Health care providers often order a different test, the IgM test. A positive IgM test, however, doesn't necessarily mean you have HSV-2, the genital herpes virus.
Why not IgM?
Herpes testing based on IgM seems like a good idea because your immune system starts producing IgM in early infection with most viruses. Theoretically, an IgM test can tell you whether sores you just developed are or aren't herpes, and it might help you figure out when you were infected.
For diagnosing HSV-2, though, the IgM test has serious shortcomings.
The IgM test doesn't distinguish one herpes virus from another. A new infection with HSV-1, which causes oral cold sores, could give you a positive herpes IgM result. If that's not taken into account, you could be told you have HSV-2 (genital herpes) when you really don't.A positive herpes IgM test could also mean you're having a recurrence of HSV-2.
Many people with HSV-2 produce new IgM every time they have a herpes outbreak. With IgM testing, there's no way to know whether you have a new infection or when you initially acquired the virus.
In contrast, IgG testing can distinguish between HSV-1 and HSV-2. It's an important distinction, because HSV-1 is less severe and quite common. Most people have antibodies to HSV-1 by the time they're grown.
The IgG tests approved by the FDA are very accurate. That means there is only a small chance that you would get a false positive or false negative result.
But there is a drawback: You have to wait four months between your first outbreak and your IgG test. Your immune system takes that long to produce IgG against herpes, so a negative result from an earlier IgG test isn't reliable.
If you're not sure whether you have HSV-2, or if you doubt the accuracy of a genital herpes diagnosis you received some time ago, it makes sense to get tested (or retested). Be sure to ask for IgG testing.