HSV-1 IgG is a blood test that detects a specific type of antibody your immune system produces in response to herpes simplex virus type 1. A positive result means your body has encountered HSV-1 at some point, though it cannot tell you when the infection occurred or where the virus is active in your body. Roughly 63.5% of adults in the United States carry HSV-1 antibodies, making a positive result extremely common.
What the Test Actually Measures
IgG stands for immunoglobulin G, a class of antibody your immune system builds over weeks after an initial infection. Unlike a swab test that looks for the virus itself in an active sore, the IgG blood test looks for evidence that your body has mounted a long-term immune response to HSV-1. These antibodies typically first appear 7 to 14 days after infection and reach their highest levels about 4 to 6 weeks later. For this reason, testing too early after a possible exposure can produce a false negative. Most guidelines suggest waiting at least 12 weeks for the most reliable result.
Modern HSV-1 IgG tests use proteins unique to type 1 (specifically a surface protein called glycoprotein G) to distinguish it from HSV-2. This means the test can tell you which type of herpes you carry, but it cannot tell you the location of the infection. HSV-1 can infect the mouth, genitals, or other areas, and a blood test has no way to determine which site is involved. Only a swab of an active lesion can answer that question.
Reading Your Results
Results are reported as an index value. A value of 1.10 or higher is considered positive by most commercial labs. Values below 0.90 are negative, and anything in between is typically labeled equivocal or indeterminate.
Here’s where it gets important: not all positive results are equally reliable. Index values that fall in the “low positive” range, roughly between 1.1 and 3.0, have a significant chance of being false positives. One study evaluating a widely used commercial assay found that 60.9% of HSV-1 results with positive index values below 3.0 turned out to be false positives when checked against more accurate methods. So if your result is positive but the index number is low, the result may not be trustworthy on its own.
For anyone with a low-positive result who needs a definitive answer, the University of Washington offers a Western blot test that serves as the gold standard for confirmation. This test is more labor-intensive but far more accurate. If your provider orders paired blood samples for seroconversion testing, the two draws should be spaced at least 12 to 16 weeks apart.
Why IgG, Not IgM
You may see references to an HSV IgM test, which in theory detects a different antibody class that appears earlier in infection. In practice, IgM testing for herpes is unreliable and widely discouraged. The sensitivity of commercially available HSV IgM kits ranges between only 50% and 70%, and specificity can drop as low as 65% depending on the assay. Many IgM results cannot be confirmed by a second test. This means an IgM test can easily miss a real infection or flag one that doesn’t exist. IgG testing, by contrast, consistently achieves sensitivity above 94% and specificity above 92% with well-validated kits.
The trade-off is timing. Because IgG antibodies take weeks to develop, the test is best suited for identifying established infections rather than diagnosing a brand-new one. If you have an active blister or sore, a direct swab (PCR or viral culture) is a faster, more precise option.
What a Positive Result Means in Practice
A confirmed positive HSV-1 IgG means the virus is present in your body permanently. HSV-1 establishes itself in nerve cells and remains dormant between outbreaks. Many people with a positive result have never noticed symptoms, or their symptoms were mild enough to go unrecognized. Cold sores on or around the lips are the most common manifestation, but plenty of carriers never develop them.
Even without visible sores, the virus can periodically reactivate and shed at the skin surface. Research on oral shedding found that the rate varies enormously between individuals, from zero to as many as 92% of days tested. Shedding episodes tend to be brief and involve small amounts of virus, but they are common enough that most HSV-1 transmission happens during periods with no visible symptoms at all.
A positive result does not tell you anything about how often you might shed virus or whether you will ever develop noticeable outbreaks. It also does not distinguish between an infection acquired in childhood (which is typical for oral HSV-1) and one acquired recently. The antibodies look the same regardless.
One More Limitation Worth Knowing
In rare cases, HSV-1 IgG antibodies can decline over time, a process called seroreversion. This means someone who was once positive could eventually test negative, even though the virus still lives in their nerve cells and could reactivate. It’s uncommon, but it underscores that a single negative result in someone with a known history of cold sores doesn’t necessarily rule out infection. If your clinical history and test results don’t match, a Western blot or direct swab during an outbreak provides more clarity.