Viral load

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Viral load, also known as viral burden, is a numerical expression of the quantity of virus in a given volume of fluid, including biological and environmental specimens. It is not to be confused with viral titre or viral titer, which depends on the assay. When an assay for measuring the infective virus particle is done (Plaque assay, Focus assay), viral titre often refers to the concentration of infectious viral particles, which is different from the total viral particles. Viral load is measured using body fluids sputum^[1] and blood plasma.^[2] As an example of environmental specimens, the viral load of norovirus can be determined from run-off water on garden produce.^[3] Norovirus has not only prolonged viral shedding and has the ability to survive in the environment but a minuscule infectious dose is required to produce infection in humans: less than 100 viral particles.^[4]

Viral load is often expressed as viral particles, (virions) or infectious particles per mL depending on the type of assay. A higher viral burden, titre, or viral load often correlates with the severity of an active viral infection. The quantity of virus per mL can be calculated by estimating the live amount of virus in an involved fluid. For example, it can be given in RNA copies per millilitre of blood plasma.

Tracking viral load is used to monitor therapy during chronic viral infections, and in immunocompromised patients such as those recovering from bone marrow or solid organ transplantation. Currently, routine testing is available for HIV-1, cytomegalovirus, hepatitis B virus, and hepatitis C virus. Viral load monitoring for HIV is of particular interest in the treatment of people with HIV, as this is continually discussed in the context of management of HIV/AIDS. An undetectable viral load does not implicate a lack of infection. HIV positive patients on long-term combination antiretroviral therapy may present with an undetectable viral load on most clinical assays since the concentration of virus particles is below the limit of detection (LOD).