Low IgM is associated with treatment failure and low platelet count. (A) Subjects with a low IgM were more likely to fail to respond to standard treatment. (B) Subjects with a low IgM had a trend toward decreased response to splenectomy. (C) There was no difference in frequency of major bleed based on IgM level. (D) Subjects with low IgM had a lower median platelet count than subjects with a normal IgM.