A herpes test result often carries significant weight, leading many to question its accuracy when the outcome is unexpected. Diagnostic tools for the Herpes Simplex Virus (HSV) are not infallible, and testing is subject to error. These errors primarily yield two types of misleading outcomes: false positives and false negatives. Understanding the reasons behind these inaccuracies is important for interpreting results and determining the next steps.
Understanding the Different Types of Herpes Tests
Testing for herpes involves two different approaches, depending on whether a person has active symptoms or is seeking to understand their exposure history.
When active lesions, such as blisters or sores, are present, healthcare providers use viral detection tests. These methods involve swabbing the lesion to collect fluid and cells, which are analyzed for the presence of the virus itself. The most common and sensitive test is the Polymerase Chain Reaction (PCR) assay, which directly detects the genetic material (DNA) of the HSV-1 or HSV-2 virus. Viral culture is an older, less sensitive method that attempts to grow the virus in a lab.
The second category is antibody testing, or serologic testing, performed via a blood sample when no lesions are visible. These tests look for the antibodies the immune system produces in response to the infection, not the virus itself. The most common blood test detects Immunoglobulin G (IgG) antibodies, which remain in the bloodstream for life after initial exposure.
IgG antibody tests are type-specific, distinguishing between HSV-1 (oral herpes) and HSV-2 (genital herpes). The Immunoglobulin M (IgM) test is not recommended because it cannot reliably differentiate between HSV types, and its presence does not necessarily indicate a recent infection, often leading to misleading results.
Reasons for a False Positive Result
A false positive result occurs when a test indicates a herpes infection in an uninfected person. This error is almost exclusively a concern with IgG antibody blood tests, which measure the body’s immune response to the virus.
The most frequent cause of a false positive is cross-reactivity. Herpes Simplex Virus belongs to a family of viruses, and antibodies produced against one virus can mistakenly react with a similar protein structure of another virus during the test. For example, high levels of HSV-1 antibodies may trigger a false positive result for HSV-2 on a standard screening test. Cross-reactivity can also occur with other common viruses in the same family, such as varicella-zoster (chickenpox and shingles) or cytomegalovirus.
Another contributor to inaccurate positive results is the index value, the numerical result provided by the blood test. Most commercial IgG tests consider a result of 1.1 or greater to be positive. However, values that fall just above this cutoff are frequently false. Low-positive results, often categorized as index values between 1.1 and 3.5 or 5.0, have a substantial probability of being incorrect, particularly for HSV-2.
Lower test specificity increases the likelihood of false positives, especially in populations where the prevalence of the virus is low. In these groups, the chance that a positive result is truly false can be as high as 50 percent for some commercial HSV-2 antibody tests. A single low-positive result should be treated with skepticism and requires further investigation.
Reasons for a False Negative Result
A false negative result indicates a person is uninfected when they are actually infected. This error can occur with both viral detection tests and antibody blood tests, but for different reasons.
For serologic (antibody) blood testing, the primary reason for a false negative is testing during the window period. After initial exposure to the virus, the immune system requires time to build up sufficient IgG antibodies for the test to detect. This period can range from a few weeks to several months. An IgG test performed too soon after exposure will be negative despite infection. Healthcare providers recommend waiting at least 12 weeks after a potential exposure before conducting a final serologic test.
For viral detection methods like PCR or culture, used on active lesions, false negatives are typically due to sampling errors. The viral load decreases rapidly as a lesion begins to heal and dry out. If the swab is taken from a crusted or nearly healed sore, the test may fail to collect enough viral material to register a positive result.
Older methods, like viral culture, are insensitive, sometimes failing to detect the virus in 25 to 70 percent of confirmed cases, especially in recurrent outbreaks. While PCR is far more sensitive, a poorly collected sample—one that does not adequately touch the moist base of a fresh lesion—can still lead to an inaccurate negative result.
What to Do After a Questionable Test Result
When a herpes test result is questionable, whether a suspected false positive or a potential false negative, the next step involves verification and retesting. This requires utilizing more precise diagnostic tools and optimizing the timing of follow-up tests.
For a suspected false positive from an IgG blood test, the most definitive step is to order a confirmatory test. The Western Blot assay is the gold standard for verifying ambiguous serologic results because it is more sensitive and specific than standard commercial tests. This test looks for multiple viral proteins instead of a single one, providing accurate confirmation of the true status.
If the initial test was negative following a known or suspected exposure, retesting is necessary to rule out a false negative caused by the window period. A follow-up IgG antibody test should be scheduled at least 12 weeks after the last possible exposure to allow for detectable antibody production.
It is important to discuss the results and any concerns with a healthcare provider experienced in interpreting HSV diagnostics. They can evaluate the result based on individual risk factors, clinical symptoms, and the specific index value of the initial test. This approach ensures decisions are based on reliable information, not a single, potentially misleading, test result.