Probability-based classification criteria for idiopathic inflammatory myopathies (IIM). Distinguishes IIM from non-IIM mimics and subcategorizes into dermatomyositis, polymyositis, IBM, and juvenile DM/PM using clinical, laboratory, histological, and imaging features.
Original Development
Lundberg IE, Tjarnlund A, Bottai M, et al.
Arthritis Rheumatol / Ann Rheum Dis, 2017
1
Muscle Biopsy
Is a muscle biopsy available for scoring?
The 2017 criteria use two separate probability tables -- one with biopsy findings (higher specificity) and one without. Select whether biopsy results are available. Both versions are clinically validated.
2
Mandatory Exclusion Check
These features exclude IIM classification if present. Verify none apply before proceeding.
✓
IBM (Inclusion Body Myositis) definitively diagnosed
IBM defined by its own validated criteria -- not classified as IIM by these 2017 criteria
✓
Other diagnosis explains the muscle weakness
Metabolic myopathy, endocrine myopathy, drug-induced myopathy, muscular dystrophy, or other confirmed alternative diagnosis
Exclusion criterion present. IIM classification criteria cannot be applied. This patient does not meet entry requirements for these criteria.
3
Age of Disease Onset
Age at first myositis symptom onset
4
Clinical Features
Constitutional / Objective Weakness
✓
0
Objective symmetric weakness of proximal limb-girdle muscles
Upper and lower extremity proximal muscle weakness present on examination
✓
0
Neck flexor weakness
Weaker neck flexors relative to neck extensors
✓
0
Leg (distal) weakness
Distal lower extremity weakness (specifically in the IBM subcategorization pathway)
Skin Manifestations
✓
0
Heliotrope rash
Violet/purple periorbital rash (pathognomonic for DM)
✓
0
Gottron papules
Erythematous to violaceous papules over MCP/PIP/DIP joints (pathognomonic for DM)
✓
0
Gottron sign
Erythema over dorsal hands, elbows, knees, medial malleoli
Other Clinical
✓
0
Dysphagia or esophageal dysmotility
Difficulty swallowing or regurgitation of food
Laboratory Values
✓
0
Elevated serum levels of muscle enzymes (CK, LDH, ALT, AST)
At least one of CK, LDH, ALT, or AST elevated above the upper limit of normal at any time during the illness
✓
0
MSA (myositis-specific antibodies) positive
Anti-Jo-1, anti-MDA5, anti-Mi-2, anti-TIF1g, anti-NXP2, anti-SAE, anti-SRP, anti-HMGCR, or other validated MSA
5
Imaging
✓
0
MRI: muscle edema consistent with myositis
STIR hyperintensity in proximal muscles on MRI consistent with active myositis
6
Muscle Biopsy Findings
Score the most significant biopsy finding present. Each item can be scored individually if multiple findings are present.
✓
0
Endomysial infiltration of mononuclear cells surrounding myofibers
T-cell infiltration pattern most characteristic of polymyositis
✓
0
Perifascicular atrophy
Characteristic of dermatomyositis -- small fibers at periphery of fascicles
✓
0
Rimmed vacuoles
Rimmed vacuoles suggesting IBM pathology (though IBM excluded by entry criteria)
✓
0
Perimysial and/or perivascular infiltration of mononuclear cells
Perimysial pattern, including dermatomyositis pattern of perivascular infiltrate
✓
0
Necrosis / necrotic fibers
Muscle fiber necrosis with or without regeneration; characteristic of necrotizing autoimmune myopathy
📊 Classification Thresholds
Probability
Classification
Clinical Meaning
< 50%
Non-IIM / Unclassified
Does not meet IIM classification. Consider alternative diagnoses.
50-74%
Possible IIM
Possible IIM. Consider further workup including biopsy, MRI, and MSA panel.
75-89%
Probable IIM
Probable IIM. Reasonable confidence for research classification. Treat with clinical judgment.
≥ 90%
Definite IIM
Definite IIM. High confidence for classification. Standard research entry criterion.
Sensitivity 87.0%, specificity 82.7% at the probable threshold (without biopsy). With biopsy: sensitivity 90.3%, specificity 90.1%.
💡 Pearls and Pitfalls
✓
MSA positivity dramatically increases classification probability. The presence of a myositis-specific antibody (particularly anti-Jo-1, anti-MDA5, anti-Mi-2, anti-TIF1g, or anti-NXP2) adds substantial probability weight and often drives the score into the probable or definite range even with limited clinical features. Always obtain a full MSA panel early.
✓
Dermatomyositis features (heliotrope rash, Gottron papules, Gottron sign) are the most heavily weighted individual clinical items. Their presence -- especially when combined with elevated enzymes -- rapidly elevates probability into the definite range. These are pathognomonic features.
⚠
IBM must be excluded before applying these criteria. Inclusion body myositis is not classified by the 2017 EULAR/ACR IIM criteria. IBM has its own validated criteria (ENMC 2013 / Lloyd et al.) and is characterized by distal weakness, finger flexor weakness, quadriceps weakness, rimmed vacuoles, and COX-deficient fibers on biopsy.
⚠
These are classification criteria, not diagnostic criteria. The 2017 criteria were designed for research classification, not clinical diagnosis. A patient with a clinical syndrome consistent with myositis who does not meet the probability threshold may still benefit from empiric treatment. Conversely, meeting criteria does not replace comprehensive clinical evaluation including cancer screening.
🔬 Evidence
The 2017 EULAR/ACR criteria were developed by an international collaboration led by Lundberg, Tjarnlund, and Bottai, using data from 976 patients with IIM and 624 non-IIM comparators across 47 centres in 21 countries. The weighted probability approach replaced the older Bohan and Peter criteria (1975), which had no formal validation. The probability model allows continuous rather than categorical classification and explicitly accommodates different diagnostic certainty levels.
View References▾
1
Lundberg IE, Tjarnlund A, Bottai M, et al.2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies and their major subgroups. Arthritis Rheumatol. 2017;69(12):2271-2282.
2
Aggarwal R, Rider LG, Ruperto N, et al.2016 American College of Rheumatology/European League Against Rheumatism criteria for minimal, moderate, and major clinical response in adult dermatomyositis and polymyositis. Arthritis Rheumatol. 2017;69(5):898-910.
For clinical decision support only. These are classification criteria designed for research use. Clinical diagnosis requires comprehensive evaluation. IBM must be excluded before applying. A positive classification does not substitute for cancer screening, which is recommended in all newly diagnosed IIM, particularly DM.