Objective To develop and validate new classification criteria for adult and juvenile idiopathic inflammatory myopathies (IIM) and their major subgroups. Methods Candidate variables were assembled from published criteria and expert opinion using consensus methodology. Data were collected from 47 rheumatology, dermatology, neurology and paediatric clinics worldwide. Several statistical methods were used to derive the classification criteria. Results Based on data from 976 IIM patients (74% adults; 26% children) and 624 non-IIM patients with mimicking conditions (82% adults; 18% children), new criteria were derived. Each item is assigned a weighted score. The total score corresponds to a probability of having IIM. Subclassification is performed using a classification tree. A probability cut-off of 55%, corresponding to a score of 5.5 (6.7 with muscle biopsy) ‘probable IIM’, had best sensitivity/specificity (87%/82% without biopsies, 93%/88% with biopsies) and is recommended as a minimum to classify a patient as having IIM. A probability of ≥90%, corresponding to a score of ≥7.5 (≥8.7 with muscle biopsy), corresponds to ‘definite IIM’. A probability of <50%, corresponding to a score of <5.3 (<6.5 with muscle biopsy), rules out IIM, leaving a probability of ≥50 to <55% as ‘possible IIM’. Conclusions The European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) classification criteria for IIM have been endorsed by international rheumatology, dermatology, neurology and paediatric groups. They employ easily accessible and operationally defined elements, and have been partially validated. They allow classification of ‘definite’, ‘probable’ and ‘possible’ IIM, in addition to the major subgroups of IIM, including juvenile IIM. They generally perform better than existing criteria.

Objective: To develop and validate new classification criteria for adult and juvenile idiopathic inflammatory myopathies (IIM) and their major subgroups.Methods: Candidate variables were assembled from published criteria and expert opinion using consensus methodology. Data were collected from 47 rheumatology, dermatology, neurology, and pediatric clinics worldwide. Several statistical methods were utilized to derive the classification criteria.Results: Based on data from 976 IIM patients (74% adults; 26% children) and 624 non‐IIM patients with mimicking conditions (82% adults; 18% children), new criteria were derived. Each item is assigned a weighted score. The total score corresponds to a probability of having IIM. Subclassification is performed using a classification tree. A probability cutoff of 55%, corresponding to a score of 5.5 (6.7 with muscle biopsy) “probable IIM,” had best sensitivity/specificity (87%/82% without biopsies, 93%/88% with biopsies) and is recommended as a minimum to classify a patient as having IIM. A probability of ≥90%, corresponding to a score of ≥7.5 (≥8.7 with muscle biopsy), corresponds to “definite IIM.” A probability of <50%, corresponding to a score of <5.3 (<6.5 with muscle biopsy), rules out IIM, leaving a probability of ≥50–<55% as “possible IIM.”Conclusion: The European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) classification criteria for IIM have been endorsed by international rheumatology, dermatology, neurology, and pediatric groups. They employ easily accessible and operationally defined elements, and have been partially validated. They allow classification of “definite,” “probable,” and “possible” IIM, in addition to the major subgroups of IIM, including juvenile IIM. They generally perform better than existing criteria.

Objective: Anti‐p155 autoantibody, which was recently described in adult patients with dermatomyositis (DM), seems to be associated with cancer in this population. We performed a systematic review and meta‐analysis to ascertain the accuracy of anti‐p155 testing for the diagnosis of cancer‐associated myositis.Methods: We searched relevant databases, with no restrictions on study design or language, for original studies that included adult patients with probable/definite DM or amyopathic DM who were evaluated for neoplasm and anti‐p155 status. Pooled sensitivity and specificity were calculated using a bivariate model. We computed the diagnostic odds ratio (OR), likelihood ratios (LRs) for positive and negative test results, positive and negative predictive values, and the summary receiver operating characteristic (SROC) curve. Statistical heterogeneity between studies was assessed using the I² statistic, and 95% confidence intervals (95% CIs) were computed for the parameters studied.Results: Six studies including a total of 312 adult patients with DM were selected. The pooled sensitivity of anti‐p155 for diagnosing cancer‐associated DM was 78% (95% CI 45–94%), and specificity was 89% (95% CI 82–93%). The diagnostic OR was 27.26 (95% CI 6.59–112.82), and LRs for positive and negative test results were 6.79 (95% CI 4.11–11.23) and 0.25 (95% CI 0.08–0.76), respectively. Heterogeneity was substantial except with regard to the LR for a positive test result. The area under the SROC curve was 0.91 (95% CI 0.88–0.93). Taking the pooled prevalence of 17% as pretest probability, anti‐p155 had a positive predictive value of 58% and a negative predictive value of 95%.Conclusion: Our findings indicate that anti‐p155 autoantibody determination is useful for diagnosing cancer‐associated myositis and guiding disease management.