To assess the performance characteristics of a radioimmunoassay (RIA) to detect N-type voltage-gated calcium channel (VGCC) autoantibodies.

VGCC antibodies were originally reported in patients with Lambert-Eaton myasthenic syndrome (LEMS) with significant association with malignancy. However, these antibodies are now recognized to be present in diverse neurologic diseases. Antibodies targeting the P/Q-type VGCC are found in most LEMS patients, while the N-type autoantibodies are less frequent but may confer diagnostic and/or prognostic value. There is limited data on the performance characteristics including correlations between methods for the detection of N-type VGCC antibodies.

One hundred and ninety-seven (n=197) sera were evaluated. These included 139 patient sera (42 positive and 97 negative for N-type VGCC antibodies) previously tested at Mayo Clinic Laboratories (MCL) and 58 controls (n=28 disease controls and n=30 “self-reported” healthy individuals). Specimens were tested by RIA using solubilized N-VGCC extracted from rabbit brain and labeled with ¹²⁵I-ω-conotoxin GVIA. Performance of the N-type VGCC antibody RIA was evaluated using in-house established reference ranges (<70 pmol/L=negative, 70-110 pmol/L=indeterminate, >110 pmol/L=positive). Method comparison (accuracy) was determined with the predicate MCL assay based on a tiered positive predictive value (PPV) approach. Assay specificity was assessed using controls.

Qualitative inter-laboratory result comparison based on tiered PPVs of the predicate assay (MCL) demonstrated 100% agreement for results >1.00 nmol/L (71% PPV; n=1), 44% agreement for results of 0.10 – 0.99 nmol/L (24% PPV; n=20) and 14% agreement for results of 0.03-0.10 nmol/L (19% PPV; n=21). Negative results showed 90% agreement (n=97). Clinical specificity was 86% and diagnostic specificity was 100%.

The overall inter-laboratory correlation was comparable; however, challenges were observed for low positive results. Collaborative efforts aimed at assessing the clinical spectrum associated with these antibodies and harmonizing testing are required.