FABP3, a Candidate Biomarker of Muscle Strength and Myositis Disease Activity in Patients with Idiopathic Inflammatory Myopathies

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Background: Identifying novel, reliable blood-based biomarkers is essential to better assess disease activity in idiopathic inflammatory myopathies (IIMs). We aimed to determine serum fatty acid binding protein 3 (FABP3) levels in IIM patients and evaluated their correlations with disease activity, muscle strength, and conventional biomarkers. Methods: This cross-sectional study enrolled 80 patients with IIM (49% polymyositis, 51% dermatomyositis) and 47 gender-matched healthy controls. Disease activity was evaluated using the IMACS core set measures, and serum FABP3 levels were determined via ELISA. Results: FABP3 levels significantly correlated with creatine kinase (CK; r = 0.42, p < 0.001), high-sensitive C-reactive protein (hsCRP; r = 0.35, p = 0.002), and physician global disease activity (PhG; r = 0.28, p = 0.013), while showing a strong negative correlation with manual muscle testing (MMT; r = −0.39, p = 0.0005). Conversely, CK and hsCRP failed to correlate with disease activity or muscle strength. Statistical analysis confirmed that FABP3 is an independent predictor of muscle strength, and correlates with PhG better than CK in patients with chronic myositis. Conclusions: FABP3 is a promising biomarker for IIM disease activity, which may enable more effective therapeutic decision-making in clinical practice.