Definition of disease duration in ankylosing spondylitis.

Two definitions of the "disease duration" of ankylosing spondylitis (AS) have been used in the literature: (1) duration since first symptoms connected with AS and (2) duration since diagnosis of AS. To investigate how useful these definitions are for research, we reinvestigated the data of four surveys completed by patients with AS in the years since 1996. We found that the majority of the patients remembered the age at first symptoms with an accuracy of <or=1 year, and that the difference between both definitions (the delay in diagnosis) was between 9.8 and 10.4 years on average, with extreme values of more than 30 years being not very seldom. Because the error made using the duration since diagnosis as "disease duration" is much larger than the inaccuracy taken into account using the duration since first symptoms, only the duration since first symptoms connected with AS should be used as definition for the term "disease duration" of AS.