A herpes IgM (immunoglobulin M) test is a blood test that can detect early herpes simplex

virus (HSV) infection. HSV is a contagious condition that causes sores around the mouth or

genitals.A positive HSV IgM blood test indicates that herpes immunoglobulin M is

present. This means the person is carrying the herpes virus. HSV IgM is often present in the

early stages of a herpes infection, but levels often decline in later stages.