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Objectives: To develop the CT Syndesmophyte Score (CTSS) for low-dose CT (ldCT) to assess structural damage in the spine of patients with ankylosing spondylitis (AS) and test its reliability., Methods: Patientswith AS in the SIAS cohort had whole spine ldCT at baseline and 2 years. Syndesmophytes were scored in coronal and sagittal planes in eight quadrants per vertebral unit (VU) as absent=0, <50% of the intervertebral disc space (IDS)=1, ≥50%=2 or bridging the IDS=3 (range 0-552). Images were scored by two readers, paired by patient, blinded to time order. Whole spine and spinal segment status and change scores were calculated. Inter-reader reliability was assessed by intraclass correlation coefficient (ICC), smallest detectable change (SDC) and frequency of scores per VU., Results: 49 patients (mean age 50 years (SD 9.8), 84% men, 88% human leucocyte antigen B27 positive) were included. Mean (SD) scores of reader 1 were: whole spine baseline status score 163 (126) and change score 16 (21), spinal segment baseline status scores 30 (41), 97 (77) and 36 (36) and change scores 2 (7), 12 (18) and 3 (4) for the cervical, thoracic and lumbar spine, respectively. Scores of reader 2 were similar. Whole spine status score ICC was 0.99 and 0.97-0.98 for spinal segments. Whole spine change score ICC was 0.77 and 0.32-0.75 for spinal segments. Whole-spine SDC was 14.4. Score distribution pattern per VU was similar between readers., Conclusions: Using the CTSS, new bone formation in the spine of patients with AS can be assessed reliably. Most progression was seen in the thoracic spine., Competing Interests: Competing interests: None declared., (© Article author(s) (or their employer(s) unless otherwise stated in the text of the article) 2018. All rights reserved. No commercial use is permitted unless otherwise expressly granted.)