Idiopathic inflammatory myopathies are a heterogeneous group of conditions characterised by skeletal muscle inflammation leading to proximal muscle weakness, often with tenderness, and occasionally dermatological manifestations. Diagnosis is clinical and is based on the Bohan and Peter criteria which comprises clinical examination, serological markers, electromyography (EMG) and muscle biopsy. This has relatively poor sensitivity and specificity. Muscle biopsy in particular has a false negative rate of 10-15% and is invasive. Clinical criteria also lack the discriminatory power to differentiate between recurrent (or breakthrough) myopathy and myopathy secondary to treatment with corticosteroid.