An initial diagnosis of myositis is usually made from conventional blood and electro-diagnostic tests and a clinical exam, but many patients are now also tested for "myositis-specific antibodies" (MSA). It is extremely rare to find these antibodies in patients without myositis, even if they have another muscle or autoimmune disease. These antibodies, and others called “myositis-associated antibodies” (MAA) were identified several years ago, and assist in confirming a diagnosis of polymyositis or dermatomyositis as well as offering some insight into the possible course of the disease.

About 50% of patients with polymyositis or dermatomyositis have myositis-specific antibodies, so when the clinical exam and testing suggest the possibility of these diseases, the presence of these antibodies can be strong supporting evidence for the diagnosis. Myositis-specific antibodies have also improved our understanding of myositis by leading to the identification of certain clinical patterns.

There are literally dozens of myositis-specific and myositis-associated antibodies identified, and researchers are learning more about them. Some of the best-known are anti-aminoacyl-tRNA synthetases, anti-Signal Recognition Particle (SRP), and anti-Mi-2: chromodomain helicase DNA binding protein 4. Several studies have noted that these serological types are identified with differences in presentation and prognosis.

• Patients with anti-aminoacyl-tRNA synthetases may have arthritis in addition to myositis, fevers, interstitial lung disease and the “mechanic’s hands” so named because of the thickening of the palms of the hands.
• Patients with anti-Signal Recognition Particle may have severe muscle weakness that comes on very rapidly, muscle aches, and cardiac involvement.
• Those with anti-Mi-2: chromodomain helicase DNA binding protein 4 may have the classic dermatomyositis skin signs, like the shawl rashes and cuticle overgrowth.

Children are much less likely to have myositis-specific autoantibodies. Only about 10 percent of children with JM show these autoantibodies.

• Children with anti-synthetase autoantibodies, which are myositis-specific and include anti-Jo-1, usually have moderate to severe muscle weakness, high muscle enzyme levels, fevers, arthritis and lung problems.
• Those with anti-Jo-1 tend to follow a chronic continuous disease course, in which children have continual clinical disease activity for a period of more than two years after diagnosis.
• Anti-SRP, also myositis-specific, is seen in children who have polymyositis with severe muscle weakness and very high muscle enzyme levels. Muscle thinning or atrophy is common, and these children often require a combination of medicines as the disease is non-responsive to many medicines on their own. Children with anti-SRP often follow a chronic continuous disease course.
• Children with anti-Mi-2 typically have mild to moderate disease and respond well to treatment, following a monocyclic (disease goes into remission within two years and does not relapse) or polycyclic (disease goes into remission but relapses at least once) course.
• Anti-p155 is the subject of much research as it is the most common autoantibody found in children with juvenile dermatomyositis. Researchers don't yet know the target of this autoantibody.
• Anti-PM/Scl is associated with lung problems and is an overlap of polymyositis and scleroderma (thus the name). Children with PM-scleroderma overlap tend to have a strongly positive antinuclear antibody.
• High levels of anti-RNP point to JM with features of different autoimmune diseases, or mixed connective tissue disease.