To assess serologic and clinical characteristics of seronegative myasthenia gravis (SNMG) patients in a multicenter Italian cohort.

MG patients who tested negative to radioimmunoassay or enzyme-linked immunosorbent assay for autoantibodies to acetylcholine receptor (AChR) or muscle-specific kinase protein (MuSK) were included; neurophysiologic findings of impaired neuromuscular transmission or a positive response to acetylcholinesterase inhibitors were required to support clinical diagnosis.

Sera were tested by L-CBA for autoantibodies to clustered adult-(A)-AChR, fetal-(F)-AChR, MuSK and lipoprotein-receptor-related protein 4 (LRP4).

A total of 156 patients were recruited from seven Italian Referral Centers: most were females (n=89) and had an early-onset phenotype (n=93). L-CBA was positive in 25/156 patients: 21 AChR, of whom 12 showed reactivity for either A-(5) or F-AChR (7) isoforms, and 4 MuSK. There were no LRP4 positive sera.

Our study confirms that L-CBA offers a diagnostic gain compared to standard assays and supports its use in the second-line screening of SNMG patients. Nonetheless, we raise the question of the real utility of LRP4 testing.

The high proportion of triple-SNMG patients to L-CBA and the lack of an alternative biomarker underlines the importance of well-defined clinical/electromyographic diagnostic criteria. Further research is warranted to better define the clinical characteristics of SNMG and help designing specific diagnostic and management guidelines.