MSTN and rheumatoid arthritis: Additionally, serum myostatin levels were directly correlated with RA duration (13 [0–45] years) (r = 0.24, P = 0.02), CRP (r = 0.48, P < 0.001), ESR (r = 0.28, P = 0.009), and DAS28-ESR (r = 0.22, P = 0.04) and were negatively correlated with SMI (r =  − 0.29, P = 0.008).