Outcome measures and statistical analyses
The primary outcome measures were the number of registered sick leave days for cases and controls at three, six, 12 and 24 months after inclusion. Data was retrieved from the Swedish Social Insurance Agency’s Micro Database for Analysing Social Insurance (MiDAS). In Sweden, sick leave benefits are granted for 25%, 50%, 75% or 100% of a working day, depending on how much the ability to work is reduced because of the diagnosis. Both gross days (number of sick days, regardless of extent of sick leave) and net days (number of sick days converted into whole sick days) were used as outcomes in this study.The Mann–Whitney test was used to investigate the difference in gross and net days between the groups, with the level of significance set to 