Certain Antibodies Predict Myositis Better Than Others

TOPLINE:

The positive predictive value (PPV) of myositis antibodies for diagnosing myositis and its related conditions differs by individual antibody type. Greater diagnostic accuracy is associated with stronger antibody band intensities and the simultaneous presence of two or more myositis antibodies.

METHODOLOGY:

• Researchers conducted this retrospective study to assess the PPVs of 17 myositis antibodies for diagnosing myositis and other myositis-related conditions (interstitial lung disease [ILD], connective tissue diseases [CTDs], and malignancy).
• They included 1068 individuals aged 18 years or older (61.4% women) who tested positive for at least one antibody in the EUROLINE myositis line blot assay or positive for anti-3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) antibody in an enzyme-linked immunosorbent assay–based test between 2015 and 2020 in Finland.
• Clinical diagnoses of myositis and other myositis-related conditions were extracted from the Care Register for Health Care between January 2013 and June 2022.
• Additionally, the impact of semiquantitative classification and the presence of two or more antibodies on their PPVs was assessed.

TAKEAWAY:

• Anti-HMGCR antibodies demonstrated the highest PPV for the diagnosis of myositis at 94%, followed by anti-melanoma differentiation–associated protein 5, anti-histidyl-transfer RNA synthetase, and anti-transcription intermediary factor 1 (TIF1)-γ (49%-54%); other myositis antibodies demonstrated predictive capacities of 18%-42%.
• Antisynthetase antibodies exhibited PPVs of 25%-47% for the diagnosis of ILD, and the diagnosis of CTDs and malignancies was common in individuals who tested positive for anti-Ro52 and anti-TIF1-γ antibodies, respectively.
• Stronger vs weaker antibody band intensity (at least one definite positive antibody vs only weak positive antibodies) was associated with higher PPVs for the diagnosis of myositis (32.6% vs 14.9%; P P
• The presence of two or more overlapping antibodies vs single antibody positivity was associated with higher PPVs for the diagnosis of myositis (45.7% vs 22.0%; P P P = .0005).

IN PRACTICE:

“Our findings may aid clinicians to use caution in the interpretation of positive myositis antibody findings,” the authors wrote.

SOURCE:

This study was led by Anne M. Kerola, MD, Institute for Molecular Medicine Finland, University of Helsinki, Helsinki, Finland. It was published online on January 20, 2025, in RMD Open.

LIMITATIONS:

The indication for myositis antibody testing was not available, and clinicians’ threshold for ordering these tests may have considerably affected the PPVs. Clinical diagnoses were extracted from healthcare registries without validation against medical records. Additionally, many rare disease phenotypes such as antisynthetase syndrome or immune-mediated necrotizing myopathy lack specific diagnostic codes, and this study lacked data on clinical variables such as muscle enzyme levels and muscle biopsy findings.

DISCLOSURES:

The costs of this study were covered by the Diagnostic Laboratory Division of the South-West Finland’s Well-Being District. Two authors reported receiving speaker fees, participating in advisory boards, and/or receiving congress sponsorship from pharmaceutical companies unrelated to this work.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.