Key takeaways
- Type-specific IgG blood tests look for antibodies to HSV-1 or HSV-2. They don't show where on the body the infection is or when it started.
- CDC says the common HerpeSelect HSV-2 test is often falsely positive at low index values (1.1–3.0).
- For low-positive HSV-2 results, CDC recommends a confirmatory test using a different method (Biokit or Western blot).
- A positive HSV-1 antibody can't tell oral infection from genital infection. Many people get oral HSV-1 in childhood.
- IgM herpes tests aren't recommended because they aren't type-specific.
You ordered a panel, the herpes line came back "positive," and the number next to it is something like 1.4 or 2.6. Before you assume the worst, know this: CDC's own guidelines say that range is where the most widely used HSV-2 blood test often gives false positives.
This guide explains what herpes blood tests measure, how to read the index value, when a result needs confirming, and what to do next. It builds on our explainer about whether standard panels include herpes.
What a herpes blood test actually measures
Herpes blood tests are type-specific IgG antibody tests. They look for antibodies your immune system makes to HSV-1 or HSV-2. According to the CDC STI Treatment Guidelines, these antibodies develop during the first weeks after infection and then last indefinitely.
That has some important consequences:
- It doesn't tell you the location. The blood test can't say whether HSV-1 is oral or genital. CDC notes that many people get oral HSV-1 in childhood, often without symptoms.
- It doesn't tell you when. A positive result can't date an infection or point to a specific partner.
- It can miss recent infections. CDC notes false negatives are more likely early on. If you think you were recently exposed, CDC suggests repeating the test 12 weeks after the presumed exposure.
- It isn't a sore test. If you have a sore right now, a swab of the sore (PCR) is the better test. It can confirm herpes and the type directly.
Reading the index value
Many labs report HSV IgG as an index value instead of a plain yes or no. The index compares how strongly your sample reacted against a cutoff. The exact cutoffs vary by lab and assay, so check your report's reference range, but results commonly fall into three bands: negative, equivocal, and positive.
The key issue is the low positive range. CDC says the HerpeSelect HSV-2 EIA "often is falsely positive at low index values (1.1–3.0)." In one study CDC cites, specificity for index values of 1.1–2.9 was only 39.8%. In plain terms, many low-positive results in that study belonged to people who didn't have HSV-2.
| HSV-2 IgG result | What it can mean | Typical next step |
|---|---|---|
| Negative | No HSV-2 antibody detected | If exposure was within ~12 weeks, consider repeating later |
| Equivocal | Borderline; can't be interpreted | Repeat or confirmatory testing |
| Low positive (about 1.1–3.0 on HerpeSelect) | True infection or false positive; both are common | Confirm with a second method before accepting a diagnosis |
| Higher positive | More likely to be true infection | Talk with a clinician; confirm if history or symptoms don't fit |
Index value isn't severity
A higher index doesn't mean a "worse" infection, more outbreaks, or higher contagiousness. It reflects how strongly the sample reacted in the test. That's useful for judging whether a result is likely true, not for judging how herpes will affect you.
How to confirm a low positive
CDC's guidance is direct. For low index values, "a confirmatory test (Biokit or Western blot) with a second method should be performed before test interpretation." It also notes the HerpeSelect immunoblot shouldn't be used for confirmation, because it uses the same antigen as the first test.
- Biokit HSV-2: a rapid test that uses a different antigen and can be ordered by some clinicians.
- Western blot: long regarded as the reference standard for HSV serology. It's offered through a specialty lab and usually needs a clinician to order it.
If neither test is available to you, CDC says people should be counseled about the limitations of the testing. A low positive with no history of symptoms and no confirmation is a result to treat with caution, not a final diagnosis.
Making sense of HSV-1 results
A positive HSV-1 IgG is common, and on its own it usually doesn't say much about genital health. CDC notes HSV-1 serology "does not distinguish between oral and genital infection," and that a genital HSV-1 diagnosis is confirmed with a virologic test (a swab) from a genital lesion. If you've never had genital symptoms, a positive HSV-1 result most often reflects oral infection, which is very common.
Why IgM herpes tests aren't recommended
Some panels still offer HSV IgM, often described as a test for "recent" infection. CDC says IgM testing for HSV-1 or HSV-2 "is not useful because IgM tests are not type specific," and IgM may be positive during a recurrent outbreak as well as a new one. If your panel shows an IgM result, don't use it to date an infection.
Who should get a herpes blood test?
CDC says routine HSV-2 screening isn't recommended for the general population. That's partly because of the false-positive problem described above. Type-specific testing can be useful in situations like these:
- Recurring genital symptoms with a negative or missing swab test.
- A clinical diagnosis of genital herpes without lab confirmation.
- A partner who has genital herpes, when knowing your own status would affect decisions.
If you have symptoms right now, see a clinician for a swab rather than ordering a blood test. Our ingrown hair vs herpes guide and symptom guide can help you describe what you're seeing.
Deciding whether to test?
Herpes IgG testing through a lab
STDcheck offers lab-based testing you order online and complete at a nearby partner lab. If you choose HSV IgG testing, keep this guide's limits in mind: a low-positive HSV-2 result should be confirmed with a different method before you accept a diagnosis.
Visit STDcheck.com →What to do next
- Write down the exact result: the test name, the index value, and the lab's reference range.
- If HSV-2 is low positive, ask a clinician about confirmatory testing with Biokit or Western blot before you treat it as a diagnosis.
- If you have a sore, get it swabbed while it's present.
- If the exposure was recent and the result is negative, consider repeating at about 12 weeks.
- If herpes is confirmed, remember it's common and manageable. Antiviral medication can reduce outbreaks and transmission. See next steps after a positive result.
And if you're questioning a result that doesn't fit your history, can STI tests be wrong? covers false positives and false negatives more broadly.
Frequently asked questions
Is an HSV-2 index of 1.5 a false positive?
It might be. CDC says the HerpeSelect HSV-2 test is often falsely positive at index values of 1.1–3.0 and recommends confirming with a second method, such as Biokit or Western blot, before interpreting the result.
Can a herpes blood test tell me if it's oral or genital?
No. The blood test detects antibodies, not location. This matters most for HSV-1, which commonly causes oral infection. A genital HSV-1 diagnosis is confirmed by swabbing a genital lesion.
How long after exposure will a herpes blood test turn positive?
Antibodies develop during the first weeks after infection, but early false negatives are possible. CDC suggests repeat testing 12 weeks after a presumed exposure if recent infection is suspected.
Should I trust a herpes IgM result?
CDC says HSV IgM testing isn't useful because it isn't type-specific, and it can be positive during recurrences. It isn't a reliable way to tell whether an infection is new.
Why isn't herpes on every STI panel?
CDC doesn't recommend routine HSV-2 screening in the general population, partly because of false positives and limited benefit for people without symptoms.
Sources
- CDC STI Treatment Guidelines: Genital Herpes
- University of Washington STD Prevention Training Center: HSV serologic testing options
Guidance changes. When timing or treatment matters, check the current linked guidance or ask a clinician.
Disclaimer: This article is for informational purposes only and does not diagnose, prescribe, or replace individualized medical care. Testing and treatment decisions should be made with a qualified healthcare provider. Some links on this page are affiliate links; we may earn a commission at no extra cost to you.