Low Prevalence and Inconsistency of LRP4-IgG Detection in Suspected Myasthenia Gravis

BACKGROUND AND OBJECTIVES: Myasthenia gravis (MG) is an autoimmune disorder primarily caused by antibodies targeting neuromuscular junction proteins, particularly the acetylcholine receptor and the muscle-specific tyrosine kinase. However, 10%-20% of patients with MG are double-seronegative (dsMG). Antibodies against low-density lipoprotein receptor-related protein 4 (LRP4-IgG) have been found in a variable proportion of dsMG cases, but their frequency and clinical relevance remain unclear because of differences in assay methodologies and study populations. In this study, we assessed the frequency of LRP4-IgG in patients with suspected MG using different cell-based assay (CBA) protocols. METHODS: In this multicenter observational study, we enrolled consecutive patients presenting with symptoms suggestive of MG. LRP4-IgG was tested by 2 centers using 3 different CBAs: live (l-CBA), methanol-fixed (mf-CBA), and paraformaldehyde-fixed (pf-CBA). Patients were classified as having MG or other disorders (ODs). Positive samples were cross-tested between centers. RESULTS: Among 684 patients (302 with MG, 382 with ODs), LRP4-IgG was detected by mf-CBA in 2% (6/302) of MG cases and in 0.52% (2/382) of OD cases. Among patients with MG, 3.9% (4/102) of dsMG and 1% (2/200) of seropositive patients tested positive. Only 50% of mf-CBA-positive cases were confirmed by pf-CBA, and none were detected by l-CBA. Cross-center testing showed partial reproducibility. DISCUSSION: LRP4-IgG detection in suspected MG is rare and inconsistent across assays, suggesting that routine testing is not currently warranted and that these antibodies are at present of limited diagnostic value. These findings highlight the need for standardized, validated LRP4-IgG assays. Future studies should focus on direct comparison and harmonization of testing protocols to clarify the clinical utility of LRP4-IgG testing.