Background: Identification of mononuclear cellular infiltrates in skeletal muscle tissue is the histological cornerstone of the diagnosis of idiopathic inflammatory myopathy (IIM). However, these infiltrates are not always present.
Objective: To determine whether MHC class I antigen expression on the sarcolemma, which is absent in normal muscle tissue, is upregulated in IIM and could serve as an additional diagnostic test.
Methods: Expression of MHC class I antigens was studied in 224 muscle samples of 61 adult patients with IIM (9 dermatomyositis, 23 polymyositis, 29 inclusion body myositis) and 163 controls (normal subjects and patients with various neuromuscular disorders) in a prospective blinded manner.
Results: The sensitivity of the test for diagnosing IIM was 78% (95% confidence interval (CI), 66% to 88%), with a specificity of 95% (91% to 98%). The sensitivity before the start of immunosuppressive treatment was 89% (76% to 96%). The sensitivity was not changed by including all patients who had been on immunosuppressive treatment for less than four weeks before muscle biopsy (sensitivity 90% (79% to 97%)). False positive results were found in only seven controls (4%), six of whom had a muscular dystrophy.
Conclusions: Detection of sarcolemmal MHC class I is a valid test for IIM. It is not affected by the short term use of immunosuppressive agents (less than four weeks) and it should be incorporated in the histological evaluation when the diagnosis of IIM is under consideration or needs to be excluded.