BackgroundNo clear-cut guidelines exist about the use of imaging procedures for the diagnosis of idiopathic inflammatory myopathies (IIM). Similarly, conflicting, and scanty data exist about Power Doppler Ultrasonography (PDUS) in this subset of patients. In this regard, we recently proposed (1) a 0-3 grey scale (GS) and Power Doppler (PD) score in a cohort of patients affected by IIM, evidencing a positive, statistically significant, correlation for PD and oedema and disease activity.ObjectivesThe aim of this study was to assess the diagnostic accuracy of our score in IIM patients compared to a control group.MethodsWe prospectively collected, since July 2020 to December 2021, all patients evaluated in Vasculitis and Myositis clinic, Rheumatology Unit, University of Siena, with a recent diagnosis of IIM, as well as patients with a previous, definite diagnosis of IIM and evaluated during follow-up or referred from other centres for a second opinion. As control group, we collected all patients affected by amyopathic dermatomyositis (DM) or who underwent myositis immunoblot or muscle biopsy for proximal limbs weakness but eventually received a diagnosis other than IIM. All patients underwent US examination of both thighs in axial and longitudinal scans.ResultsForty-five IIM patients (11 anti-synthetase syndrome, 20 DM, 12 PM, 2 scleromyositis) and twenty-six controls were included. During the observational period, 7, 8, 1 and 1 patients underwent PDUS twice, three, four and five times, respectivelyAssessing area under the receiver operating characteristic (AUROC) curve analysis, IIM patients and control group were distinguished according to PD sum, Oedema sum, atrophy sum and CRP values (Figure 1a). The best cut-off value for PD sum values was 0.5 (70.2% SE and 83.3% SP), for Oedema sum 1.5 (74.5% SE and 79.2% SP), atrophy sum 0.5 (63.4% SE and 65.4% SP) and CRP was 0.22 mg/dL (61.5% SE and 75% SP).Figure 1.Stratifying IIM population into two groups according to disease activity (PhGA≥2), AUROC curve analysis allow to distinguish these groups according to PD and oedema sum and CRP values (Figure 1b) and the best cut-off values was 1.5 (69.6% SE and 76.9% SP), 2.5 (52.2% SE and 92.3% SP) and 0.55 mg/dL (66.7% SE and 88.9% SP), respectively.Testing the IIM group versus control as dependent variable by logistic regression, with PD sum, oedema sum, atrophy sum, CRP, CPK and myoglobin as independent variables, the AUROC was 0.976. From the probability associated with the Chi-square tests, the Type II analysis showed the variable that most influences the IIM diagnosis was PD sum and oedema sum (p=0.017 and p=0.013, respectively) (Figure 1c).ConclusionGS and PDUS have proven an overall good diagnostic accuracy in distinguishing between IIM and myositis mimicker. In particular, even low values of PD (sum 1.5) display a good sensitivity and specificity and, together with oedema, elevated CRP values and myositis-specific and associated antibodies, may be considered a reliable tool for a definite diagnosis of IIM.References[1]Conticini E, Falsetti P, Al Khayyat SG et al. A novel grey scale and Power Doppler ultrasonographic score for idiopathic inflammatory myopathies: Siena Myositis Ultrasound Grading Scale. Rheumatology (Oxford). 2021 Dec 24;61(1):185-194.Disclosure of InterestsNone declared