HSV-2 is a common infection. For individuals without active symptoms, blood tests are the primary way to determine past exposure and infection status. The accuracy of these tests is a frequent concern, as a positive result has significant implications for a person’s health and relationships. Understanding the methods and limitations of serological testing is necessary to interpret results correctly. The reliability of HSV-2 blood tests is influenced by the type of test used and the timing of sample collection.
The Two Main Types of Antibody Blood Tests
Blood tests for HSV-2 look for the antibodies the body produces in response to the infection, specifically Immunoglobulin G (IgG) and Immunoglobulin M (IgM).
The type-specific glycoprotein G (gG) antibody tests, which detect IgG antibodies, are the standard screening tool for HSV-2. IgG antibodies are produced after initial infection and remain in the blood for life, indicating a past or current infection. The ability of gG tests to differentiate between HSV-1 and HSV-2 makes them the preferred method for determining serostatus.
IgM antibody tests are generally not recommended for diagnosing HSV-2 infection. Although IgM antibodies are the first type produced after initial exposure, they may disappear or reappear over time. Crucially, they cannot reliably distinguish between HSV-1 and HSV-2, leading to a high rate of false positive results.
Understanding Test Reliability Metrics
The accuracy of the preferred IgG blood tests is described using two metrics: sensitivity and specificity. Sensitivity measures the test’s ability to correctly identify people who truly have the virus (true positives). Specificity measures its ability to correctly identify people who do not have the virus (true negatives). A test with high sensitivity results in fewer false negatives.
The specificity of commercial gG-based HSV-2 IgG tests is high, often ranging from 96% to nearly 100%. This means the test is effective at ruling out the infection when it is not present. Sensitivity, however, can vary more widely between different commercial tests, with reported ranges from 85% to 99.5%.
The reliability of a positive result, known as the positive predictive value, is influenced by how common the infection is in the tested population. Even a highly specific test can yield a higher rate of false positives where HSV-2 is rare. In low-prevalence settings, some assays show a positive predictive value such that nearly one in three positive results could be false.
Why Accuracy Varies: The Role of Timing and Index Values
Two main factors cause the accuracy of HSV-2 IgG tests to fluctuate: the window period and low index values. The window period is the time between initial exposure and when the body produces enough antibodies for the test to detect. Testing too early, before antibodies reach a detectable level, can produce a false negative result.
Detectable IgG antibodies usually appear within three weeks of infection, but this process can take up to six months or longer. This variability means a negative result shortly after potential exposure does not definitively rule out infection, often requiring repeat testing.
Index values introduce the second variability, particularly concerning false positive results. Commercial IgG tests provide a numerical result, or index value, where a value above a threshold (typically 1.1) is considered positive. Low positive index values, often between 1.1 and 3.5, are the most common cause of false positives.
These low positive results often occur due to cross-reactivity, where antibodies fighting HSV-1 react with HSV-2 antigens in the test. Studies show that a positive HSV-2 IgG result with a low index value can have up to a 50% chance of being incorrect. This ambiguity diminishes the certainty of the result and necessitates further action.
Confirming Indeterminate or Low Positive Results
When an initial commercial HSV-2 IgG test yields a low positive index value or is unexpected, confirmatory testing is necessary. The gold standard for confirming uncertain serological results is the Western Blot assay, typically performed at specialized reference laboratories. The Western Blot is a comprehensive test that looks for antibodies against multiple viral proteins, offering a definitive determination of infection status.
However, the Western Blot is not widely available, is more expensive, and has a longer turnaround time than standard commercial assays. An alternative confirmatory method is the HSV-2 IgG inhibition assay. This enzyme-linked immunoassay measures the specific binding of antibodies to the HSV-2 glycoprotein G and can be more sensitive for detecting recent infections.
Repeat testing after the window period has fully closed is also a common recommendation for low positive results. If the initial test was performed shortly after exposure, a repeat test at 12 weeks or more can allow antibody levels to rise or confirm a false positive if the result reverts to negative. A discussion with a healthcare provider, considering symptoms and risk factors, is paramount in interpreting the result and determining next steps.