CLassification of Axial SpondyloarthritiS Inception Cohort

The overall study design will follow a similar format to that used to develop the ASAS axial SpA classification criteria. All consecutive patients referred to a rheumatologist with current undiagnosed back pain of ≥3 months duration with onset ≤45 years of age will comprise the prospective cohort. The sample size of 500 from North America (minimum of 300 from the US) and 500 from outside North America is aimed at ensuring that a sufficient number of patients will have axSpA and also permit the option to conduct 5-year follow up to determine predictive validity of the classification criteria. The clinical assessment data, including the history and physical examination, will be recorded by the rheumatologist in the electronic case report form (eCRF). This will be accessed through an online portal. The rheumatologist will complete 5 global assessments to determine the presence/absence of axSpA. The first will be completed immediately after the first clinical assessment at the end of the patient encounter and will incorporate details of the history and physical exam. This is aimed at ascertaining which clinical features are considered most important in formulating the rheumatologist's opinion regarding the diagnosis of axSpA. The second will be conducted once the C-Reactive Protein (CRP), and HLA-B27 data are made available. The third will be completed after the pelvic radiograph has been reviewed by the rheumatologist. The fourth will be completed after the rheumatologist has reviewed the report of the pelvic MRI scan provided by the local radiologist. The fifth will be completed after central review of the anonymized radiograph and pelvic MRI scan and feedback to both the rheumatologist and radiologist. In ASAS centers outside North America, it may not be possible to have an assessment of each separate step as indicated if the rheumatologist already receives information on lab and imaging results in the referral later. In that circumstance, only global assessments 4 and 5 will be used as external standards. Patients will then be followed according to the discretion of the rheumatologist and appropriate standards of clinical practice. Patient contact information will be obtained and patient consent obtained through the informed consent to allow contact with the patient after 5 years of follow up.