Serum Calprotectin - What is the Scope of Clinical Application?

BACKGROUND: Calprotectin (CLP), a heterodimer of S100A8 and S100A9, is a calcium-binding protein with key intracellular and extracellular roles, especially in inflammatory processes. Predominantly expressed by neutrophils and monocytes, CLP is released in response to infection or inflammation and serves as a potent antimicrobial and pro-inflammatory mediator. METHODS: We performed a systematic search of electronic databases to identify studies evaluating serum CLP in inflammatory diseases. RESULTS: Serum CLP levels are elevated in numerous inflammatory conditions, making it a valuable biomarker for disease activity, prognosis, and therapeutic monitoring. In rheumatoid arthritis (RA), CLP reflects disease severity more accurately than conventional markers like CRP and ESR, correlates with radiographic progression, and is strongly expressed at inflammation sites. In juvenile idiopathic arthritis (JIA), serum CLP levels are significantly higher in active, treatment-naïve patients and correlate well with clinical activity. In spondyloarthritis (SpA), especially ankylosing spondylitis, CLP levels tend to be elevated, though results vary among studies. In inflammatory bowel disease (IBD), CLP is proposed as a non-invasive marker for disease burden and response to treatment. It is especially useful in systemic inflammation assessment. Elevated CLP levels are also observed in psoriasis, Behçet's disease, ANCA-associated vasculitis, and preeclampsia. CLP has emerged as a promising prognostic marker in bacterial infection and coronavirus disease 2019 (COVID-19), with higher levels correlating with ICU admission and disease severity. CONCLUSIONS: Serum CLP is a promising inflammatory biomarker, though disease specificity remains limited.