What Does a Positive Herpes IgG Test Mean?

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Herpes IgG tests are one type of blood test for the herpes simplex virus (HSV). Herpes blood tests look for the body's immune reaction to a herpes infection. They don't search directly for the virus. Because the immune reaction takes time to develop after the time of infection, it's not immediately detectable. In fact, depending on the type of test used, it can take up to four months to become positive on an HSV IgG test.

Tip: If you have herpes symptoms, go to the doctor right away. Sores can be tested directly. You don't have to wait for the immune system to respond. This type of testing can be more accurate if the sore is tested quickly enough. 

How the Test Works

When a person becomes infected, the immune system tries to fight off the infection. That's true not just for herpes but for any pathogen, Part of that process involves the production of antibodies. These proteins are specific for each infection they are fighting.

For a new type of infection, it takes time for the body to make strong antibodies. The body can make several kinds of antibodies to fight infections. The two types that herpes blood tests look for are IgG and IgM. Herpes IgM antibodies usually are detectable by herpes blood tests within 7-10 days after initial infection. IgM levels stay high for approximately two weeks. After that, they usually decline.

Therefore IgM testing is primarily considered to be useful for detecting acute infection. However antibody levels also sometimes go up during an outbreak.

In contrast, HSV IgG antibodies do not show up until slightly later after initial infection. A positive herpes IgG test, if the test result is accurate, means that your body has been infected with the herpes simplex virus.

Furthermore, type specific HSV IgG tests can be used to distinguish between HSV-1 and HSV-2. Type-specific tests are far more accurate than non-type-specific​ tests, however, they can not detect whether a particular infection is oral or genital. The only way to determine that is to watch for symptoms. 

HSV-1 usually infects the mouth, causing oral herpes, and HSV-2 usually infects the genitals. However, there are a growing number of people with genital HSV-1.  That's why herpes IgG and IgM tests can only tell you that you have been infected. They can't show where.

Test Results & Timing of Infection

If you test positive for HSV IgG but not IgM, then your herpes infection is probably not recent. You've probably been infected for at least two months. Individuals with newer infections are more likely to test positive for both herpes IgG and IgM. They might also be positive for herpes IgM alone. The converse, however, isn't true. Positive herpes IgG and IgM results together do not necessarily mean you were infected recently. Between 30 and 70 percent of patients with recurrent herpes infections will test positive for herpes IgM.

Test Results and Timing of Infection
  Positive IgGNegative IgG
Positive IgM Infection date indeterminate Acute/Recent infection
Negative IgM Established Infection No infection detected

Test Accuracy

It is possible to have a false positive or false negative result. That's true for either a HSV IgG or HSV IgM test. Therefore, don't panic if your herpes blood test results do not agree with your known risk factors and sexual history. Instead, talk to your doctor about possible issues with testing. Diagnostic testing isn't perfect. You also may not be accurately assessing your risk. Many people do not understand that herpes can be transmitted even when their partner has no symptoms or does not know they are infected.


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