Background and Objective Syndesmophytes formation and complete fusion of the total spine are common characteristics leading to functional impairment and disability in ankylosing spondylitis (AS) patients. Predictors for progression of structural damage are smoking, elevated levels of acute phase reactants and the presence of syndesmophytes at baseline. These predictors identify increased risk for progression at group level but their specificity is not strong enough to be used as biomarkers in individual patients. We recently demonstrated that calprotectin, a heterodimer of S100A8 and S100A9, expressed and secreted during monocyte infiltration into inflamed tissues, is a good biomarker of treatment responses in AS. Here, we aimed to determine if calprotectin levels are predictive for radiographic spinal progression in AS patients.
Materials and Methods 76 AS patients were selected from German Spondyloarthritis Inception Cohort (GESPIC). Spinal radiographs were scored by two readers using the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS) system. Subsequently, anteroposterior views of the lumbar spine were scored for presence of syndesmophytes. Radiographic spinal progression was defined as 1) mSASSS worsening by ≥2 units after 2 years, and 2) development of a new syndesmophyte or progression of existing syndesmophytes after 2 years. Serum calprotectin levels were determined by ELISA.
Results High calprotectin levels were associated with mSASSS worsening over two years in AS, with an Area Under the Curve (AUC) of 0.740 (95% CI 0.614–0.866; P = 0.004). The odds ratio (OR) for radiographic spinal progression (mSASSS worsening by ≥2 units) in patients with calprotectin serum level >0.5 µg/ml was 6.2 (95% CI 1.6–24.2, P = 0.009). The association between calprotectin levels and mSASSS progression remained significant after adjusting for other independent risk factors (syndesmophytes at baseline, elevated C-reactive protein (CRP), and smoking): OR = 5.5 (95% CI 1.2–25.8; P = 0.030). Analysis of syndesmophyte formation and/or progression as outcome for structural damage yielded similar results: calprotectin levels were significantly associated with progression of syndesmophytes. The AUC was 0.670 (95% CI 0.520–0.819; P = 0.031). The predictive value of calprotectin was independent but similar to that of CRP.
Conclusions Calprotectin is an independent predictor for radiographic spinal progression in AS.