Normally we use anti-IgM to detect IgM antibodies. And, although we have
seen positives for IgM antibodies with anti-IgG secondaries,  we have also
had negatives if we use anti-IgG on IgM antibodies. But, in reply to a
question, a company rep said we should be able to use either since IgM is
just a pentamer of IgG (but I seem to remember that the globulin in IgM is
slightly different than true IgG). What is the experience of others in these
types of cases?

Tim Morken
CDC, Atlanta