CRP and rheumatoid arthritis: Our research group has demonstrated that serum calprotectin stratifies disease activity more accurately than C-reactive protein (CRP) or the erythrocyte sedimentation rate (ESR) in patients receiving tumor necrosis factor inhibitors (TNFi) [10] or tocilizumab [11], and strongly correlates with power Doppler ultrasound synovitis (PDUS) in RA and PsA patients with low disease activity [12].