What are the cutoff criteria for the European League Against Rheumatism (EULAR) American College of Rheumatology (ACR) classification of immune inflammatory myopathy?

EULAR/ACR Classification Criteria Cutoffs for Immune Inflammatory Myopathy

The 2017 EULAR/ACR classification criteria use a probability-based scoring system with three cutoff thresholds: ≥55% probability (score ≥5.5 without biopsy or ≥6.7 with biopsy) for "probable IIM," ≥50% to <55% probability (score 5.3-5.4 without biopsy or 6.5-6.6 with biopsy) for "possible IIM," and ≥90% probability (score ≥7.5 without biopsy or ≥8.7 with biopsy) for "definite IIM." 1

Classification Thresholds

Probable IIM (Recommended Minimum Cutoff)

• Probability: ≥55% 1
• Score without muscle biopsy: ≥5.5 (range 5.5-5.7 for optimal sensitivity/specificity balance) 1
• Score with muscle biopsy: ≥6.7 (range 6.7-7.6 for optimal sensitivity/specificity balance) 1
• This represents the recommended minimum threshold for classifying a patient as having IIM in research studies 1

Possible IIM

• Probability: ≥50% to <55% 1
• Score without muscle biopsy: 5.3-5.4 1
• Score with muscle biopsy: 6.5-6.6 1
• This category represents patients with borderline probability who may warrant further evaluation 1

Definite IIM (High Specificity Cutoff)

• Probability: ≥90% 1
• Score without muscle biopsy: ≥7.5 1
• Score with muscle biopsy: ≥8.7 1
• This threshold is recommended when high specificity is required, such as in clinical trials with strict inclusion criteria 1

Scoring System Components

The criteria use weighted point values for clinical features, with different scores depending on whether muscle biopsy data is available 1:

Age of Onset

• Age 18-39 years: 1.3 points (without biopsy) or 1.5 points (with biopsy) 1
• Age ≥40 years: 2.1 points (without biopsy) or 2.2 points (with biopsy) 1

Muscle Weakness Patterns

• Proximal upper extremity weakness: 0.7 points (both scenarios) 1
• Proximal lower extremity weakness: 0.8 points (without biopsy) or 0.5 points (with biopsy) 1
• Neck flexors weaker than extensors: 1.9 points (without biopsy) or 1.6 points (with biopsy) 1
• Proximal leg muscles weaker than distal: 0.9 points (without biopsy) or 1.2 points (with biopsy) 1

Skin Manifestations (Highest Weighted Features)

• Heliotrope rash: 3.1 points (without biopsy) or 3.2 points (with biopsy) 1
• Gottron's papules: 2.1 points (without biopsy) or 2.7 points (with biopsy) 1
• Gottron's sign: 3.3 points (without biopsy) or 3.7 points (with biopsy) 1

Laboratory Findings

• Anti-Jo-1 antibody positive: 3.9 points (without biopsy) or 3.8 points (with biopsy) 1
• Elevated CK, LDH, AST, or ALT: 1.3 points (without biopsy) or 1.4 points (with biopsy) 1

Other Clinical Features

• Dysphagia or esophageal dysmotility: 0.7 points (without biopsy) or 0.6 points (with biopsy) 1

Critical Application Points

When Muscle Biopsy is Not Required

Patients with pathognomonic skin rashes (heliotrope rash, Gottron's papules, or Gottron's sign) can be accurately classified without muscle biopsy data, as these features carry high point values (3.1-3.7 points) 1

Reporting Requirements

Both the descriptive term ("possible," "probable," or "definite"), the probability percentage, and the aggregated score should be reported in research studies for transparency 1

Validation Performance

External validation studies demonstrate 85% sensitivity and 92% specificity at the ≥55% probability cutoff in Mexican populations 2, though sensitivity may be lower (around 80%) in certain subgroups like immune-mediated necrotizing myopathy 3, 4

Important Caveats

Subgroup Classification

After meeting the ≥55% probability threshold for IIM, patients can be further subclassified using a classification tree into dermatomyositis, polymyositis (including IMNM), inclusion body myositis, juvenile dermatomyositis, or amyopathic dermatomyositis 1, 5

Limitations with Specific Antibodies

The original criteria only included anti-Jo-1 antibody due to insufficient data on other myositis-specific antibodies at the time of development 1. Adding extended myositis antibody panels (anti-Mi-2, anti-SRP, anti-PL-7, anti-PL-12, anti-MDA5, anti-TIF1-γ, anti-HMGCR) can reclassify patients from "no IIM" to "probable" or "definite" categories, improving diagnostic accuracy 2

Classification vs. Diagnosis

These are classification criteria designed for research and clinical trial enrollment, not diagnostic criteria for individual patient care 1. Some IMNM cases with mild phenotypes may not meet classification thresholds despite having positive myositis-specific antibodies, making antibody testing crucial for accurate diagnosis 3