshop recommendations for the various objective tests are very often not strictly followed. Population based studies tend to identify cases with less severe disease and higher prevalence rates. Furthermore, there seems to be little correlation between dryness symptoms reported by subjects and objective hypofunction of lachrymal and/or salivary glands. Possible explanations to these findings include selection bias towards more severe cases in clinical (university) studies and a low predictive value for some criteria in population studies.