What Does a Positive Herpes IgM Test Result Mean?

doctor taking blood
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Many doctors will tell patients with a positive herpes IgM test that their test result means they were recently infected with herpes (HSV). This is because anti-HSV IgM antibodies are thought to peak shortly after an initial infection and then recede. In fact, that is the common wisdom for IgM antibodies in general. Compared to IgM, herpes IgG antibodies develop more slowly. However, they are expected to remain high throughout the course of an infection.

Therefore, a positive HSV IgG test is thought to mean that the detected infection is at least several months old. 

Does a Positive HSV IgM Test Mean You Were Recently Infected?

Interestingly, research suggests that the common wisdom about herpes blood test results may not be accurate. Individuals who were recently infected with herpes DO tend to have positive HSV IgM tests. However, 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.

Herpes IgM tests are more likely to be positive in early herpes infections than herpes IgG tests. This is because IgM develops more quickly than IgG. However, a positive HSV 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.

It's also possible that you could have a positive HSV IgM result if you're having a recurrence. 

How to Interpret Your Results

If you have a positive herpes IgM test and a negative herpes IgG test, then it is more likely that your results signal a recent infection. However, it is possible to misinterpret test results, as there is the possibility that a result could be a  false positive.

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.

Detectable levels of herpes IgG take longer to develop than detectable levels of herpes IgM. However, even 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, don't run to the doctor. 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. 

Tip: If you have symptoms that look like a new herpes infection, head to the doctor. They can test the sores for virus right away. You don't need to wait for the immune response to catch up. 


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