What blood tests are used to diagnose myositis (inflammation of the muscles)?

From the Guidelines

Blood tests for myositis should include creatine kinase (CK), transaminases (AST, ALT), lactate dehydrogenase (LDH), aldolase, troponin, and inflammatory markers (ESR and CRP) to evaluate muscle inflammation and myocardial involvement. These tests are essential in diagnosing and monitoring myositis, as recommended by the American Society of Clinical Oncology clinical practice guideline 1. The guideline suggests that blood testing should be part of the diagnostic work-up, including CK, transaminases, LDH, and aldolase, which can be elevated in myositis. Additionally, troponin and other cardiac testing, such as echocardiogram, may be necessary to evaluate myocardial involvement. Inflammatory markers like ESR and CRP should also be checked to assess overall inflammation.

Some key points to consider when ordering blood tests for myositis include:

• CK is a crucial marker for muscle damage and inflammation
• Transaminases (AST, ALT) and LDH can be elevated in myositis
• Aldolase is another enzyme that can be elevated in myositis
• Troponin is necessary to evaluate myocardial involvement
• Inflammatory markers (ESR and CRP) help assess overall inflammation
• Myositis-specific antibodies, such as anti-Jo-1, anti-Mi-2, and anti-SRP, can help identify specific types of inflammatory myopathies and guide treatment, although they are not mentioned in the guideline 1.

It is essential to interpret these test results together with clinical symptoms and muscle strength assessment, as no single test definitively diagnoses myositis. Regular monitoring of these markers helps track disease activity and treatment response. If blood tests suggest myositis, additional testing like electromyography (EMG) or muscle biopsy may be needed for confirmation, as recommended in the guideline 1.

From the Research

Blood Tests for Myositis

• Blood tests can be used to support the diagnosis of myositis, with several studies identifying specific autoantibodies and laboratory test abnormalities associated with the condition 2, 3, 4, 5, 6.
• Myositis-specific autoantibodies, such as anti-TIF1, anti-NXP2, anti-MDA5, anti-SAE, anti-HMGCR, and anti-cN1A, have been identified and are regarded as key biomarkers for diagnosis 2.
• Laboratory test abnormalities common in myositis patients include elevated serum activities of creatine kinase, alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, aldolase, and myoglobin levels 3, 4, 5.
• Other frequent abnormalities include elevated high white blood cell counts, low lymphocyte counts, low hematocrit levels, low albumin levels, high creatine kinase MB isoenzyme fractions, high erythrocyte sedimentation rates, and high IgM and IgG levels 3.
• The presence of specific autoantibodies and laboratory test abnormalities can differ among clinical, gender, racial, and age groups, and can be used to support diagnosis and monitor disease course 2, 3, 6.

Specific Blood Tests

• Serum creatine kinase (CK) is considered a biological marker of myopathies, but selective elevation of serum aldolase with normal CK has been reported in some myopathies, including dermatomyositis and immune-mediated myopathy 5.
• Elevated aldolase levels can be found in a variety of myopathies, and most cases are treatable 5.
• Myositis autoantibodies, such as those directed against eukaryotic initiation factor 3 and heat shock factor 1, have been recently discovered and may provide additional tools for diagnosis and management 6.

Clinical Associations

• The presence of specific autoantibodies and laboratory test abnormalities can be associated with distinct clinical manifestations, such as skin disease, internal organ involvement, and cancer-associated dermatomyositis 2, 3, 6.
• The association of anti-TIF1γ with cancer-associated dermatomyositis dependent on age has been confirmed in several large cohorts 6.
• Further mechanisms, including autoantigen processing and presentation with adaptive T-cell help, may determine the final autoantibody specificity and are likely to include the need for CD4-positive T cells specific for histidyl tRNA synthetase protein 6.