What is the negative predictive value of a negative extended myositis panel in dermatomyositis patients?

The short answer is "not great, but it depends" since the more nuanced answer is contingent on:

1. the constellation of myositis-specific antibodies (MSA) included on the panel,
2. the nitty-gritty of how the commercial lab is checking the MSA included (this detail can change between vendors, or even within the same vendor over time)
3. and what your pre-test suspicion for autoimmune myositis is based on the clinical picture (and whether the clinical myositis phenotype matches up with the MSA included in the panel)

For what it's worth, we previously looked at this question by analyzing the performance of all commercial laboratory MSA panels ordered through the University of Pennsylvania healthcare system between 2011-2016 (Gandiga et al., PMID ...