Enzyme immunoassay for measuring antibodies against skeletal muscle in patients with myasthenia gravis.

We developed a highly sensitive enzyme immunoassay (EIA) for measuring IgG and IgM antibodies against human skeletal muscle (SM) component and tested sera from 100 patients with myasthenia gravis (MG), 59 with thymoma and 41 without thymoma. We found that the frequency of anti-SM IgG antibodies was significantly higher in MG patients with (81%) than without (37%) thymoma. The titers of the anti-SM IgG antibodies measured by EIA correlated well with those measured by RIA (r = 0.81, P less than 0.01). We also found that 12% of the myasthenic patients with thymoma and 15% without it had anti-SM IgM antibodies. There was no correlation between the titers of the IgG and IgM antibodies. Our EIA provides a measure of anti-SM antibodies that is of comparable sensitivity to that of RIA.