Objectives To evaluate determinants of productivity costs in ankylosing spondylitis (AS) in three European countries.

Methods 216 consecutive prevalent AS out-patients started a 2 year longitudinal cost-of-illness study. AS-related health resource utilisation was collected by questionnaires every 2 months. Productivity costs were calculated by friction cost (FC) method and human capital (HC) approach. Cox proportional hazard analysis with costs as dependent variable was used to assess if ?country? was a determinant of productivity costs after controlling for age, gender, disease duration and time integrated disease activity (BASDAI:0–10; higher values indicating higher disease activity) if FC were the outcome or baseline physical function (BASFI:0–10; higher values indicating worse function) if HC costs were the outcome.

Results 209 patients completed follow-up. In the Netherlands (NL), (F) and (B) respectively, 70, 69 and 74% of patients were male, mean age 46 (sd:12), 38 (sd: 12) and 42 (sd: 14) yr, mean disease duration 24 (sd:12), 14 (sd:9) and 13 (sd:10) yr and 34, 13 and 24% had manual professions. 196 patients were in working age and 113 patients had a paid job.

Conclusion In AS, productivity costs determined by FC and HC approach are significantly associated with the country studied. This finding has important consequences for the generalizability of health economic studies.