When a herpes test indicates a positive result, but the individual does not actually have the infection, this is known as a false positive. This outcome can cause distress and unnecessary concern. While herpes tests are generally reliable, various factors can contribute to inaccurate positive readings. Understanding these reasons helps clarify why such results occur and guides appropriate next steps.
Understanding Herpes Tests
Different types of tests detect herpes, either by looking for the virus itself or the body’s immune response. Understanding how these tests work helps explain why false positive results can happen.
Antibody tests, or blood tests, identify proteins called antibodies that the immune system produces in response to a herpes simplex virus (HSV) infection. Immunoglobulin G (IgG) tests are widely used as they detect antibodies that typically appear weeks after exposure and remain in the blood for life, indicating a past or current infection. Immunoglobulin M (IgM) tests detect earlier antibodies, but they are not recommended for reliable diagnosis due to their unreliability and frequent false results. Type-specific antibody tests can differentiate between HSV-1 (often associated with oral herpes) and HSV-2 (the primary cause of genital herpes).
Viral culture involves taking a fluid sample directly from a suspicious sore or blister. This sample is sent to a laboratory to grow the virus. This method is most effective when lesions are fresh and active, ideally within 48 hours of appearance, as viral levels decrease as sores heal. Polymerase Chain Reaction (PCR) tests also detect viral genetic material (DNA) in a sample from a sore or blood. PCR tests are highly sensitive and accurate, making them a preferred method, especially when symptoms are present.
Key Reasons for Inaccurate Results
False positive herpes test results can arise from several factors, leading to distress. Understanding these causes is important for interpreting a positive herpes test result.
Cross-reactivity is a frequent cause of false positive results, especially with antibody tests. The immune system can produce antibodies that react to similar proteins found in different viruses. Antibodies generated in response to other common herpesviruses, such as HSV-1 (oral herpes), Varicella-Zoster Virus (chickenpox and shingles), Cytomegalovirus (CMV), or Epstein-Barr Virus (EBV), can mistakenly react with antigens used in HSV-2 antibody tests. This means a person might test positive for HSV-2 even if they only have antibodies to one of these other related viruses. False positives due to cross-reactivity are common when initial HSV-2 IgG index values are in the low positive range, typically between 1.10 and 3.50.
No medical test is completely without error. Tests have inherent limitations in their sensitivity (ability to correctly identify those with the condition) and specificity (ability to correctly identify those without the condition). Even a test with high specificity can produce false positives, especially in populations where the disease prevalence is low. For example, some commercially available HSV-2 IgG tests can yield false positive rates as high as 50% in low-prevalence populations. This means a certain number of false positives are expected in groups with a low likelihood of infection.
The timing of a herpes test can also influence result accuracy. For antibody tests, if performed too soon after exposure, the body may not have produced enough antibodies, leading to a false negative. Conversely, fluctuating or very low antibody levels can sometimes be misinterpreted, contributing to ambiguous or false positive readings.
In rare instances, laboratory errors or human mistakes can lead to inaccurate test results. These include improper handling or storage of samples, mislabeling, or technical malfunctions. While laboratories follow strict protocols, these factors remain a potential, though uncommon, source of false positives.
What to Do After a Positive Test
Receiving a positive herpes test result can be unsettling, but further steps can clarify the diagnosis, especially if a false positive is suspected. Consulting with a healthcare provider is the initial and most important step.
Confirmation testing is often recommended, particularly if the initial result is ambiguous or falls within the low-positive range on an antibody test. The Western Blot test is widely regarded as the gold standard for confirming herpes diagnoses due to its high accuracy. This specialized test can help differentiate true positive results from false positives. In some cases, your healthcare provider may suggest repeating the standard antibody test after several weeks to re-evaluate an equivocal result.
Discussing your specific test values with a healthcare provider is important for proper interpretation. They can analyze numerical index values, as low positive IgG index values often warrant additional investigation to rule out a false positive. Considering your symptoms and exposure history is also an important part of this discussion. You should share whether you have experienced any symptoms or potential exposures that align with the test results.
A positive test result can lead to significant emotional reactions. It is important to acknowledge these feelings and seek support if needed. Open communication with your healthcare provider about your concerns can help you navigate the diagnostic process.