Inclusion body myositis (IBM) is the most common subtype of autoimmune myopathy in patients older than the age of 50 years. Several diagnostic criteria have been proposed for IBM based on expert opinion and consensus groups. Their use in clinical practice is however limited due to low sensitivity. The European Neuromuscular Centre (ENMC) 2011 clinically defined diagnostic criteria have a high specificity of greater than 99% to diagnose IBM, but like other criteria, its sensitivity is low at 57%. ENMC 2011 Inclusion Body Myositis Diagnostic Criteria Mandatory Features: 1. Age of onset later than 45 years. 2. Duration of symptoms more than 12 months. 3. Serum creatine kinase level, not more than 15 times the upper limit of normal. Clinical Features: 1. A weakness of quadriceps more than hip flexors. 2. A weakness of finger flexors more than shoulder abductors. Pathological Features: 1. Endomysial inflammatory infiltrate. 2. Rimmed vacuoles. 3. Protein accumulation or 15- to 18-nm filaments. 4. Upregulation of MHC class I. Classification Criteria: Clinicopathologically defined IBM: Mandatory criteria + one or both of the clinical criteria plus 1, 2, and 3 of the pathological criteria Clinically defined IBM: Mandatory criteria plus all clinical criteria plus one or more, but not all the pathological criteria Probable IBM: Mandatory criteria plus one clinical criterion plus one or more, but not all the pathological criteria.