Tu91 - Evaluation of Indirect Immunofluorescent Assays for LGI1 Antibody Detection in Patients with Anti-LGI1 Encephalitis

LGI1 antibodies (LGI1-abs) are probably the most common cause of autoimmune limbic encephalitis and the second most common cause of autoimmune encephalitis. Therefore, detection of these antibodies is important to establish a definitive diagnosis of anti-LGI1 encephalitis. Herein, we assess the clinical performance of two antigen-specific indirect immunofluorescent assays (IIFA) for LGI1-abs in serum and CSF: 1) commercial IIFA with cells transfected with a GPI-LGI1 construct, and 2) in-house IIFA with HEK293 cells co-transfected with LGI1 and ADAM23 constructs. We also evaluated if cerebrospinal fluid (CSF) or serum is the most suitable sample to detect LGI1 antibodies by Indirect Immuno Histochemistry (IIHC) as screening assay.

We retrospectively examined paired serum/CSF of 70 patients known to be LGI1-abs positive by IICH in at least one of the samples. Among the 70 sera, 69 (98.5%) were positive by IIHQ, 66/70 (94.3%) by commercial IIFA, and 58/70 (82.8%) by in-house IIFA. Regarding the CSF samples, 68/70 (97.1%) were positive by IIHC, 58/70 (82.8%) by commercial IIFA, and 70/70 (100%) by in-house IIFA.

The performance of brain tissue IIHC as a diagnostic screening method is independent of the sample source (serum or CSF). A negative test in CSF by commercial IIFA does not exclude the presence of LGI1-abs in patients with suspected anti-LGI1 encephalitis. Based in our experience, LGI1-ADAM23 IIFA using CSF is the most sensitive method to study patients with suspected LGI-1 encephalitis.