French Cohort of Undifferentiated Spondyloarthritis

"Radiological examinations:~* Standard x-ray: they include an x-ray of the cervical, thoracic, and lumbar spine (side-on), of the lumbar spine from the front and an x-ray of the pelvis from the front~* MRI scan: are taken of the cervical, thoracic and lumbar spine and sacroiliac joints.~Bone densitometry: evaluation of bone densitometry of the lumbar spine and femoral neck.~Enthesis ultrasound: bilateral evaluation of entheseal insertions for the Achilles tendon, patella tendons and epicondyle tendons.~Biological tests :~* Genetic test : DNA will be collected at the first visit from all centers. RNA will be collected at the inclusion visit in AP-HP centres only.~* Serum will be collected by all centres at the initial visit, then in M6, M12 and M24 for the first two years of the study, then in M60.~* Urine will be collected by all centres at the initial visit, then in M24 for the first two years of the study, then in M60."