Viral Culture

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What is viral culture?

Viral culture is a method that doctors use to grow and detect any viruses that may be present in an STD testing sample or other biological sample. For example, it could be used to grow herpes virus from a herpes sore. Viral culture is slightly more involved than bacterial culture. This is because, unlike bacteria, viruses cannot replicate on their own. A bacterial culture simply involves incubating the sample in an appropriate media (solution) where the bacteria can grow. In contrast, viral culture requires using the sample to infect susceptible cells. The virus is then allowed to grow and replicate within those cells until it reaches detectable levels.

Viral culture can take various amounts of time depending on:

  • What virus is being cultured
  • What types of cells are being used
  • The techniques used in the culture process

However, regardless of the precise methodology, viral culture is slow. In particular, viral culture is generally a more time-consuming process than nucleic-acid amplification testing (NAAT). It also requires a greater degree of skill. This skill is needed both in the laboratory and from the clinician taking the sample. Why? Skill is required in order for the virus to remain infectious and intact. Because of this, viral culture may not be available at all STD testing sites.

As with many types of STD testing, a viral culture is often only the first step in diagnosing an unknown infection. After a virus has been cultured it still needs to be identified. A virus can be identified using a variety of techniques. These include including nucleic acid tests, antibody-based tests, and electron microscopy.

Viral culture is also used in research settings. It is a way for scientists to produce large amounts of virus in vitro. However, some types of viruses can be extremely difficult to grow and purify in the lab.

Examples:

Viral culture and/or nucleic-acid amplification testing of material from visible sores is the gold standard for herpes testing. However, this type of testing can only done in people who have symptomatic genital sores. Screening asymptomatic individuals for genital or oral herpes requires the use of blood tests. These detect antibodies against specific strains of the virus rather than the virus itself. Finding such blood tests may be difficult, however. Some doctors are unaware of their existence. Other doctors are reluctant to use these tests because of the risk of a false positive combined with herpes stigma.

Herpes viral culture can be done using a variety of cell types. Unfortunately, it is not equally effective at all stages of a herpes outbreak. Viral culture is very good at detecting the herpes virus in vesicular and pustular herpes lesions (>90 percent). It is far less effective at detecting the virus in ulcerative lesions (~70 percent). The detection rate in lesions that have started to crust over falls to only 27 percent. In addition, the speed at which samples are transported for testing, and whether they are refrigerated properly, can also have significant effects on the effectiveness of viral culture.

Viral culture are held up as the gold standard of herpes testing. However, it can be a difficult test to do well. For a number of reasons, it is possible to have a negative viral culture result even when someone clearly has herpes symptoms and is positive via blood test. Such false negatives may occur, for example, if testing was done at an inappropriate stage of the outbreak. They can also occur if the sample was improperly transported and stored. As such, it may be the gold standard, but it's not always the best choice.

Note: People looking for definitive herpes test results are generally advised to visit the doctor as soon as an outbreak begins. Doing so gives their physician the greatest chance of being able to test the sores at the point when viral culture is most effective.

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