Diagnostic value of synovial calprotectin (S100A8/A9) in differential diagnosis of juvenile idiopathic arthritis and pigmented villonodular synovitis in children: preliminary results of a single-center study

BACKGROUND: Juvenile idiopathic arthritis is the most common chronic inflammatory musculoskeletal disease in children. Its most prevalent clinical subtype is oligoarthritis. Pigmented villonodular synovitis, also known as tenosynovial giant cell tumor, is a rare benign synovial disorder, which may clinically resemble oligoarthritis. Differential diagnosis between juvenile idiopathic arthritis and pigmented villonodular synovitis is challenging. Intra-articular steroids used in juvenile idiopathic arthritis therapy may induce negative effects in patients with pigmented villonodular synovitis. Orthopedic and surgical procedures used to rule out pigmented villonodular synovitis are burdensome for children. Magnetic resonance imaging may yield similar findings for both conditions at early stages. Several studies revealed that serum calprotectin is a promising biomarker for juvenile idiopathic arthritis. AIM: To assess synovial fluid calprotectin concentrations in children with oligoarthritis and pigmented villonodular synovitis. METHODS: The synovial fluid concentrations of calprotectin, interleukin-6, and tumor necrosis factor-alpha in 42 children with oligoarthritis and 12 children with diffuse pigmented villonodular synovitis of the knee joint were obtained by enzyme-linked immunosorbent assay. In patients with juvenile idiopathic arthritis, cytokine levels were determined at disease onset, and prior to therapeutic and diagnostic arthroscopy in those with pigmented villonodular synovitis. RESULTS: Synovial calprotectin significantly increased in children with oligoarthritis (108 [28.2; 237] μg/mL) compared to those with pigmented villonodular synovitis (1.53 [1.26; 1.69] μg/mL; p 0.001). No statistically significant differences were found in synovial tumor necrosis factor-alpha and interleukin-6 concentrations between patients with juvenile idiopathic arthritis and those with pigmented villonodular synovitis. ROC analysis showed a synovial calprotectin threshold of 2.9 μg/mL for the diagnosis of juvenile idiopathic arthritis (AUC = 0.996 ± 0.00479; 95% CI: 0.926–1.000). CONCLUSION: In children, the differential diagnosis of oligoarthritis is often complicated by clinically similar nonrheumatic joint disorders. The main synovial proinflammatory markers cannot be used for the differential diagnosis of juvenile idiopathic arthritis and pigmented villonodular synovitis. Synovial calprotectin concentration is a promising biomarker of juvenile idiopathic arthritis.