Multicenter Validation of the 2025 Classification Criteria for Axial Spondyloarthritis

This is a multicenter, observational, cross-sectional study conducted across 4 hospitals in China, led by the Department of Rheumatology and Immunology at Chinese PLA General Hospital (First Medical Center).

Study Design:

• Prospective enrollment of eligible patients over 12 months
• No intervention; all diagnostic and treatment decisions are made by clinicians based on clinical judgment
• Data collection through standardized electronic case report forms (eCRF) using EpiData platform

Study Population:

• Adults with chronic back pain (CBP) duration ≥3 months
• Back pain onset age ≤45 years
• Patients presenting to rheumatologists without confirmed diagnosis of axSpA, AS, or other rheumatic diseases
• Patients willing to complete full diagnostic assessments and provide informed consent

Assessments:

• Clinical history and physical examination
• Laboratory tests: complete blood count, liver and renal function, HLA-B27, ESR, CRP
• Imaging: pelvic and whole-spine X-ray, sacroiliac joint MRI, sacroiliac joint CT; additional imaging (hip/spine MRI or CT, joint ultrasound, bone scan) as clinically indicated

Diagnostic Evaluation:

• Clinicians will establish a gold-standard diagnosis based on comprehensive clinical, laboratory, and imaging findings
• Both 2025 ASAS-SPARTAN and 2009 ASAS classification criteria will be applied to each patient
• Statistical analysis will evaluate sensitivity, specificity, and concordance between criteria

Sample Size: 247 patients (calculated using PASS 2021 software for one-sample sensitivity and specificity confidence intervals, based on expected sensitivity 75%, specificity 90%, and prevalence 0.7)

Statistical Methods:

• Continuous variables: mean ± standard deviation, independent sample t-test
• Categorical variables: relative frequencies, Chi-square test
• Multivariate logistic regression and Cox regression for prognostic factor analysis
• P < 0.05 considered statistically significant

Data Management:

• Standardized eCRF with defined variable definitions and coding rules
• Password-protected online data entry platform
• De-identification of personal information for publication and data sharing
• Compliance with GCP, Declaration of Helsinki, and Chinese regulatory requirements

Inclusion criteria

• 1. Presence of current chronic back pain with duration ≥3 months and without a definitive diagnosis 2. Age at onset of back pain ≤45 years 3. Presentation to a rheumatologist for evaluation of suspected axial spondyloarthritis or related conditions, without a confirmed diagnosis of axSpA or other rheumatic diseases 4. Willingness to complete comprehensive diagnostic assessments including clinical history, physical examination, CRP testing, HLA-B27 testing, pelvic X-ray, and sacroiliac joint MRI (recent imaging may be acceptable if completed within appropriate timeframe) 5. Voluntary participation with provision of signed electronic informed consent

Exclusion criteria

• 1. Inability to communicate due to cognitive impairment, visual impairment, or other factors affecting self-assessment 2. Confirmed diagnosis of axial spondyloarthritis (axSpA), ankylosing spondylitis (AS), or other established rheumatic diseases (e.g., rheumatoid arthritis, psoriatic arthritis) 3. Back pain duration <3 months or age at onset >45 years 4. Refusal or inability to complete full diagnostic evaluation due to medical contraindications (e.g., metallic implants preventing MRI, severe hepatic or renal dysfunction precluding laboratory testing) 5. Presence of clearly identified non-SpA causes of chronic back pain with established diagnosis, including lumbar disc herniation, spinal stenosis, spinal tumors, infectious spondylitis, or osteoporotic fractures 6. Failure to provide informed consent, inability to cooperate with study follow-up, or inability to provide complete data