Background/Purpose: The identification of myositis specific and associated autoantibodies occurring in idiopathic inflammatory myopathies (IIMs) has improved classification and prognosis determinations.   With commercial availability, these autoantibodies are being ordered outside of the research setting in routine clinical practice. While these autoantibodies are reported to occur in approximately 30% of IIMs, real world positivity rates and ordering patterns have not been described.  We aimed to determine the positivity rate within our tertiary health system, and the proportional ordering of the myositis panel between specialists.

Methods: We included all Duke University Health System patients who had a myositis autoantibody panel (including anti-Jo-1, PL-7, PL-12, EJ, OJ, Mi-2, SRP, PM/Scl, Ku and U2-snRNP) ordered between October 2014 and December 2015.  We recorded specific autoantibody positivity for the institution and by ordering specialty.  Ordering provider was classified as either adult or pediatric and grouped by provider specialty.  Specialty groups included pulmonary, rheumatology, neurology, dermatology, cardiovascular, emergency medicine and general medicine. 

Results: Out of 378 myositis antibody panels ordered, 79 (20.9%) returned positive (Table).  Overall, 59% of positive tests were myositis specific autoantibodies.  Among positive tests, anti-synthetase antibodies were the most common (29.1%) followed by anti-Mi-2 (27.8%).  Of the 5 assayed, only 3 anti-synthetase antibodies were detected in this population, anti-Jo-1, anti-PL-7, and anti-PL-12.  Combined, adult pulmonary, rheumatology and neurology ordered 85.7% of the antibody panels; adult pulmonary ordered the most tests (242; 64%).  Positivity rates were 50% for adult cardiovascular and general pediatrics (4 panels ordered), followed by adult rheumatology (31.7%; 13 tests).  Six patients were positive for two different antibodies; anti-Mi-2 occurred in 4 of the 6.  The panel was performed twice in 4 patients; in 1 patient, an initially negative panel later found anti-U2-snRNP.

Conclusion: This is the first study to determine the positivity rate of the myositis antibody panel in real-world use in a tertiary referral center.  Within our multi-specialty practice, the majority of testing was performed by adult pulmonary, followed by rheumatology and neurology.  Our results may reflect the presence of an active interstitial lung disease clinic.  Of interest, anti-Jo-1 had a nearly equal prevalence to the other anti-synthetase antibodies in this population.  Both anti-Mi-2 and anti-Ku antibodies were seen across the majority of ordering specialties, while the anti-PM/Scl antibody was primarily found by pulmonology.  Overall, our test positivity rates were similar to prior reports in disease-specific populations.  Further characterization of this cohort will help guide future use of this testing.