What Does a Positive Herpes IgG Test Mean?

herpes testing

Illustration by Cindy Chung, Verywell

Herpes IgG tests are one type of blood test for the herpes simplex virus (HSV). Herpes blood tests, including the herpes IgG test, 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.

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. Testing a herpes sore can be more accurate than testing your blood, but only if the sore is tested quickly enough. 

Purpose of Test

A herpes IgG test is a blood test to see if you've been infected by herpes. Your doctor will take a blood sample and then send it to a lab for analysis. This test is generally used if you have had herpes symptoms, or think you may have been exposed to herpes. Many doctors prefer to test sores directly for the herpes virus. However, a herpes blood test can be used in someone who doesn't have sores or whose sores are too old for testing.

The biggest limitation of a herpes IgG test is that this test may not become positive until several months after someone has become infected. This is because of 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.

Interpreting Results

Results for a herpes IgG test will usually be back within a week. In general, the results will be reported as positive, negative, or equivocal. A positive test means that IgG was detected. An equivocal test means that the results were unclear.

It is possible to have a false positive or false negative result. That's true for either ​an 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.

It can be particularly difficult to understand herpes blood test results if they're different on multiple tests. However, herpes IgG and IgM test for different phases of infection.

  Positive IgG Negative IgG
Positive IgM Infection date indeterminate Acute/Recent infection
Negative IgM Established Infection No infection detected

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.

A Word From Verywell

It can be very scary to wait for results on a herpes blood test. There is a lot of stigma associated with herpes infection. Many people would simply rather not know if they have herpes. In fact, the emotional devastation that can be associated with a positive herpes test is one reason that some doctors do not offer herpes blood tests to people without symptoms. Because there a reasonably high false positive rate in areas with few herpes infections, these doctors don't want to risk causing that kind of pain if there isn't likely to be a significant benefit. 

It's important to remember that you can live a long and happy life with herpes. However, if you test positive for herpes, make certain you get the support that you need. The diagnosis can seem overwhelming, but you're not alone. Herpes is a common virus. It affects more than one in six Americans. Many people will never even know they're infected. Those not in that fortunate group can take comfort in the fact that their symptoms will usually get better with time.

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