Herpes Simplex Virus type 2 (HSV-2) can cause concern, and a positive test result can be distressing. While blood tests are commonly used to detect HSV-2, questions often arise regarding their accuracy, particularly the possibility of a false positive result. Understanding these tests and the factors that influence their outcomes is important for clarity on HSV-2 status and making informed health decisions.
Understanding HSV-2 Blood Tests
HSV-2 blood tests detect antibodies, proteins produced by the immune system in response to a viral infection. These tests do not directly look for the virus itself. The two main types of antibodies are Immunoglobulin G (IgG) and Immunoglobulin M (IgM). IgG antibodies typically appear a few weeks after initial infection and remain in the bloodstream for a lifetime, indicating past exposure. IgM antibodies are among the first to appear, but they may disappear or reappear during recurrent outbreaks, making them less reliable for diagnosing long-term or new infections.
Reliable herpes blood tests focus on detecting IgG antibodies, specifically those targeting glycoprotein G-2 (gG-2), which helps differentiate HSV-2 from HSV-1. The time it takes for IgG antibodies to reach detectable levels can vary among individuals. While some tests may detect antibodies as early as 2-3 weeks, it can take up to 12-16 weeks for IgG antibodies to fully develop. This “window period” is an important consideration, as testing too soon can lead to a false negative result.
Other diagnostic methods, such as viral culture or polymerase chain reaction (PCR) tests, directly detect the virus from active lesions. These methods are used when a person has visible sores or symptoms, confirming the presence of the virus at the infection site. Blood tests, however, are often used for individuals without symptoms seeking to determine previous HSV-2 exposure.
Factors Leading to Inaccurate Results
HSV-2 antibody blood tests have limitations, and false positive results can occur. One common reason is cross-reactivity with antibodies produced against HSV-1, the virus that typically causes oral herpes. Because HSV-1 and HSV-2 are closely related, HSV-1 antibodies can sometimes react with HSV-2 antigens in the test, leading to a false positive. This is particularly observed with lower index values, which represent the concentration of antibodies detected. Some commercial tests show significantly lower specificity for index values between 1.1 and 3.0.
The use of IgM tests for HSV-2 diagnosis is not recommended due to their high potential for false positive results and inability to distinguish between new and recurrent infections. IgM antibodies can persist or reappear during outbreaks, making them unreliable markers. Relying solely on an IgM test can lead to misinterpretations.
Borderline or “low positive” results, often in the index value range of 1.1 to 3.5, also pose challenges. These results are more likely to be false positives compared to higher index values. Many HSV-2 positive results from commercial immunoassays, especially those without symptoms and low index values, are incorrect when compared to a confirmatory Western Blot test. Human error in laboratory processing, such as improper sample handling, can also contribute to inaccurate test outcomes.
Steps to Confirm a Diagnosis
If an HSV-2 blood test result is positive, especially if it falls within the low positive or borderline range, or if there are no symptoms, seeking further evaluation is important. Discuss results with a healthcare provider, who can consider your complete medical history, including any symptoms or potential exposures. A single test result should not be the sole basis for a definitive diagnosis, particularly when accuracy concerns exist.
Confirmatory testing is often recommended to clarify uncertain results. The Western Blot test is considered the gold standard for confirming HSV-2 antibody presence. This test detects antibodies to multiple viral proteins, offering a more comprehensive and accurate assessment than standard commercial immunoassays. While the Western Blot can be costly and may not be readily available, it provides reliable confirmation. Other confirmatory tests can also improve the accuracy of initial results.
Repeating the IgG antibody test after about 12 weeks from potential exposure or the initial test can also be helpful, especially if the first test was performed too early. This allows sufficient time for antibody levels to develop. Clinical evaluation, which involves assessing any symptoms or a history of lesions, is important in confirming a diagnosis. If active lesions are present, a viral culture or PCR test from the lesion can directly confirm the virus and its type.