Herpes simplex virus (HSV), which includes both HSV-1 and HSV-2, is a common viral infection worldwide. While many people with the virus experience no symptoms, others may have recurrent outbreaks of painful sores. Determining exposure to HSV requires laboratory testing, but the results can often appear confusing. This guide clarifies the meaning behind your herpes test report, helping you distinguish between an active infection and past exposure.
The Two Main Types of Herpes Testing
Herpes testing is divided into two categories that look for fundamentally different things: the active virus itself or the body’s immune response. When an outbreak is present, the preferred method is a viral detection test, which involves swabbing a sore or blister. This process, often a Polymerase Chain Reaction (PCR) test, looks directly for the genetic material of the herpes simplex virus within the sample. PCR is sensitive and can accurately identify which type of virus, HSV-1 or HSV-2, is causing the active lesion.
When a person has no visible symptoms, a blood test, known as a serology test, is used to check for antibodies. Antibodies are proteins the immune system creates in response to a viral infection, and the most reliable tests look specifically for immunoglobulin G (IgG) antibodies. IgG antibodies remain in the bloodstream for a lifetime after initial infection, meaning a positive blood test indicates past exposure, not necessarily a current outbreak. These type-specific IgG blood tests can distinguish between HSV-1 and HSV-2 exposure, providing a clear picture of a person’s overall status.
Interpreting Positive, Negative, and Indeterminate Results
The results from a viral detection test, such as a swab PCR, are reported as either Positive or Negative. A Positive result confirms the active virus was present in the lesion at the time of testing, indicating an active outbreak. Conversely, a Negative swab result suggests the sore was not caused by HSV, or that the sample was taken too late for the virus to be detected.
For the IgG antibody blood test, results fall into three categories: Negative, Positive, or Indeterminate (sometimes called Equivocal). A Negative blood result means no detectable IgG antibodies were found for that specific virus type, suggesting no prior exposure. This result can be falsely reassuring if the test was performed too soon after exposure, before antibodies had time to develop. A Positive blood result confirms the presence of antibodies, meaning a person has been infected with that HSV type at some point.
An Indeterminate or Equivocal result occurs almost exclusively with the blood test. This means the level of antibodies detected is too close to the cutoff threshold to be definitively positive or negative. This result can happen if the infection is very recent and antibody levels are still rising, or if the test is registering low-level cross-reactivity with other viruses. If a result is indeterminate, a healthcare provider typically recommends retesting in four to six weeks to allow more time for antibodies to reach detectable levels.
Understanding Index Values and Test Accuracy
When you receive an IgG blood test result, it includes a numerical figure known as an index value, which represents the concentration of antibodies in your blood. This index value is crucial for interpreting the test’s reliability, particularly when the result is positive. For many standard tests, an index value of 0.9 or less is considered Negative, and a value of 1.1 or higher is considered Positive. The zone between 0.91 and 1.09 is the Equivocal range, where the result is inconclusive and retesting is suggested.
The accuracy of a positive result is directly related to the index value. A high positive result, such as an index value greater than 3.5 or 5.0, suggests a true infection. However, a low-positive index value, often between 1.1 and 3.5, has a higher chance of being a false positive. In these ambiguous cases, a specialized test, such as the Western Blot assay, may be recommended to confirm the diagnosis with greater certainty.
Testing too early after potential exposure significantly affects accuracy due to the window period. This is the time it takes for the body to produce a detectable level of IgG antibodies, which can take anywhere from three to twelve weeks after infection. Testing before this period is complete can result in a false negative, despite the person having contracted the virus. For the most accurate serology result, it is recommended to wait about 12 to 16 weeks after the last possible exposure before being tested.