Concordance of low titer positive myositis-specific autoantibodies and myositis-associated autoantibodies on subsequent myositis panel testing.

Nikola Wilk¹, Jocelyn Hui Ching Chow², Ian Smith³, Ronald A. Booth⁴, Nancy Maltez¹, Catherine Ivory¹ and Jodi Warman-Chardon⁵, ¹Department of Medicine, Division of Rheumatology, The Ottawa Hospital, University of Ottawa, Ottawa, Canada., Ottawa, ON, Canada, ²Department of Medicine, Division of General Internal Medicine, The Ottawa Hospital, University of Ottawa, Ottawa, Canada., Ottawa, ON, Canada, ³Department of Medicine, Division of Neurology, The Ottawa Hospital, University of Ottawa, Ottawa, Canada., Ottawa, ON, Canada, ⁴Department of Pathology and Laboratory Medicine, The Ottawa Hospital, University of Ottawa & Eastern Ontario Regional Laboratory Association, Ottawa, ON, Canada., Ottawa, ON, Canada, ⁵Department of Medicine, Division of Neurology, The Ottawa Hospital, University of Ottawa, Ottawa, Canada. Department of Medicine and Cellular and Molecular Medicine, Children’s Hospital of Eastern Ontario, University of Ottawa, Ottawa, ON, Canada., Ottawa, ON, Canada

Meeting: ACR Convergence 2025

Keywords: Measurement, Myositis

Session Information

Date: Tuesday, October 28, 2025

Title: (2052–2078) Muscle Biology, Myositis & Myopathies – Basic & Clinical Science Poster III

Session Type: Poster Session C

Session Time: 10:30AM-12:30PM

Background/Purpose: Idiopathic inflammatory myopathies (IIMs) are a heterogeneous group of autoimmune connective tissue diseases that commonly affect multiple systems. Myositis-specific autoantibodies (MSA) and myositis-associated autoantibodies (MAA) can be identified in up to 70% of patients with IIM. The clinical significance of multi-specific immunoassays, especially regarding the clinical relevance of low titer positive MSA/MAA, and their consistency in repeat testing, remains to be determined.

Methods: Using an electronic database, we identified all patients who had a myositis panel assessed through Eastern Ontario Regional Laboratory Association between June 2019 and October 2024. This myositis panel includes 18 autoantibodies detected by a semi-quantitative immunoblot assay, with autoantibodies reported in terms of low, moderate, and high titers (EUROIMMUN Medizinische Labordiagnostika AG, Lübeck Germany). We performed a retrospective chart review on the subset of patients with low positive titers of MSA/MAA and multiple myositis panels performed. Concordance was determined by the stability of the antibody result between sequential testing.

Results: 1478 patients had a myositis panel done, and 166 patients were positive with low-titer MSA or MAA antibodies. A total of 25 patients had two or more myositis panels performed within this timeframe. In these patients with repeat testing, 1056 individual MSA/MAA were measured, and the overall concordance was 95.8% (573/598) [Table 1]. Of the 1056 individual MSA/MAA, 102 were in patients with at least one MSA/MAA positive antibody result. Five patients had concomitant moderate/high positive titer MSA/MAA, though these values were excluded from the primary analysis. Concordance limited to positive low-titer findings on at least one time point was much lower at 48% (24/50) [Table 2]. Among low-titer MSA/MAA, the highest concordance was observed for Ro (4/6, 67%), PL12 (2/2, 100%), and Jo-1 (4/4, 100%), whereas concordance was < 50% for PL7 (0/3, 0%), Ku (2/6, 33%), SAE (2/5, 40%), OJ (0/1, 0%), MDA-5 (0/2, 0%) and SRP (0/2, 0%) antibodies [Table 2]. 68% of the repeat patients (17/25) did not develop any IIM and were deemed to have false positive antibodies on their myositis panel [Table 3].

Conclusion: The concordance of a positive low titer positive MSA and MAA on subsequent myositis panel testing was found to be overall low (48%) within our patient population. Only 3 of 20 patients limited to low-positive titers received an IIM diagnosis, highlighting the high frequency of clinically false positives among low positive titers. Further studies are needed to understand the clinical significance of low positive titers and repeat testing using myositis panels

Table 1 | Overall concordance among patients with positive titer MSA/MAA

Table 2 | Concordance of low-titer MSA/MAA

Table 3 | Idiopathic inflammatory myopathy diagnosis

Disclosures: N. Wilk: None; J. Chow: None; I. Smith: None; R. Booth: None; N. Maltez: None; C. Ivory: None; J. Warman-Chardon: None.

To cite this abstract in AMA style:

Wilk N, Chow J, Smith I, Booth R, Maltez N, Ivory C, Warman-Chardon J. Concordance of low titer positive myositis-specific autoantibodies and myositis-associated autoantibodies on subsequent myositis panel testing. [abstract]. Arthritis Rheumatol. 2025; 77 (suppl 9). https://acrabstracts.org/abstract/concordance-of-low-titer-positive-myositis-specific-autoantibodies-and-myositis-associated-autoantibodies-on-subsequent-myositis-panel-testing/. Accessed .

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