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. The IgM test does not detect herpes directly. Instead, it looks for IgM, a type of antibody. This is a protein produced by the immune system in response to a herpes infection.
Herpes IgM levels tend to go up early in an infection, and levels often go down with time. This means a positive herpes IgM result is most often associated with a new infection. However, individuals with recurrent herpes infections may also have positive herpes IgM blood results.
Individuals may not have a positive immunoglobulin G (IgG) test, an antibody test that detects IgG, until they've been infected for at least several months. However, IgG levels tend to remain high throughout the course of an infection, while IgM levels are more variable.
This means that if an individual has positive results on both IgG and IgM tests, they may be having a recurrent herpes episode, instead of a new infection. A positive IgM test alone is more likely to be a new infection.
Why is the IgM HSV Antibodies test no longer recommended?
The Centers for Disease Control and Prevention, the Infectious Diseases Society of America, and the American Society for Microbiology all recommend the use of the IgG antibodies test over the IgM HSV test.
IgG tests are preferred because:
- For herpes, IgG and IgM antibodies show up around the same time — normally, IgM antibodies appear first.
- IgM antibodies may only last a few months and may not be made after the first outbreak — but IgG antibodies last indefinitely and are made during all outbreaks.
- IgG antibodies can distinguish between HSV-1 and HSV-2 and IgM antibodies can’t.
Herpes Simplex Virus 1 and 2 (IgM), Type-Specific Antibodies
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