Background Increased titres of serum IgM antibodies to GM1 ganglioside are often associated with multifocal motor neuropathy (MMN). Testing for IgM antibodies to other antigens including GM2, the mixture of GM1 and galactocerebroside (GM1/GalC) and the disulfated heparin disaccharide NS6S were reported to increase the sensitivity of antibody testing in MMN even if it is unclear whether the specificity and positive (PPV) or negative predictive value (NPV) for MMN were also affected.

Methods We measured IgM antibodies to GM1, GM2, galactocerebroside, GM1/GalC and NS6S in 40 consecutive patients with MMN and 142 controls with other neuropathies or related diseases and compared their sensitivity, specificity and PPV for MMN.

Results With the only exception of anti-GM2 and, partially, anti-NS6S antibodies, IgM antibodies to the antigens tested were more frequent in MMN than in controls. Increased titres of anti-GM1 IgM were found in 48% of MMN patients with a specificity of 93% and PPV for MMN of 66%. Anti-GM1/GalC antibodies were present in all anti-GM1 positive MMN patients and in 11 additional patients (28%) with MMN raising the sensitivity of antibody testing to 75%. The specificity (85%) and PPV (59%) for MMN was, however, moderately reduced compared to anti-GM1 IgM, even if they rose with increasing anti-GM1/GalC titres. IgM antibodies to GM2, NS6S and galactocerebroside were found in 8%, 23% and 60% of MMN patients but had a low specificity and PPV for MMN.

Conclusions Testing for anti-GM1/GalC IgM significantly increased the sensitivity of antibody testing in MMN compared to anti-GM1 alone (p=0.021) and may represent a preferred option for GM1 reactivity testing in MMN.