Background/Purpose: EULAR/ACR recently approved classification criteria for idiopathic inflammatory myopathies (IIM) with 93% sensitivity and 88% specificity. An acknowledged limitation of the study is the absence of controls/comparators in the external validation cohort, prompting the authors to call for further validation studies in different populations.

We sought to 1) evaluate the performance of the criteria in an Australian cohort of adult patients with suspected IIM; 2) determine the level of agreement between the EULAR/ACR criteria and traditional criteria (Bohan and Peter, and Targoff); 3) determine the optimal cut-point for Australian patients; 4) assess the effect of including MRI of the musculature or an extended panel of antibodies, as additional ‘risk factors’; 5) assess the effect of extending “endomysial infiltration of mononuclear cells surrounding but not invading myofibres” to include invasion of myofibres.

Methods: Data were collected retrospectively on all consecutive patients referred for muscle biopsy to two large tertiary teaching hospitals. Area under the receiver operator curve (AUC) was calculated to assess the overall performance of the EULAR/ACR criteria. The Youden method was used to determine the optimal cut point. Patients were scored for ‘risk of IMM’ according to the EULAR/ACR criteria and dichotomized into probable/definite and negative/possible groups. The predictive accuracy of the EULAR/ACR criteria was assessed by evaluating the sensitivity and specificity.

Results: Eighty seven of 204 patients had IIM. Overall, the EULAR/ACR criteria had an outstanding level of discrimination (AUC=0.90). Application of the criteria in this cohort however, showed lower sensitivity (71% vs. 93%) but comparable specificity (89% vs. 88%). The optimal cut point of 5.23 (sensitivity 91%, specificity 75%) for this sample was lower than the EULAR/ACR cut point of 6.7. The EULAR/ACR criteria had a moderate agreement with Bohan and Peter (kappa=0.45, 95%CI=0.28, 0.62, p<0.001) and Targoff (kappa=0.40, 95%CI=0.23, 0.57, p<0.001). Inclusion of MRI of the musculature (AUC=0.86, 95%CI=0.79, 0.93), or non-Jo1 myositis specific antibodies (AUC=0.84, 95%CI=0.77, 0.91) improved the ability of the model to identify IIM compared with the EULAR/ACR criteria alone (AUC=0.80, 95%CI=0.75, 0.86). Furthermore, extending the criteria so that endomysial infiltration of mononuclear cells included invasion of myofibres, resulted in a model with a comparable level of accuracy to the original criteria (both AUCs=0.90). However, the sensitivity was slightly improved (75% vs. 71%) while the specificity remained the same (89% vs. 89%).

Conclusion: The EULAR/ACR criteria had an outstanding discrimination between IIM and non-IIM. Application of the criteria to an Australian cohort has shown comparable specificity but lower sensitivity, and lower optimal cut-point than reported. There is moderate agreement with traditional criteria. Addition of MRI of the musculature, or non-Jo1 myositis specific antibodies, improved the ability of the model to identify IIM. Extending the criteria so that endomysial infiltration of mononuclear cells included invasion of myofibres may improve sensitivity.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/performance-of-the-2017-european-league-against-rheumatism-american-college-of-rheumatology-eular-acr-classification-criteria-for-adult-idiopathic-inflammatory-myopathies-iim-in-an-australian-co/