Evaluation of a New Skeletal Troponin I Assay in Patients with Idiopathic Inflammatory Myopathies

Background: The current biomarkers for diagnosis and monitoring of injured and diseased skeletal muscles, such as creatine kinase (CK), have limited tissue specificity and incapability to differentiate between pathological and physiological changes. Thus, new biomarkers with improved diagnostic accuracy are needed. Our aim was to develop and validate a novel assay for skeletal troponin I (skTnI), and to assess its clinical performance in patients with idiopathic inflammatory myopathies (IIM). Methods: A two-step fluoroimmunoassay was used to analyze samples from healthy reference individuals (n = 140), patients with trauma (n = 151), and patients with IIM (n = 61). Results: The limit of detection was 1.2 ng/mL, and the upper reference limit (90th percentile) was 5.2 ng/mL. The median skTnI concentrations were <limit of detection (LoD), 2.7 ng/mL, and 8.6 ng/mL in reference, trauma, and IIM cohorts, respectively. Differences in measured skTnI levels were statistically significant between all three study cohorts (Kruskal-Wallis P<0.001; Mann-Whitney P<0.001 for all). skTnI and CK had a strong positive correlation (Spearman's r = 0.771, P<0.001), and the longitudinal changes in skTnI mirrored those observed with CK. Conclusions: With the skTnI assay, patients with IIM were identified from healthy individuals and from patients with traumatic muscular injuries. When compared to CK, skTnI appeared to be more accurate in managing patients with low-grade IIM disease activities. The developed assay serves as a reliable analytical tool for the assessment of diagnostic accuracy of skTnI in the diagnosis and monitoring of myopathies.