Moving instead of asking? Performance-based tests and BASFI-questionnaire measure different aspects of physical function in ankylosing spondylitis

In this cross-sectional study 126 AS patients completed the BASFI questionnaire and eight performance-based tests based on BASFI-items. Each test received three scores: one for performance (time or points) and a score for exertion and pain experienced during performance (using modified Borg-scale and VAS 0-100 mm, respectively). Linear regression analyses were used to assess the associations between the BASFI questionnaire and performance-based tests.The univariable association between performance and BASFI-score was moderate with a R-square of 0.31 and Beta of 0.56 (p's < 0.05). In a multivariable analysis, the association between performance, exertion and pain on the one hand and BASFI-score on the other was assessed; R-square increased to 0.54: the Beta's for exertion and pain during performance were 0.38 and 0.26, respectively; the Beta for performance decreased to 0.19 (p's < 0.05).This study demonstrates that alongside actual performance, patients seem to incorporate exertion and pain in their assessment of perceived physical function on the BASFI questionnaire. Performance-based tests could provide an objective outcome measurement for the evaluation of physical function and give relevant new information in addition to the BASFI questionnaire.Limitations in physical function due to inflammation, pain and reduced spinal mobility are an important feature of ankylosing spondylitis (AS). Physical function is, therefore, considered an important outcome measure for evaluating the disease course and the effectiveness of interventions in AS patients [1,2]. The preferred assessment methods for evaluating limitations in physical function in AS patients are self-reported outcome measures such as the Bath Ankylosing Spondylitis Functional Index (BASFI) [3] or the Dougados Functional Index (DFI) [4]. Both indexes are disease-specific outcome parameters that have been proven to have adequate clinimetric properties and to be reflective of patients' perspectives [5-8].The BAS