Spondyloarthritis Inception Cohort of Southern Denmark (SPINCODE): a protocol for a single centre prospective observational cohort study in patients suspected of axial spondyloarthritis.

introductionSpondyloarthritis (SpA) comprises a heterogeneous group of chronic inflammatory diseases. Axial SpA (axSpA) predominantly involves the sacroiliac joints and the spine and typically presents with back pain and stiffness. Modern imaging techniques, especially MRI, allow earlier detection of axSpA compared to conventional radiography, but imaging findings still need cautious interpretation to facilitate a clinical diagnosis.This cohort study of patients with suspected SpA is designed to (1) compare demographic, clinical, laboratory, imaging data and patient-reported outcomes (PROs) of patients diagnosed with or without axSpA (2) to identify prognostic factors for the diagnosis of axSpA and high disease activity (3) to compare performance of different imaging modalities for establishing a diagnosis of axSpA. METHODS AND ANALYSIS: The study is a 2-year prospective observational cohort study of consecutive patients with low back pain ≥3 months and ≤3 years and symptom onset at age 18-45 years, suspected of axSpA.Clinical visits are conducted at baseline, week 6, 12 (telephone consultation), 24, 48, 72 and 96. Demographics, clinical data, laboratory findings and PROs are collected. Radiography (only at baseline), MRI, low-dose computed tomography (ldCT) and dual-energy CT (DECT) are performed at baseline and prior to the last study visit. Clinical, laboratory and imaging data are prior to visits at week 6 and week 96 assessed at a multidisciplinary team (MDT) conference involving a musculoskeletal radiologist and a rheumatologist, where a diagnosis of axSpA or not will be determined.Descriptive statistics are used to summarise clinical, PROs and imaging data. Comparisons between axSpA patients and non-axSpA patients are made cross-sectionally at baseline and week 96. We want to investigate and compare the diagnostic performance, the ability to detect structural and inflammatory lesions and the incremental diagnostic value of ldCT and DECT compared to MRI and radiography. ETHICS AND DISSEMINATION: The ethical committee of the Region of Southern Denmark (S-20230055) has approved the study. The study is registered at clinicaltrials.gov (NCT06337513). Results will be presented at both national and international conferences, as well as published in peer-reviewed journals.