Background::Follistatin-like 1 (FSTL1) plays both pro-inflammatory and anti-inflammatory roles in the inflammatory processes. We investigated whether serum FSTL1 could predict the current anti-neutrophil cytoplasmic antibody-associated vasculitis (AAV)-specific indices.Methods::We randomly selected 74 patients with AAV from a prospective and observational cohort of Korean patients with AAV. Clinical and laboratory data and AAV-specific indices were recorded. FSTL1 concentration was determined using the stored sera. The lowest tertile of the short-form 36-item health survey (SF-36) was defined as the current low SF-36. The cutoffs of serum FSTL1 for the current low SF-36 physical component summary (PCS) and SF-36 mental component summary (MCS) were extrapolated by the receiver operator characteristic curve.Results::The median age was 62.5 years (55.4% were women). Serum FSTL1 was significantly correlated with SF-36 PCS (n r = -0.374), SF-36 MCS (n r = -0.377), and C-reactive protein (CRP) (n r = 0.307), but not with Birmingham vasculitis activity score (BVAS). In the multivariable linear regression analyses, BVAS, CRP, and serum FSTL1 were independently associated with the current SF-36 PCS (n β = -0.255, n β = -0.430, and n β = -0.266, respectively) and the current SF-36 MCS (n β = -0.234, n β=-0.229, and n β= -0.296, respectively). Patients with serum FSTL1 ≥779.8 pg/mL and those with serum FSTL1 ≥841.6 pg/mL exhibited a significantly higher risk of having the current low SF-36 PCS and SF-36 MCS than those without (relative risk 7.583 and 6.200, respectively).n Conclusion::Serum FSTL1 could predict the current functional status in AAV patients.“,”Background::Follistatin-like 1 (FSTL1) plays both pro-inflammatory and anti-inflammatory roles in the inflammatory processes. We investigated whether serum FSTL1 could predict the current anti-neutrophil cytoplasmic antibody-associated vasculitis (AAV)-specific indices.Methods::We randomly selected 74 patients with AAV from a prospective and observational cohort of Korean patients with AAV. Clinical and laboratory data and AAV-specific indices were recorded. FSTL1 concentration was determined using the stored sera. The lowest tertile of the short-form 36-item health survey (SF-36) was defined as the current low SF-36. The cutoffs of serum FSTL1 for the current low SF-36 physical component summary (PCS) and SF-36 mental component summary (MCS) were extrapolated by the receiver operator characteristic curve.Results::The median age was 62.5 years (55.4% were women). Serum FSTL1 was significantly correlated with SF-36 PCS (n r = -0.374), SF-36 MCS (n r = -0.377), and C-reactive protein (CRP) (n r = 0.307), but not with Birmingham vasculitis activity score (BVAS). In the multivariable linear regression analyses, BVAS, CRP, and serum FSTL1 were independently associated with the current SF-36 PCS (n β = -0.255, n β = -0.430, and n β = -0.266, respectively) and the current SF-36 MCS (n β = -0.234, n β=-0.229, and n β= -0.296, respectively). Patients with serum FSTL1 ≥779.8 pg/mL and those with serum FSTL1 ≥841.6 pg/mL exhibited a significantly higher risk of having the current low SF-36 PCS and SF-36 MCS than those without (relative risk 7.583 and 6.200, respectively).n Conclusion::Serum FSTL1 could predict the current functional status in AAV patients.