Myositis Antibody Panel

The myositis antibody panel detects myositis specific antibodies (MSA) and myositis associated antibodies (MAA) by line immunoassay, dot blot, ELISA or immunoprecipitation.

Idiopathic inflammatory myopathies (dermatomyositis, antisynthetase syndrome, immune mediated necrotising myopathy, polymyositis, inclusion body myositis, overlap myositis) can be classified by the antibody present, and each antibody predicts a distinctive clinical phenotype, organ risk (interstitial lung disease, cancer), response to treatment and prognosis.

ANA by immunofluorescence is negative in many of these, so the panel is needed when the ANA is negative and myositis is suspected.

Idiopathic inflammatory myopathy affects about 5 to 10 in every 100 000 people, with a female predominance, bimodal onset in childhood and the fifth to sixth decade. A myositis specific antibody is found in 60 to 70% of patients.

Components.

• Anti-Jo-1 (histidyl-tRNA synthetase) and other antisynthetase antibodies (PL-7, PL-12, EJ, OJ, KS, Zo, Ha). Antisynthetase syndrome: myositis, interstitial lung disease (in 70 to 90%), arthritis, mechanic's hands, Raynaud phenomenon and fever. Anti-Jo-1 is the commonest (15 to 30% of myositis). Non-Jo-1 antibodies carry more lung and less muscle disease.
• Anti-Mi-2. Classic dermatomyositis with prominent V-sign, shawl sign and cuticular overgrowth, good response to steroids and a low cancer risk.
• Anti-TIF1-gamma (p155/140). Dermatomyositis in adults with a cancer association (20 to 40% of adults), and severe skin disease. In children it is not cancer associated.
• Anti-NXP2 (MJ). Calcinosis and severe skin disease in juveniles, and cancer risk in adults, with dysphagia and oedema.
• Anti-MDA5. Clinically amyopathic dermatomyositis with skin ulcers, palmar papules and rapidly progressive interstitial lung disease (high early mortality, 30 to 50% in East Asian cohorts), with high ferritin.
• Anti-SAE1/2. Dermatomyositis with skin disease first, later muscle involvement and dysphagia.
• Anti-SRP (signal recognition particle). Immune mediated necrotising myopathy with severe weakness, very high creatine kinase and poor response to therapy.
• Anti-HMGCR. Necrotising myopathy associated with statin exposure (two thirds of adults) and persistent after stopping, with very high creatine kinase.
• Anti-cN1A (NT5C1A). Inclusion body myositis (sensitivity 33 to 70%, specificity over 90%) with finger flexor and quadriceps weakness.
• Myositis associated antibodies. Anti-Ro52 (with interstitial lung disease, adds risk of severe lung involvement when combined with anti-Jo-1), anti-PM-Scl (myositis with systemic sclerosis overlap), anti-Ku (myositis with lupus or systemic sclerosis overlap), anti-U1-RNP (mixed connective tissue disease) and anti-U3-RNP.

Indications. Proximal muscle weakness with raised creatine kinase. Dermatomyositis rash (heliotrope, Gottron papules, shawl sign, periungual changes) with or without weakness. Unexplained interstitial lung disease with arthritis, fever or mechanic's hands. Persistent weakness after statin withdrawal. Necrotising myopathy on biopsy. Dysphagia with raised muscle enzymes. Cancer associated myositis screening. Not for isolated myalgia or fatigue.

Specimen. Serum 1 mL, no fasting. Stable at 2 to 8 °C for days and frozen long term. Collected before immunosuppression where possible.

Reference values. Reported as negative, weak positive, moderate positive or strong positive (line immunoassay, band intensity), or in units for ELISA (cut offs set by each manufacturer).

Interpretation.

• A strong positive for a specific antibody with a compatible phenotype is highly specific (over 95%) and defines the syndrome.
• Weak positives are often false positives and do not define a syndrome. Confirmation by a second method or repeat is needed, especially for anti-Mi-2, anti-SAE, anti-cN1A and anti-Ku.
• Negative panel does not exclude myositis (30 to 40% of patients have no known antibody), and biopsy, electromyography and magnetic resonance imaging decide.
• Antibody with lung disease: anti-MDA5 and antisynthetase antibodies need early high resolution CT and lung function, and anti-MDA5 with high ferritin requires aggressive combined immunosuppression.
• Antibody with cancer risk: anti-TIF1-gamma and anti-NXP2 (adult onset, over 40 years) call for cancer screening at diagnosis and for 3 years (CT of chest, abdomen and pelvis, mammography, colonoscopy or cervical screening, and consideration of PET-CT), and anti-SAE1 and anti-HMGCR (statin unexposed) are lesser associations.
• Antibody titres are not used as routine disease monitoring, with the exception of anti-HMGCR in some centres.

Critical values and findings needing immediate action.

• Anti-MDA5 positive with rapidly worsening dyspnoea, hypoxia and ground glass opacities: rapidly progressive interstitial lung disease, intensive care, high dose steroids, calcineurin inhibitor and cyclophosphamide.
• Creatine kinase over 10 times the upper limit with dysphagia, respiratory weakness or myoglobinuria: severe myositis or necrotising myopathy, hospital admission, swallowing assessment and forced vital capacity monitoring.
• Positive anti-TIF1-gamma or anti-NXP2 in an adult: urgent cancer screening.
• Anti-HMGCR positive: stop the statin and start immunosuppression.
• Myocarditis suspected (arrhythmia, troponin rise, heart failure): urgent cardiac assessment.

Interferences and pitfalls.

• ANA by immunofluorescence is negative or cytoplasmic in many antisynthetase and anti-MDA5 cases, so a negative ANA does not exclude myositis.
• Line immunoassay false positives are frequent for weak bands, and correlation with phenotype is required. Panels vary by manufacturer in the antibodies included.
• Antibody tests do not replace muscle biopsy, electromyography, MRI of muscle and creatine kinase.
• Creatine kinase can be normal in dermatomyositis, anti-MDA5 disease and long standing disease, and raised by exercise, trauma, hypothyroidism, statins and viral illness.
• Statin associated autoimmune myopathy is distinguished from statin intolerance by anti-HMGCR, since intolerance resolves on stopping and autoimmune disease persists.
• Treatment with steroids and immunosuppressants may lower titres.
• A positive antibody in a child needs paediatric rheumatology review, and cancer risk is lower.

Special groups.

• Children: juvenile dermatomyositis, with anti-NXP2 (calcinosis), anti-TIF1-gamma (skin disease) and anti-MDA5 (lung and ulcers) subtypes. Cancer screening is not routine.
• Pregnancy: active disease increases foetal loss, and planning for remission is advised.
• Older adults: cancer associated myositis is more frequent, and inclusion body myositis occurs after 50 years.
• Regions: access to the full panel is limited, and endemic infections (viral, parasitic, tuberculous myositis) must be excluded.

Follow up tests. Creatine kinase, aldolase, aspartate and alanine aminotransferase, lactate dehydrogenase, ferritin, troponin, ANA and ENA, electromyography, MRI of the thighs, muscle biopsy, high resolution CT chest, pulmonary function tests with carbon monoxide transfer factor, swallowing study, echocardiography and ECG, thyroid function, tumour imaging and markers, and HIV, hepatitis B and C serology.