Rickettsia IgM antibody testing is a serological test used to detect the presence of IgM antibodies specific to the Rickettsia bacterial antigen. Rickettsia is bacteria causing Rocky Mountain spotted fever, typhus and rickettsialpox. The immune system produces IgM antibodies, the earliest-formed antibodies. Their presence indicates acute infection. The test is helpful in diagnosing recent exposure to rickettsia and active infection and guiding treatment decisions.