What Does a Positive Herpes IgM Test Result Mean?

What to expect when getting a herpes IgM test

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What is a Herpes IgM Test?

A herpes IgM test is a type of blood test that can be used to detect an early herpes herpes (HSV) infection. This test does not detect herpes directly. Instead, it looks for a type of antibody that can be produced in response to a herpes infection - IgM.

Purpose of a Herpes IgM Test

A herpes IgM test is usually to learn to confirm that a person has herpes after they've had an outbreak. The test is a simple blood test, and results are usually available within a week. Although it takes time for this test to become positive, herpes IgM is one of the first types of antibody to be made after an infection. Therefore it is the earliest blood test that can be used to detect herpes. A direct swab test of a herpes lesion may be able to detect the virus sooner. However, there is only a limited period of time when virus can be directly detected in a sore.

Interpreting Herpes IgM Test Results

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

Does a Positive Test Mean a New Infection?

The common wisdom suggests that a herpes IgM blood test means that someone has been recently infected with herpes. However, this is not the whole story. Herpes IgM levels tend to go up early in an infection, and and levels often go down with time. This means a positive herpes IgM result is most often associated with a new infection. However, that is not the only time when IgM may be detected.

Individuals who were recently infected with herpes do tend to have positive HSV IgM tests. So do many people with recurrent herpes infections. Between 30 and 70 percent of people with recurrent herpes are positive on HSV IgM tests, depending on the test and the study. In other words, an IgM test, without any other data, does not prove that infection a herpes infection is recent. That's particularly true if it is accompanied by a positive herpes IgG test.

People will not become positive on an HSV IgG test until they've been infected for at least several months. However, IgG levels are expected to remain high throughout the course of an infection, while IgM levels are more variable. Therefore, if someone has positive results on both IgG and IgM tests, they may be having a recurrence. It is almost certainly not a new infection. A positive IgM test alone is more likely to be a new infection.

False Positive Herpes Test Results

False positives are not uncommon on herpes blood tests, particularly in areas where relatively few people have herpes. You might not be infected at all, or you might be infected with a different virus that stimulates antibodies that cross-react with the herpes test -- like the Epstein Barr virus or parvovirus. Therefore, if you have no symptoms, you may want to go back for IgG testing at a later date. If you do have symptoms, your doctor can test the lesions for herpes directly. There is no need to wait for an antibody response.

Because of the risk of false positive results, doctors may be reluctant to offer a herpes IgM test to individuals who have not had herpes symptoms. A positive result is more likely to be accurate in individuals who have had symptoms of a herpes outbreak.

A Word from Verywell

If you think you might have been exposed to herpes, you may not want to run to the doctor right away. That's because herpes IgM antibodies can take up to ten days to develop after primary infection with the virus. Therefore, if you believe you have been exposed but have no symptoms, it's best to wait at least two weeks before getting tested. It could be wise to wait even longer, depending on which tests are available in your area. You may also want to go for a repeat test after 6 months if you do not undergo regular screening.That way you're more likely to catch an infection, without worrying about missing the window in which the tests are accurate.