Line Blot Immunoassay in Inflammatory Myopathies: Diagnostic Accuracy and Factors Predicting Positive Results in Routine Clinical Practice

Fergus To¹, Clara Ventín Rodríguez ², Shuayb Elkhalifa ³, James Lilleker ⁴ and Hector Chinoy ⁴, ¹University of British Columbia, Vancouver, Canada, ²Complejo Hospitalario Universitario de A Coruña, A Coruña, Spain, ³(3) Department of Immunology, Salford Royal NHS Foundation, Salford, United Kingdom, ⁴University of Manchester, Manchester, United Kingdom

Meeting: 2019 ACR/ARP Annual Meeting

Keywords: Antibodies, inflammatory myositis, Myositis, quality improvement and polymyositis/dermatomyositis (PM/DM)

Session Information

Date: Monday, November 11, 2019

Session Title: Muscle Biology, Myositis & Myopathies Poster II

Session Type: Poster Session (Monday)

Session Time: 9:00AM-11:00AM

Background/Purpose: To evaluate the real-world accuracy of a line blot immunoassay (LIA) for myositis-specific (MSA) and myositis-associated (MSA) autoantibody testing in patients at a tertiary rheumatology and neurology centre.

Methods: Adults tested with the EUROLINE Inflammatory Myopathies 16 Ag LIA at Salford Royal NHS Foundation Trust between January 1, 2016-July 30, 2018 were identified. Cases were reviewed to determine if LIA results were true or false positives. Accuracy was calculated for the autoantibodies (Abs), stratified into strong and weak positivity. Factors associated with true positive results were evaluated.

Results: In total, 342 patients were analysed. 67/342 (19.6%) had a final diagnosis of IIM, 120/342 (35.1%) with connective tissue disease (CTD) without IIM, and 121/342 (35.4%) without IIM or CTD. In IIM patients, 50/71 (70.4%) Abs detected were strongly positive and 48/50 (96.0%; 19 MSAs, 29 MAAs) of these were true positives. 21/71 (29.6%; 7 MSAs, 14 MAAs) were weakly positive. Only 15/21 (71.4%; 3 MSAs, 12 MAAs) of these were true positives. In CTD without IIM patients, 31/61 (51.0%; 5 MSAs, 26 MAAs) Abs detected were strong positives. Only 24/31 (77.4%; 0 MSAs, 24 MAAs) were true positives. 30/61 (49.2%; 13 MSAs, 17 MAAs) were weakly positive and 16/30 (53.3%; 0 MSAs, 16 MAAs) of these were true positives. In patients without CTD or IIM, 46 Abs (24 MSAs, 22 MAAs) were detected. All were false positives, 17/46 (37.0%; 7 MSAs 10 MAAs) of which were strong positives. Individual Abs specificities were 98.2-100% for weakly positive Abs and 97.5-100% for strongly positive Abs. The odds ratio (OR) of a true positive result was significantly associated with a pre-test working diagnosis of IIM (OR 50.8, 95%CI 13.66-189.22, p< 0.001) and strongly positive antibody results (OR 4.38, 95%CI 2.32-8.26, p< 0.001).

Conclusion: We demonstrated a high diagnostic specificity for IIM in a real-world setting using a commercially available LIA. However, a significant burden of false positive results was evident in those with low pre-test likelihood of IIM and for weak positive Ab results.

Disclosure: F. To, None; C. Ventín Rodríguez, None; S. Elkhalifa, None; J. Lilleker, None; H. Chinoy, None.

To cite this abstract in AMA style:

To F, Ventín Rodríguez C, Elkhalifa S, Lilleker J, Chinoy H. Line Blot Immunoassay in Inflammatory Myopathies: Diagnostic Accuracy and Factors Predicting Positive Results in Routine Clinical Practice [abstract]. Arthritis Rheumatol. 2019; 71 (suppl 10). https://acrabstracts.org/abstract/line-blot-immunoassay-in-inflammatory-myopathies-diagnostic-accuracy-and-factors-predicting-positive-results-in-routine-clinical-practice/. Accessed October 23, 2020.

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