Elevated Serum Follistatin-Like Protein 1 Levels Correlate with Disease Activity and Severity in Behçet's Disease: A Case-Control Study.

Objective: Behçet's disease (BD) is chronic multisystem inflammatory condition with wide spectrum of clinical manifestations, lacking specific laboratory biomarkers for diagnosis or monitoring. Follistatin-like protein 1 (FSTL1), having dual roles in inflammation, remains largely unexplored in BD. The present study aimed to evaluate FSTL1 levels in the serum of BD patients and investigate their association with disease activity and severity. Methods: This prospective, hospital-based case-control study included 40 BD patients, the control group consisted of 40 healthy individuals age and sex matched with the patients. FSTL1 concentrations were measured using a double-antibody sandwich ELISA. The disease activity and cumulative disease related damage were assessed using the Behçet's Disease Current Activity Form (BDCAF), and the Behçet's Syndrome Overall Damage Index (BODI). Results: The median serum FSTL1 concentrations were significantly elevated in BD patients in comparison to controls :4.05 (1.9 - 9.1), 3.55 (2.2 - 4.8) ng/ml; p = 0.024, r (rank-biserial correlation) = 0.036 represent very small effect size. FSTL1 levels also showed moderate positive correlations with both BDCAF (r = 0.397) and BODI (r = 0.391) scores, suggesting that higher FSTL1 levels reflect increased inflammatory activity and tissue involvement. Conclusion: Our study demonstrates an elevation in serum FSTL1 levels in BD and positively correlate with disease BD activity and cumulative damage. FSTL1 may reflect inflammatory burden in BD, but further validation is needed to establish causality.