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In 1997 we studied labour force participation among 658 Dutch patients with ankylosing spondylitis (AS).1 In those who had a paid job before onset of disease (n=529), age and sex adjusted withdrawal rate was 3.0 times (95% CI 2.5 to 3.6) higher than expected in the general Dutch population.2 Within patients with AS, those with a manual job had a 2.3 (95% CI 1.5 to 3.4) times increased risk of withdrawal compared with those with a non-manual job after correction for age at onset of disease, gender, educational level, and coping strategies.2 However, the question remained whether a manual job was a specific risk factor for withdrawal from work in patients with AS or if a manual job is a non-specific risk factor for withdrawal in every working subject.
Recently, Dutch population figures for 1998 on withdrawal from the labour force, for the same work categories as used in our study, became available (Dutch Bureau of Statistics). Therefore, we were then able to calculate the job adjusted ratios for withdrawal from the labour force in patients with AS by comparing the job-specific annual withdrawals in patients with those from the general population. Five major classes of work were distinguished: agriculture, industry, transport, commerce, and services/management. The first three classes were later grouped as manual jobs and the last two classes as non-manual jobs. This classification has some limitations because jobs like housekeepers and waiters are also included within the servicing jobs and therefore considered as non-manual jobs. However, this was similar in patients and in controls. All data were stratified by gender. The 95% CIs for the ratios were calculated using Poisson's distribution.
Table 1 presents for each job class, the observed rates of withdrawal from the labour force as well as the ratios when compared with the general population. While the overall risk for a patient with AS to withdraw from the labour force is 3.0 (95% CI 2.5 to 3.6) times higher than expected in the general population, this risk is 4.9 (95% CI 3.5 to 5.9) times higher for those with a manual job versus 2.2 (95% CI 1.6 to 2.7) times higher for those with a non-manual job. The observed effect was stronger in male than in female patients, probably because of the smaller number of female patients in the study sample. In the general population withdrawal rates were not significantly different among the professional classes studied either for men or for women.
We conclude that patients with AS with a manual job have an increased risk for withdrawal from the labour force, also when adjusted for job related withdrawal in the general population.
The authors acknowledge the Dutch rheumatologists participating in the Standardised Diagnositic Register of Rheumatic Diseases and all patients who took part in this study.