Viral load

Overview
Viral load is a measure of the severity of a viral infection, and can be calculated by estimating the amount of virus in an involved body fluid, for example, it can be given in RNA copies per milliliter of blood plasma. Determination of viral load is part of the therapy monitoring during chronic viral infections and in immunocompromised patients, e.g. after bone marrow or solid organ transplantation. Currently, routine testing is available for HIV-1, cytomegalovirus, hepatitis B virus, and hepatitis C virus.

HIV viral load test
Several different HIV viral load tests have been developed, and three are currently approved for use in the US:


 * Amplicor HIV-1 Monitor test (Hoffman-La Roche), better known as the PCR test
 * NucliSens HIV-1 QT, or NASBA (bioMerieux)
 * Versant/Quantiplex HIV-1 RNA, or bDNA (Chiron/Bayer)

These tests have been approved by the Food and Drug Administration in the United States for use in monitoring the health of people with HIV, in conjunction with other markers. Higher numbers in the viral load tests indicate an increased risk of getting sick from opportunistic diseases. These tests are also approved for monitoring the effects of anti-HIV therapy, to track viral suppression and detect treatment failure. Successful combination antiretroviral therapy should give a fall in viral load of 1.5 to 2 logs (30-100 fold) within six weeks, with the viral load falling below the limit of detection within four to six months.

An inexpensive, largely manual HIV viral load assay has also been developed which depends on measuring virus-associated reverse transcriptase (RT) activity (ExaVir Load Version 2)([Cavidi AB], Uppsala, Sweden). This test has a lower limit of detection of 400 RNA copies/ml. The test was developed primarily for use in resource-limited environments, such as southern Africa.

Viral load tests can also be used to diagnose HIV infection, especially in children under 18 months born to mothers with HIV, where the presence of maternal antibodies prevents the use of antibody-based (ELISA) diagnostic tests.

Persons with HIV are most contagious during the earliest stages of infection, when an antibody test would yield a negative result. Therefore, the importance of viral load testing is deemed important for yielding an earlier HIV diagnosis. Since persons are most contagious during early infection, widespread testing could provide significant public health benefits.

Results
The results of these tests are usually given as number of HIV RNA copies per milliliter (ml) of blood. The PCR test may give the number of HIV RNA copies per 0.05/ml, so one would multiply the result by 20 to get the standard result.