What Does a Positive Herpes IgM Test Result Mean?

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.

A sample tube filled with blood being held by a gloved hand.
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This article explains the purpose of the herpes IgM test. It also discusses what the results mean, as well as retesting considerations.

What Is the Purpose of a Herpes IgM Test?

The herpes IgM test is a simple blood test and results are usually available within a week.

Keep in mind that:

  • This test may be done to confirm that a person has herpes after they've had a recent outbreak.
  • Herpes IgM is one of the first types of antibody that appears after infection, so this blood test is the earliest one that can be used to detect herpes.
  • The herpes IgM antibodies can take up to 10 days to develop after a primary infection with the virus.

If you believe you have been exposed but have no symptoms, you should wait at least two weeks before getting tested. You may want to wait even longer depending on which tests are available in your area.

The Centers for Disease Control and Prevention (CDC) does not recommend HSV IgM testing because it may give a positive result during a recurrent oral or genital herpes episode. Instead, they recommend specific testing to determine which HSV you may have.

How Are the Results Interpreted?

Results of a herpes IgM test are usually reported as positive, negative, or equivocal.

Herpes IgM test results:

  • A positive test result means that herpes IgM was detected in the sample.
  • A negative result means that no IgM was detected.
  • An equivocal test result means that the test was unable to determine whether or not there was an antibody present.

Is It a New or Recurrent 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.

Is It Possible to Get a False Positive Result?

A false positive is when a test says you have a condition that you actually don't have. This can happen with the herpes IgM blood test.

In some cases you may not be infected at all. However, you may have a different virus that triggers a positive on this blood test. This may include the Epstein-Barr virus, which is in the herpes virus family, or parvovirus, a contagious virus that can lead to flu-like symptoms.

If you don't have symptoms, you may want to go back for IgG testing at a later date. If you do have symptoms, your healthcare provider can test the sores for herpes directly with a swab test.

A positive result is more likely to be accurate if you have or have had symptoms of a herpes outbreak.

Should I Get Tested Again?

You may want to go for a repeat test after six months if you do not undergo regular sexually transmitted infection (STI) screening. In doing so, you're more likely to identify an infection without missing the window in which the tests are accurate.

Speak with your healthcare provider about when it's best for you to get tested, and if there are other tests you should consider.

Summary

A herpes IgM test can detect early HSV infection by looking for IgM antibodies. Your body produces IgM and IgG antibodies after HSV infection. The IgM test can detect HSV earlier than the IgG test.

A positive IgM test might mean you have a new or recurrent HSV infection. If you have a positive IgM test and a negative IgG test, you likely have a new infection.

To verify your test result, consider following up with your healthcare provider in six months for repeat testing.

Frequently Asked Questions

  • What is HSV IgG and HSV IgM?

    HSV IgG and HSV IgM are antibodies produced to fight the herpes simplex virus. Blood tests can look for these antibodies to diagnose herpes.

  • What does HSV IgM positive mean?

    A positive HSV IgM blood test indicates that herpes immunoglobulin M is present. This means the person is carrying the herpes virus. A false positive is possible.

  • What does HSV IgG positive mean?

    A positive HSV IgG blood test indicates that herpes immunoglobulin G is present. This means the person is carrying the herpes virus. An HSV IgG test will not be positive in the early stages of an infection. 

  • Does a positive HSV IgM test mean a herpes infection is new?

    It may. However, if HSV IgM and HSV IgG are both positive, it indicates that the herpes virus has been present for at least several months.

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  1. Centers for Disease Control and Prevention. Genital herpes.

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