FSTL1 and rheumatoid arthritis: The serum FSTL1 levels in RF-negative RA patients (median, 27.1 μg/l, 25th to 75th percentile, 11.8 to 70.0 μg/l) and ACPA-negative RA patients (median, 25.8 μg/l, 25th to 75th percentile, 13.3 to 71.8 μg/l) were higher than those in HC.