A herpes culture is a laboratory test used to diagnose an active herpes simplex virus (HSV) infection by attempting to grow the virus from a sample taken directly from a sore. This method confirms the presence of live, replicating virus particles within the lesion fluid or tissue. The culture test provides direct evidence of viral activity and is used when a person presents with visible skin lesions or ulcers.
How the Viral Culture is Performed
The process begins when a healthcare provider collects the sample from a fresh, active lesion, such as a moist vesicle or blister. They use a sterile swab to firmly rub the base of the lesion, often after unroofing a blister, to collect fluid and epithelial cells where the virus is actively replicating. Proper timing and technique are paramount because the presence of live virus is much higher in newer lesions.
Once collected, the swab is immediately placed into a specialized transport medium and kept cool, typically at 4 degrees Celsius, to preserve the virus’s viability. In the laboratory, the sample is inoculated onto a monolayer of susceptible cells, such as Vero or HEp-2 cell lines, which act as living hosts. The technician incubates and monitors the cells over several days to see if the virus successfully infects and reproduces.
If the herpes virus is present, it will begin to destroy the host cell monolayer, resulting in the cytopathic effect. This destruction is visible under a microscope as distinct changes. These include the enlargement and ballooning of infected cells, the formation of multinucleated giant cells where several infected cells fuse, and the appearance of specific intranuclear inclusion bodies. Observing these characteristic effects confirms HSV activity in the original sample.
Understanding Positive and Negative Results
A positive result indicates that the herpes simplex virus was successfully isolated and grown in the laboratory from the lesion sample. This outcome confirms an active infection. Once the virus is cultured, further testing can determine the specific strain, identifying it as either HSV-1 (often associated with oral herpes) or HSV-2 (the most common cause of genital herpes).
Technicians must wait for the virus to multiply enough to cause a visible change. A positive result may be reported quickly, sometimes within 48 hours. However, a final negative result may require up to 7 to 10 days of observation to ensure no slow-growing virus is missed. This extended waiting period can delay treatment or diagnosis.
A negative culture result means that no virus was isolated or grew in the lab, but this does not always rule out a herpes infection. This outcome is often a false negative. If the sample was collected from a lesion that had already begun to heal, or if the virus was damaged during transport, the culture may fail to grow the virus even if HSV caused the sore.
Limitations and Modern Alternatives
The herpes culture test has low sensitivity, meaning it frequently produces false-negative results. The test’s ability to detect the virus diminishes rapidly as a lesion progresses from a fluid-filled blister to an ulcer or a crusted sore. Sensitivity may drop to as low as 25% for a lesion that is already crusted over.
This low sensitivity and the long turnaround time have led to the culture test being largely superseded by more advanced molecular techniques. The Polymerase Chain Reaction (PCR) test is the primary modern alternative, offering significantly more reliable diagnosis of active infections. PCR works by directly detecting the virus’s genetic material (DNA) by amplifying minute traces found in the lesion sample.
PCR is much faster, often providing results within hours, and is considerably more sensitive than the viral culture. This makes PCR the preferred method for diagnosing initial infections and for testing older or atypical lesions where the viral load is lower. For individuals who suspect prior exposure but have no active lesions, a serology blood test is used to detect HSV antibodies, revealing past exposure rather than a current outbreak.