Can a Herpes Blood Test be WRONG?
Yes. A herpes blood test can be wrong — and understanding what your result actually means matters before assuming you have herpes.
If you had routine STI testing and were surprised to see a positive result for herpes simplex virus (HSV), you’re not alone. Herpes blood testing is one of the most misunderstood areas of STI testing, and a positive blood test does not always provide a straightforward diagnosis.
I’m Dr. Andi Toufexis, a board-certified family medicine physician and founder of Your Doctor Bestie. Here’s what I want you to know before you panic over a positive HSV blood test.
First: What Does a Herpes Blood Test Actually Test For?
The most commonly used herpes blood test is a type-specific HSV IgG antibody test.
Unlike a swab taken directly from a suspicious sore or blister, a blood test does not detect the herpes virus itself. It looks for antibodies your immune system has produced in response to HSV.
Type-specific testing generally reports results separately for:
HSV-1, which commonly causes oral herpes but can also cause genital herpes
HSV-2, which is more strongly associated with genital herpes
This distinction is important because a blood test cannot tell you where on your body the infection is located.
For example, a positive HSV-1 blood test could reflect an oral infection acquired years ago — even during childhood. The blood test alone cannot tell you whether HSV-1 is oral or genital.
So, Can a Positive Herpes Blood Test Be a False Positive?
Yes.
False-positive results are a recognized limitation of herpes blood testing, particularly with some HSV-2 IgG tests when the reported result is only slightly above the laboratory’s positive cutoff.
This is one reason major medical organizations do not recommend routine herpes blood screening for most people without symptoms.
A test can technically come back “positive” while the overall clinical picture is much more complicated.
And this is where one number becomes particularly important.
What Is the HSV Index Value?
Many HSV IgG tests report something called an index value.
You might see a result that looks something like:
HSV-2 IgG: 1.7
or
HSV-2 IgG: 8.4
Both may be labeled “positive” by the laboratory, but those two results should not necessarily be interpreted the same way.
Historically, many commonly used assays have classified results approximately like this:
Below 0.9: negative
0.9–1.1: equivocal
Above 1.1: positive
However, the laboratory cutoff is not the same thing as the probability that you truly have HSV.
The CDC specifically warns that one commonly used HSV-2 enzyme immunoassay can produce false-positive results at low index values, particularly between 1.1 and 3.0. Because of the poor specificity in this range, the CDC recommends confirmation with a second testing method before interpreting a low-positive result as evidence of HSV-2 infection.
That means seeing the word POSITIVE on your patient portal is not always the end of the story.
What If My HSV-2 Index Value Is Low?
If your HSV-2 result falls into the low-positive range, don’t interpret the number in isolation.
Your healthcare provider may need to consider:
● Whether you’ve ever had symptoms consistent with genital herpes
● Why the test was ordered
● Your previous HSV testing
● How recently a possible exposure occurred
● Which specific assay was used
● Whether confirmatory testing is available
Depending on the situation, a second test using a different method may help clarify the result.
What Is Confirmatory Herpes Testing?
Confirmatory testing uses a different testing method to determine whether the antibodies detected on the initial screening test truly represent HSV infection.
Depending on availability and circumstances, options may include another type-specific assay, an HSV-2 inhibition assay, or the University of Washington HSV Western Blot, which is widely considered the reference standard for HSV antibody testing.
Not everyone with a positive herpes blood test needs a Western blot. But if you have a low-positive result — particularly when you’ve never had symptoms — it’s reasonable to discuss whether confirmation is appropriate before assuming you have a lifelong infection.
Can a Herpes Blood Test Be Falsely Negative Too?
Yes.
Antibodies don’t appear immediately after exposure. Your immune system needs time to develop detectable levels of HSV antibodies.
If testing is performed too soon after a new infection, the result may be negative even though infection has occurred.
This is sometimes called the window period.
If a recent HSV exposure is suspected, your healthcare provider may recommend repeating type-specific antibody testing after enough time has passed for antibodies to develop.
And if you currently have a new blister, ulcer, or other suspicious lesion, don’t rely on a blood test to determine whether that lesion is herpes.
Whenever possible, a fresh lesion should be tested directly using a PCR/NAAT swab, because this looks for HSV at the site itself.
Why Isn’t Herpes Blood Testing Part of Routine STI Screening?
This surprises a lot of people.
When you ask for “everything” during an STI screening, herpes testing isn’t necessarily supposed to be included.
The CDC does not recommend routine HSV-2 serologic screening of the general population, and the U.S. Preventive Services Task Force recommends against routine serologic screening for genital herpes in asymptomatic adolescents and adults, including pregnant people.
Why?
One major reason is the potential for false-positive results and the harms that can follow an incorrect diagnosis.
Testing can absolutely be useful in certain situations. But more testing isn’t automatically better testing.
The right test, ordered for the right reason, at the right time matters.
I Got a Positive Herpes Blood Test. What Should I Do Now?
First, look at the actual result rather than just the word “positive.”
Find out:
1. Was the result HSV-1 or HSV-2?
2. Was this an IgG test?
3. What was the index value?
4. Have you ever had symptoms?
5. Was the test performed because of a known exposure, or was it simply included with routine labs?
6. Does your result warrant confirmatory testing?
A positive result may ultimately represent a true HSV infection. But especially with certain low-positive HSV-2 results, it deserves appropriate interpretation before you make major decisions about your health, relationships, or treatment.
The Bottom Line
Yes, herpes blood tests can be wrong.
A positive HSV antibody result doesn’t tell you when you acquired HSV, who you acquired it from, and — particularly with HSV-1 — where the infection is located.
And some low-positive HSV-2 IgG results can be falsely positive and may require confirmation.
If you received a herpes test result you weren’t expecting, don’t stop at the red POSITIVE flag in your patient portal.
Look at the test. Look at the index value. And understand what the result can — and cannot — actually tell you.
Want to Understand Your Herpes Results?
If you’re staring at an HSV test result and trying to figure out what the numbers actually mean, I’ve created resources specifically for this.
The Herpes Handbook is my comprehensive guide to herpes testing, diagnosis, treatment, transmission, disclosure, pregnancy, relationships, and the questions people commonly have after an HSV diagnosis.
For a faster deep dive specifically into blood testing, The HSV Index Value Mini-Guide walks you through understanding IgG index values, low-positive results, false positives, and when additional testing may be worth discussing with your healthcare provider.
No fear. No shame. Just evidence-based information you can actually understand.
Written by Dr. Andi Toufexis, DO — Board-Certified Family Medicine Physician
This article is for educational purposes and is not a substitute for individualized medical care.