A positive herpes test result can be a source of significant anxiety, prompting a search for clarity regarding the diagnosis. A preliminary positive result does not automatically confirm an infection, as the possibility of a false positive exists, particularly with certain laboratory screens. Understanding the different testing methods and the science behind the results helps individuals navigate the next steps with confidence.
Which Herpes Tests Can Yield False Positives?
The testing approach for herpes simplex virus (HSV) generally falls into two categories: direct viral detection and antibody detection. Tests that look for the active virus, such as Polymerase Chain Reaction (PCR) or viral culture, are typically performed by swabbing a visible lesion or sore. These direct detection methods look for the presence of the virus’s DNA or live virus and are highly reliable during an active outbreak.
False positives are overwhelmingly associated with blood tests that detect antibodies, known as serological tests. These tests, specifically Immunoglobulin G (IgG) antibody assays, measure the body’s immune response to past exposure rather than the virus itself. While useful for diagnosis in people without active symptoms, the accuracy of IgG tests varies significantly.
The limitations of antibody screening mean a person may test positive without having the infection. This issue is particularly pronounced for tests detecting antibodies to Herpes Simplex Virus type 2 (HSV-2), which causes most cases of genital herpes. Experts recommend that commercially available IgG antibody tests should not be the sole basis for diagnosis, especially for individuals at a low risk of infection.
Mechanisms Behind False Positive Results
False positive results in herpes antibody tests arise primarily from the test’s inability to perfectly distinguish between antibodies. The most common reason for this misidentification is cross-reactivity. This occurs when antibodies produced in response to one pathogen accidentally bind to the antigens of a different, but structurally similar, pathogen in the test.
The herpes virus family is large, and antibodies to common related viruses can trigger a false positive for HSV-2. For example, antibodies to Herpes Simplex Virus type 1 (HSV-1, which causes most oral herpes) or Varicella Zoster Virus (VZV) may cross-react with the HSV-2 antigens. This confusion leads the test to signal a positive result for HSV-2 when the individual is only infected with the related virus.
False positives are most common when the test result falls into the low positive range, often called a “low-titer” result. Serological tests report an index value, which is the ratio of the antibody level detected to the test’s cutoff point. A result is considered positive if the index value is 1.1 or higher. However, in the low range (typically 1.1 to 3.5), the chance of a false positive is significantly higher, with studies showing nearly half of these results may not be confirmed upon further testing.
Confirmation Testing and Result Interpretation
When a commercially available IgG test yields a positive result, especially in the low-titer range, confirmation testing is necessary. The gold standard for resolving ambiguous antibody results is the Western Blot (WB) test. This highly accurate method separates viral proteins and detects antibodies against multiple specific proteins, providing a much more precise antibody profile than standard screening tests.
The Western Blot offers high sensitivity and specificity. If a preliminary test result is low-positive, a healthcare provider will recommend the WB to confirm or rule out the diagnosis. Since standard screening tests produce a high rate of false positives for HSV-2, a single low-positive result should never be considered definitive proof of infection.
If the initial test was performed too early after potential exposure, the body may not have produced enough antibodies yet, a period known as seroconversion. A repeat IgG test is recommended 12 to 16 weeks later to allow sufficient time for antibodies to reach detectable levels. Complex results, such as a low index value or an indeterminate Western Blot, require consultation with a knowledgeable healthcare provider or an infectious disease specialist for appropriate interpretation and guidance.