Testing for herpes simplex virus (HSV), which includes HSV-1 (oral herpes) and HSV-2 (genital herpes), is not always a straightforward process. Results can be complex, especially for individuals without an active outbreak. Accurate interpretation requires understanding how tests work and what their limitations are. Reliability depends heavily on the test type, the timing relative to exposure, and the patient’s risk factors.
Understanding the Different Types of Tests
Herpes testing methods fall into two broad categories: direct viral detection, used during an active outbreak, and antibody detection, which measures the body’s immune response. When a visible sore is present, direct viral detection is the preferred diagnostic approach. The most sensitive method is the Nucleic Acid Amplification Test (NAAT), typically the Polymerase Chain Reaction (PCR) test, which detects the virus’s genetic material.
The PCR test uses a swab taken directly from a lesion and is highly effective at identifying both HSV-1 and HSV-2 types. A less sensitive option is the viral culture, which attempts to grow the virus from the swab sample. Since the virus is most active early on, the accuracy of any swab test declines significantly if not taken within the first 48 hours of symptoms appearing.
If no symptoms are present, a blood test is used to detect antibodies, which are proteins the immune system creates in response to infection. These tests measure the body’s long-term immune memory rather than the virus itself. The most widely used are type-specific serologic tests that detect immunoglobulin G (IgG) antibodies. IgG antibodies appear weeks to months after infection and remain detectable for the rest of a person’s life, indicating past or present infection.
Some commercial tests also look for immunoglobulin M (IgM) antibodies, the first antibodies produced after an infection. However, health organizations widely discourage IgM testing because it is not reliably type-specific and can reappear during recurrent outbreaks. IgM tests often cannot distinguish between HSV-1 and HSV-2, frequently leading to false positive results for HSV-2.
Defining Test Accuracy
The performance of any medical test, including those for herpes, is measured using sensitivity and specificity. Sensitivity refers to a test’s ability to correctly identify people who truly have the infection (the true positive rate). A test with high sensitivity has a very low number of false negative results, meaning it is unlikely to miss an actual infection.
Specificity is the measure of a test’s ability to correctly identify people who do not have the infection (the true negative rate). High specificity ensures the test is unlikely to produce a false positive result. Commercial IgG blood tests, often based on Enzyme-Linked Immunosorbent Assay (ELISA) technology, generally have high sensitivity, sometimes reaching 99 percent for HSV-2, but their specificity can be lower. While a negative result is usually reliable, a positive result may need further investigation.
Factors That Impact Result Reliability
The timing of a blood test is a common factor leading to unreliable results, particularly false negatives. After initial exposure, the body requires time to produce enough IgG antibodies for detection, known as the “window period.” This period can range from weeks to months, so experts suggest retesting 12 to 16 weeks after the last possible exposure for maximum accuracy. Cross-reactivity is also a challenge with commercial HSV-2 IgG tests, often leading to false positive results.
Cross-reactivity occurs when the test mistakenly identifies antibodies for a different virus, such as HSV-1, as HSV-2 antibodies. This is problematic for individuals whose results fall into a “low-positive” range, typically with an index value between 1.1 and 3.5. For viral detection tests (PCR and viral culture), the location and age of the lesion influence reliability. Swabbing a healing or small lesion may not yield enough viral material, increasing the chance of a false negative result. For example, viral culture is often positive in only about 30 percent of recurrent outbreaks.
Interpreting Your Results
Interpreting a herpes test requires considering the test’s statistical reliability alongside the patient’s individual risk factors. A positive antibody test in an individual with a low likelihood of infection is more likely to be a false positive than the same result in a high-risk individual. For HSV-2, a low-positive index value (often 1.1 to 3.5) on a commercial IgG test carries a substantial risk of being inaccurate. In these ambiguous situations, or when a patient receives a low-positive result, confirmatory testing is necessary.
The gold standard for confirming an HSV antibody diagnosis is the Western Blot test, which is a specialized laboratory procedure. This test is significantly more accurate because it looks for a wider array of viral proteins, providing a comprehensive profile of the immune response compared to standard commercial tests. The Western Blot has greater than 99 percent sensitivity and specificity and is used to definitively rule out a false positive from initial screening. A diagnosis should never be made based solely on a single, low-positive screening result without considering the full clinical picture.