What Does a Positive Herpes IgM Test Result Mean?

doctor taking blood
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Question: Does a Positive Herpes IgM Test Mean I Was Recently Infected with Herpes?

Answer: Not really.

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 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. 

Interestingly, recent 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. All this shows is that a positive herpes IgM test alone does not prove that an infection is recent. That's particularly true if it is accompanied by a positive herpes IgG test.

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 false positive test results. 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 - or 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.

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. 

Sources:

Hashido M, Kawana T. "Herpes simplex virus-specific IgM, IgA and IgG subclass antibody responses in primary and nonprimary genital herpes patients." Microbiol Immunol. 1997;41(5):415-20.

Ho DW, Field PR, Sjögren-Jansson E, Jeansson S, Cunningham AL. "Indirect ELISA for the detection of HSV-2 specific IgG and IgM antibodies with glycoprotein G (gG-2)." J Virol Methods. 1992 Mar;36(3):249-64.

Morrow, R. and Friedrich, D. "Performance of a novel test for IgM and IgG antibodies in subjects with culture-documented genital herpes simplex virus-1 or -2 infection" Clin Microbiol Infect 2006; 12: 463–469

Page J, Taylor J, Tideman RL, Seifert C, Marks C, Cunningham A, Mindel A. "Is HSV serology useful for the management of first episode genital herpes?" Sex Transm Infect. 2003 Aug;79(4):276-9.

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