Routine screening blood tests for herpes are not recommended by the Centers for Disease Control. When I say routine screening I mean a scenario such as, “I want to get checked for everything, ok?”

Blood tests for herpes do have a place in specific clinical situations, but that’s for another post.

But say you did get a herpes blood test (even though it’s not recommended many people seem to get it done) and it’s positive for something called IgM antibodies. Armed with these results your doctor proceeds to tell you that these results mean you caught herpes recently, so you start to freak out.

Stop right there.

A positive herpes IgM test means nothing. I’ve posted about this before but enough people continue to leave comments on my herpes posts asking about their positive IgM test that it bears repeating.

A herpes IgM test is never, ever, ever indicated. Not ever, because it doesn’t mean much of anything.

“Impossible,” you say. “My doctor ordered it, so it must be a valid test.”

Well, no.

Let’s review antibodies. When we are exposed to infections we make antibodies. Antibodies are proteins produced by the immune system that fight bacteria and viruses, hopefully containing these infections. IgM antibodies are made first, within weeks, but they don’t hang around. Think of them as first responders. IgG antibodies take longer to produce, but once present you typically have them for life.

However, with herpes this response system is a little different. The herpes virus is never cleared by the immune system. Once you stop shedding the virus it takes up shop in the nerves (the dorsal root ganglia to be specific). From time to time the virus is reactivated, either producing ulcers or just asymptomatic shedding of the virus. How often the virus is reactivated and how much virus is shed depends on many factors.

It is this ability to stay dormant and then reactivate that is a defining characteristic of herpes viruses, but this also affects IgM production. When herpes is reactivated, even though it is not a brand new infection, the body often produces IgM. So, the presence of IgM antibodies against herpes don’t tell you if this is a recent infection or a reactivation and so is of no use clinically. In addition, the IgM tests offered by commercial labs have not been well vetted as type specific, meaning they may not reliably distinguish between IgM for herpes type 1 and type 2 (and remember, by the age of 30 almost everyone is positive for heroes type 1).

Just because a lab offers a test doesn’t mean that the test is useful for your medical care or reliable.

Despite this, many people get tested for herpes IgM.

If you are getting a blood test for herpes, ask your doctor specifically what test that means. If your doctor is ordering an IgM test then go somewhere else, because it means they don’t know enough about herpes to be testing you in the first place.

Disclaimer:  This post is not direct medical advice.

Jennifer Gunter is an obstetrician-gynecologist and author of The Preemie Primer. She blogs at her self-titled site, Dr. Jen Gunter.