The diagnostic value of urinary N-terminal fragment of titin for skeletal muscle damage in idiopathic inflammatory myopathy

Objectives N-terminal fragment of titin (N-titin) is a marker of sarcomere damage in striated muscles; however, its value in patients with IIM (idiopathic inflammatory myopathy) is unclear. This study aimed to investigate the diagnostic value of N-titin for skeletal muscle damage in patients with IIM. Methods Urine samples from 62 patients with IIM, 59 patients with other CTD diseases, and 29 healthy controls were collected to detect N-titin by ELISA assays. Clinical features and laboratory data were all included in logistic regression analysis to obtain the independent predictive factor for skeletal muscle damage. Results Urinary N-titin level of the IIM group [168.3 (19.0, 1279.0) pmol/mg cr] was significantly higher than that in CTD controls [2.80 (1.53, 3.60)] and healthy controls [1.83 (1.09, 2.95)] (P < 0.001). IIM patients with skeletal muscle injury had a significantly higher level of urinary N-titin [1001.0, (181.8, 1977.0)] than those without [9.3, (5.8, 23.9)] (P < 0.001). The N-titin level was strongly correlated with CK (r = 0.907, P < 0.001) and muscle disease activity assessment scores by Spearman correlation analysis. After adjusting for the anti-MDA5 antibody and cardiac troponin T, N-titin was shown to independently predict skeletal muscle damage in patients with IIM (odds ratio = 1.035, 95% CI: 1.002, 1.069, P = 0.039). The cut-off value of urinary N-titin to diagnose skeletal muscle damage was 89.9 pmol/mg Cr, with a sensitivity of 87.8% and a specificity of 100% (AUC = 0.971, 95% CI: 0.938, 1.000, P < 0.001). Conclusion Urinary N-titin is a non-invasive and independent predictive factor for determining skeletal muscle damage in patients with IIM.