Herpes Simplex Virus 1 (HSV-1) IgM is a type of blood test used to detect the presence of IgM antibodies in the blood that are specific to HSV-1. HSV-1 is a virus that commonly causes oral herpes, characterized by cold sores or fever blisters around the mouth and on the face. IgM antibodies are one of the body's immune responses to a recent infection, so the presence of HSV-1 IgM antibodies in a blood sample may indicate a recent or active HSV-1 infection.