A Proposed Serum Calprotectin IgG Cut-Off Level for Diagnosing Inflammatory Arthritis

Background: The identification and validation of soluble markers provide significant opportunities for managing patients with rheumatic diseases, and calprotectin may be an alternative laboratory biomarker of inflammatory rheumatoid arthritis (RA) and psoriatic arthritis (PsA) even though its levels may vary considerably. The aim of this study was to propose a calprotectin cut-off value that would be useful for distinguishing patients with inflammatory arthritis or noninflammatory arthritis (NIA) in clinical practice. Methods: A commercial enzyme-linked immunosorbent assay was used to measure serum calprotectin levels in patients with RA, ankylosing spondylitis (AS), PsA and controls with NIA. All of the patients had been treated with biological disease-modifying anti-rheumatic drugs (DMARDs) for about 12 months after previous failure on conventional DMARDs. Results: Receiver operating characteristic (ROC) analysis showed that serum calprotectin levels significantly differentiated the samples of the patients with inflammatory rheumatic disease from those of the controls. A serum calprotectin level of > 0.9 μg/mL (the optimal predictive cut-off value in the ROC analysis) had a sensitivity of 95.3%, a specificity of 82.2%, a positive likelihood ratio (LR) of 5.35 and a negative LR of 0.057. Conclusions: Our findings suggest that serum calprotectin levels are useful in clinical practice to distinguish patients with inflammatory arthritis and NIA. Further studies of a larger population are suggested.