Tests
1st tests to order
creatine kinase
Test
Serum creatine kinase is the most sensitive diagnostic and monitoring marker, although its levels can be normal in some idiopathic inflammatory myopathy patients.[1][4] Creatine kinase can rise up to 50 times the upper normal limit in polymyositis, dermatomyositis, overlap myositis, and antisynthetase syndrome. In patients with immune-mediated necrotizing myopathy creatine kinase levels can be markedly elevated at up to 100 times the upper normal limit, but creatine kinase levels tend to be normal or mildly raised at no more than 10 times the upper normal limit in patients with inclusion body myositis. It is important to note that serum creatine kinase can also be raised in patients with rhabdomyolysis, infections, toxin-related myopathy, metabolic diseases, neurogenic myopathy, and congenital myopathy which therefore have to be carefully excluded
Result
elevated creatine kinase
electromyograph
Test
May show myopathic motor units on voluntary activity.
These abnormalities are not specific for idiopathic inflammatory myopathy; they may be seen in a variety of myopathies and only in the acute phase of the disease.[77]
Result
short duration, low amplitude, polyphasic units with early recruitment on voluntary activity; diffuse spontaneous activity with fibrillation and positive sharp waves at rest
muscle biopsy
Test
Muscle biopsy can be used to confirm muscle inflammation, identify the subtype of idiopathic inflammatory myopathy, and exclude other potential causes of myopathy.[72][77]
Result
polymyositis: endomysial inflammatory infiltrates, muscle necrosis, atrophy, muscle fiber regeneration; dermatomyositis: perifascicular atrophy, perivascular/perimysial inflammation; inclusion body myositis: endomysial inflammatory infiltrate, fiber size variability, fiber necrosis, rimmed vacuoles
myositis-specific and associated autoantibodies
Test
Myositis-specific and/or myositis-associated autoantibodies (MSAs/MAAs) are found in up to 70% of patients with idiopathic inflammatory myopathy.[4][6]
Immunoprecipitation is the gold-standard for MSA/MAA testing, but it is too time consuming to be considered suitable for routine testing, other available tests (e.g., enzyme-linked immunosorbent assay, addressable laser bead immunoassay, and immunoblotting techniques) do not detect the full repertoire of MSAs, show variable results in terms of performance, and lack a standardized approach.[4][6]
The majority of MSAs will yield a positive antinuclear antibody but the staining pattern on indirect immunofluorescence is not distinctive enough to confirm the antibody specificity. Certain MSAs (especially those associated with antisynthetase syndrome) can yield a negative or only weakly positive ANA test.[6]
Result
type-specific antibodies present
magnetic resonance imaging
Test
Performed to assess inflammation in muscles.
May be helpful in selecting a site for muscle biopsy.[72][81]
Result
increased signal or edema that can be seen in inflammatory processes; the changes are not specific; presence of significant fatty infiltration of selected muscles may indicate hereditary myopathy; muscle inflammation is indicated when high signal uptake is observed on T2 fat suppression and short-tau inversion recovery (STIR) images representing muscle edema, or T1 images show fatty infiltration as high signal uptake, indicative of chronic injury.
aldolase
lactate dehydrogenase
alanine aminotransferase
Test
May be elevated but is less specific for muscle injury than creatine kinase.
Result
elevated
aspartate aminotransferase
Test
May be elevated but is less specific for muscle injury than creatine kinase.
Result
elevated
myoglobin
Test
May be elevated but is less specific for muscle injury.
A sensitive index of the integrity of muscle fibers.
Useful in the assessment of disease activity and serves as a guide during treatment.[87]
Result
elevated
Tests to consider
erythrocyte sedimentation rate
Test
May be elevated with concurrent arthritis, serositis, infection. Less commonly elevated in patients with only myositis.
Result
may be elevated
C-reactive protein
Test
May be elevated with concurrent arthritis, serositis, infection. Less commonly elevated in patients with only myositis.
Result
may be elevated
antinuclear antibodies
Test
May be positive in patients with idiopathic inflammatory myopathy. Look out especially for a cytoplasmic speckle or nucleolar pattern.
Result
may be positive
serum creatinine
Test
May be reduced with loss of muscle bulk.
Result
may be reduced
high resolution computed tomography of the lungs
pulmonary function test
Test
If the extramuscular manifestation of interstitial lung disease is suspected, a pulmonary function test may help to establish how much damage to the lungs has occurred.[82]
Result
may show decreased lung volume, capacity, rates of flow, and gas exchange
diffusion capacity
Test
If the extramuscular manifestation interstitial lung disease is suspected, a diffusion capacity test may help to establish how much damage to the lungs has occurred.[82]
Result
may show decreased diffusion capacity; normal >75% to 140%; mild: 60% to LLN (lower limit of normal); moderate: 40% to 60%; severe: <40%
serum ferritin
Test
May be raised over and above that seen with an acute-phase response in IIM-associated interstitial lung disease.
Result
may be elevated in those with interstitial lung disease
troponins
Test
Troponin level may help to confirm cardiac involvement in patients with idiopathic inflammatory myopathy.
Result
may be elevated.
electrocardiography
Test
Electrocardiography may help to confirm cardiac involvement in patients with idiopathic inflammatory myopathy.
Result
may show arrhythmia
echocardiography
Test
Echocardiography may help to confirm cardiac involvement in patients with idiopathic inflammatory myopathy.
Result
may show reduced left ventricle ejection fraction
cardiac magnetic resonance imaging
Test
Cardiac MRI may help to confirm cardiac involvement in patients with idiopathic inflammatory myopathy.
Result
normal: >50% thickening (end diastolic thickness is >50% of systolic), normal movement; hypokinetic: <50% thickening, decreased movement; akinetic: no thickening, decreased or no movement; dyskinetic: no thickening, paradoxical movement
fluorodeoxyglucose positron emission tomography (FDG-PET)/computed tomography
Test
Useful for malignancy screening and can also highlight myositis disease activity.[88]
Result
highlights myositis disease activity and may show underlying malignancy
muscle ultrasound scan
Test
Muscle ultrasound is an easily available imaging technique to assess patients with idiopathic inflammatory myopathy (IIM). It has high sensitivity for the detection of biopsy proven IIM, with high positive and negative predicative values when performed by an experienced practitioner.[2]
Result
acute IIM - mild "see through" echogenicity with normal or increased muscle bulk, likely indicative of edema; chronic IIM - the muscle appears more atrophied (increased echogenicity with reduced bulk); dermatomyositis - fascial thickness, indicative of fasciitis, is increased; inclusion body myositis - higher echogenicity is seen in the flexor digitorum profundus compared to flexor carpi ulnaris, known as the "FDP-FCU echogenicity contrast"
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