The short answer is "not great, but it depends" since the more nuanced answer is contingent on: the constellation of myositis-specific antibodies (MSA) included on the panel, the nitty-gritty of how the commercial lab is checking the MSA included (this detail can change between vendors, or...

The negative predictive value of the panel is not high. About half of my dermatomyositis patients have negative antibodies. Diagnosis is made based on clinical presentation with compatible/classic signs, skin/muscle pathology, MRI, and muscle enzymes in addition to antibodies. Of note, about 80% of ...