Objective To investigate which spinal mobility measures (SMMs) are most frequently impaired in patients with ankylosing spondylitis (AS), whether a hierarchy of impairment can be established, and whether assessing fewer measures sufficiently captures impairment in spinal mobility. MethodsPatients from the Outcome in Ankylosing Spondylitis International Study were followed up for 12 years. SMMs were considered impaired when falling below predefined cutoffs, derived from normal individuals. The proportion of patients in whom each SMM was impaired was calculated using baseline observation. In patients with 1 impaired SMM, we investigated how often impairment in spinal mobility would be missed if only a fixed number of SMMs was assessed. Analyses were repeated using all 12-year observations. ResultsA total of 216 patients were included (70% males). Lateral spinal flexion (LSF) was the most frequently impaired measure, followed by the modified Schober (mSchober) test, tragus-to-wall, cervical rotation, intermalleolar distance, and chest expansion measures, respectively. This hierarchy was strikingly consistent over time, and independent of sex, symptom duration, and presence of syndesmophytes. In patients with 1 impaired SMM, LSF was impaired most frequently (86%), followed by the mSchober test (58%). If only LSF was measured, 14% of patients with impairment in any SMM would be missed; if additionally the mSchober test was measured, 9% would be missed. ConclusionLSF followed by the mSchober test are the most frequently impaired mobility measures in AS, reflecting an earlier involvement of the lumbar spine, followed by involvement of the thoracic and cervical spine. In clinical practice LSF and the mSchober test suffice to screen impairment in spinal mobility.