Line blot immunoassays in idiopathic inflammatory myopathies: retrospective review of diagnostic accuracy and factors predicting true positive results

Fergus To; Clara Ventín-Rodríguez; Shuayb Elkhalifa; James B Lilleker; Hector Chinoy

doi:10.1186/s41927-020-00132-9

Line blot immunoassays in idiopathic inflammatory myopathies: retrospective review of diagnostic accuracy and factors predicting true positive results

BMC Rheumatol. 2020 Jul 20:4:28. doi: 10.1186/s41927-020-00132-9. eCollection 2020.

Authors

Fergus To¹, Clara Ventín-Rodríguez², Shuayb Elkhalifa³, James B Lilleker^{4

5}, Hector Chinoy^{6

7

8}

Affiliations

¹ Division of Rheumatology, University of British Columbia Division of Rheumatology, University of British Columbia, 802 - 1200 Burrard Street, Vancouver, BC V6Z 2C7 Canada.
² Complejo Hospitalario Universitario de A Coruña, Xubias de Arriba, 84, 15006 A Coruña, Spain.
³ Department of Immunology, Salford Royal NHS Foundation Trust, 2nd floor, Turnberg Building, Stott Lane, Salford, M68HD UK.
⁴ Centre for Musculoskeletal Research, School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Stopford Building, 99 Oxford Rd, Manchester, M13 9PG UK.
⁵ Manchester Centre for Clinical Neurosciences, Salford Royal NHS Foundation Trust, Stott Lane, Salford, M68HD UK.
⁶ National Institute for Health Research Manchester Biomedical Research Centre, Manchester University NHS Foundation Trust, The University of Manchester, Manchester, UK.
⁷ Department of Rheumatology, Salford Royal NHS Foundation Trust, Manchester Academic Health Science Centre, Stott Lane, Salford, M68HD UK.
⁸ Centre for Musculoskeletal Research, Manchester Academic Health Science Centre, The University of Manchester, Manchester, UK.

Abstract

Background: Line blot immunoassays (LIA) for myositis-specific (MSA) and myositis-associated (MAA) autoantibodies have become commercially available. In the largest study of this kind, we evaluated the clinical performance of a widely used LIA for MSAs and MAAs.

Methods: Adults tested for MSA/MAA by LIA at a tertiary myositis centre (January 2016-July 2018) were identified. According to expert-defined diagnoses, true and false positive rates were calculated for strongly and weakly positive autoantibody results within three cohorts: idiopathic inflammatory myopathy (IIM), connective tissue disease (CTD) without myositis, and non-CTD/IIM. Factors associated with true positivity were determined.

Results: We analysed 342 cases. 67 (19.6%) had IIM, in whom 71 autoantibodies were detected (50 strong positives [70.4%], 21 weak positives [29.6%]). Of the strong positives, 48/50 (96.0%; 19 MSAs, 29 MAAs) were deemed true positives. Of the weak positives, 15/21 (71.4%; 3 MSAs, 12 MAAs) were deemed true positives.In CTD without myositis cases (n = 120), 31/61 (51.0%; 5 MSAs, 26 MAAs) autoantibodies were strongly positive, with 24/31 (77.4%; 0 MSAs, 24 MAAs) true positives. 30/61 (49.2%; 13 MSAs, 17 MAAs) were weakly positive, with 16/30 (53.3%; 0 MSAs, 16 MAAs) true positives. In non-CTD/IIM cases (n = 155), all 24 MSAs and 22 MAAs were false positives; these results included 17 (37.0%; 7 MSAs, 10 MAAs) strong positives.Individual autoantibody specificities were > 98.2 and > 97.5% for weakly and strongly positive results, respectively. True positivity was associated with high pre-test for IIM (odds ratio 50.8, 95% CI 13.7-189.2, p < 0.001) and strong positive (versus weak positive) results (4.4, 2.3-8.3, p < 0.001).

Conclusions: We demonstrated the high specificity of a myositis LIA in a clinical setting. However, a significant burden of false positive results was evident in those with a low pre-test likelihood of IIM and for weakly positive autoantibodies.

Keywords: Idiopathic inflammatory myopathy; Immunoblotting; myositis; autoantibodies; Inflammatory muscle disease; Line blot immunoassay.

Grants and funding

MR/N003322/1/MRC_/Medical Research Council/United Kingdom