To validate and verify an SPR-based assay for detecting anti-MuSK Ab in serum in the diagnosis of myasthenia gravis.

Similar to the laboratory diagnosis of AChR MG, MuSK MG is chiefly diagnosed by the detection of MuSK antibodies in patient sera using a radioimmunoprecipitation assay (RIPA). Despite their excellent sensitivities and specificities, RIPAs have a major limitation as a result of the use of radioactive labels. Thus novel label-free assays are attractive alternatives in clinical laboratory diagnoses. SPR is a cutting-edge biosensor platform that detects biomolecular interactions in real time without the need for radio- or enzyme- labels. The resultant sensorgrams provide information on active concentration, affinity and equilibrium constants, stoichiometry and binding specificity. Most importantly, as it does not involve washing steps, Biacore™ is an optimal analytical platform for detecting both high and low affinity antibodies, although the clinical relevance of the low affinity antibodies are not well-understood.

The SPR assay was fully validated with 41 samples to establish performance characteristics such as sensitivity, specificity, accuracy, recovery and interference. We further cross-validated and verified the assay using 80 blinded samples from Oxford University that had been assayed by RIPA. 

Analytical sensitivity, specificity and accuracy were determined to be 100%, 94% and 95%, respectively.  We did not observe any interference in positive serum samples spiked with hemoglobulin and lipid emulsion (1 – 6mg/ml), and a dose-dependent interference with high concentrations of bilirubin (> 5mg/ml). Among the 80 samples used for verification, 20 tested positive and 50 tested negative for both SPR and RIPA. Ten samples tested positive for SPR but were negative for RIPA. Further SPR inhibition and antibody isotyping analyses of these discrepant samples demonstrated that they are true positives.

The SPR assay has superior diagnostic attributes for the detection and characterization of anti-MuSK antibodies in MG.