In the majority of patients, serum levels of CK and other muscle enzymes, such as aldolase, are elevated. Serum CK levels tend to be higher in PM than in DM because of the widespread single-fiber necrosis in the former (as described in the following section on pathologic changes). However, in DM, if there are infarcts in muscle, CK levels will be moderately elevated as well. The sedimentation rate is normal or mildly elevated in both diseases.
It has been appreciated that some cases of PM and DM are associated with autoantibodies in the blood. Some of these are undoubtedly nonspecific markers of an autoimmune or inflammatory state (see Brouwer et al), but others may be of pathogenetic significance or are markers for syndromes with multiorgan damage that extends beyond muscle. Tests for circulating rheumatoid factor or antinuclear antibody (ANA) are positive in fewer than half of cases. A high titer of ANA, in conjunction with elevated antiribonuclear antibodies, suggests the coexistence of systemic lupus or mixed connective tissue disease. It must be emphasized, however, that absent or low-titer ANA and a normal sedimentation rate do not exclude the diagnosis of PM, a fact that limits their diagnostic usefulness. Other antibodies can be found on occasion that are directed against constituents of a nucleolar protein complex (PM-Scl) and ribonucleoproteins (Ro/SS-A and La/SS-B).
Of greater interest are the findings that 20 to 30 percent of patients with DM have antibodies against various cellular components of muscle, in particular, antibodies directed against cytoplasmic transfer ribonucleic acid (tRNA) synthetases (anti-Jo1), or against the tRNA itself. These are found when the myositis is coupled with an expanded illness that involves other tissues. The clinical disorders associated with these antibodies usually combine myositis with (1) interstitial lung disease but also (2) arthritis, (3) Raynaud syndrome, and (4) thickening of the skin of the hands (“mechanic’s hands”). Following from the designation of the main type of antibody, these have been termed synthetase syndromes.
A proportion of cases of severe, necrotizing inflammatory myositis show specific antibodies that are directed against a cytoplasmic ribonucleoprotein complex (SRP), or against a protein complex that is a nuclear helicase (Mi-2). These are now classified as separate entities from DM and in some series have carried a heightened risk of cardiac muscle inflammatory involvement. Similarly, in the category of necrotizing inflammatory myositis, a proportion of patients display antibodies to HMGCR, the target of statin drugs, but may also be present without exposure to these drugs. Although these various autoantibodies, with the possible exception of anti-Jo1, have not been especially useful as primary diagnostic tools, they do have a role in refining diagnosis. For example, a positive Jo1 antibody, although too uncommon to use as a screening test, argues against the diagnosis of inclusion body myopathy (which has been associated with a different set of autoantibodies as discussed further on) and its presence raises concern about the later development of interstitial lung disease. The presence of these antibodies also underscores the role for the humoral immune system in the pathogenesis of inflammatory myositis and raises opportunities for investigation discussed as follows.
Myoglobinuria can be detected in the majority of patients with most forms of myositis, particularly a necrotizing form, provided that a sensitive immunoassay procedure is used, but this test is not routinely performed.