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Abstract Background Lambert-Eaton myasthenic syndrome (LEMS) is caused by IgG-mediated disruption of certain voltage-gated Ca2+-channel (VGCC) functions and diminished peripheral motor nerve-stimulated release of acetylcholine by presynaptic terminals at the neuromuscular junction and selected autonomic synapses. Both P/Q-type (Cav2.1) and N-type (Cav.2) channels are the targets of IgG-mediated autoimmunity in LEMS. Functional presynaptic VGCCs at the healthy mature mammalian neuromuscular junction are P/Q-type. About 60% of LEMS cases are paraneoplastic, of which a majority are associated with small-cell lung cancer (SCLC-LEMS). LEMS can also have a non-paraneoplastic etiology (idiopathic, non-tumor LEMS [NT-LEMS]). It is important to have reliable autoantibody biomarkers to aid the serological diagnosis of LEMS and to distinguish between SCLC-LEMS and NT-LEMS, because the treatment and prognosis differ. Previous studies have reported seropositivity for SOX1/AGNA-1-IgG in ∼ 64% of SCLC-LEMS cases, but in no NT-LEMS cases. When coexisting with P/Q-type VGCC IgG in LEMS patients, IgGs of both N-type VGCC and SOX1/AGNA-1 IgG specificities have been suggested as useful biomarkers for predicting SCLC. Here we compared the clinical utility of serological tests for P/Q-type and N-type VGCCs and SOX1/AGNA-1 IgGs as diagnostic aids for LEMS and for differentiating paraneoplastic from idiopathic LEMS. Methods This is a retrospective study of 102 patients with a clinical (electrophysiologically confirmed) diagnosis of LEMS who were tested for both P/Q-type and N-type VGCC IgGs by radioimmunoprecipitation assays performed at the Mayo Clinic Neuroimmunology Laboratory. 95 patients had comprehensive clinical screening for cancer; 66 had NT-LEMS, 21 had SCLC-LEMS and 8 had LEMS with non-SCLC cancers. Historical data were reviewed retrospectively. Of the NT-LEMS and SCLC-LEMS patients, 76 were tested for SOX1/AGNA-1 IgG. 122 healthy sex and age-matched controls were tested. Results Among the 102 LEMS patients, 92 (90%) were P/Q-VGCC IgG positive (P/Q+) and 26 (26%) were N-VGCC IgG positive (N+); 67 (66%) were P/Q+ only, 1 (0.9%) was N+ only, 25 (25%) were P/Q+N+, and 9 (9%) were seronegative for both. Three of the 122 healthy control subjects (2.5%) were N+ only, the rest were seronegative. The frequency of isolated N-type VGCC seropositivity did not differ significantly for LEMS patients and healthy controls (P = 0.69). All 21 SCLC-LEMS cases (100%) were P/Q-VGCC IgG positive, compared with 58 of the 66 NT-LEMS cases (88%). The rate of N-type VGCC IgG positivity in P/Q+ LEMS patients did not differ significantly for SCLC-LEMS (5 of 21) and NT-LEMS (13 of 58) diagnoses (24% vs 22%, P = 0.90). The frequency of SOX1/AGNA-1 IgG was 35% for the SCLC-LEMS cases (7 of 20), but none of the NT-LEMS cases (n = 56) was SOX1/AGNA-1 IgG positive. Conclusion N-type VGCC improved neither sensitivity for LEMS nor positive predictive value for cancer and has limited diagnostic utility in assessing patients with suspected LEMS. PQ-type VGCC IgG negativity does not exclude a diagnosis of LEMS. SOX1/AGNA-1 positivity has high specificity but low sensitivity for SCLC. Therefore, while antibody testing confirms a diagnosis of autoimmune LEMS, electrophysiology (to confirm neurological diagnosis) and body imaging (for cancer detection) remain important for the diagnostic evaluation.