Viral load

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 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.

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. [DHHS Panel on Antiretroviral Guidelines for Adults and Adolescents. Guidelines for the Use of Antiretroviral Agents in HIV-1-Infected Adults and Adolescents. May 4, 2006. (available for download from [http://www.aidsinfo.nih.gov/OrderPublication/OrderPubsBrowseSearchResultsTable.aspx?from=G&searchString=&MenuItem=Guidelines AIDSInfo] )]

An affordable, largely manual test, which has the European regulatory approval for clinical use, is also used for viral load monitoring (ExaVir Load from Cavidi AB, http://www.cavidi.com). It depends on measuring virus-associated reverse transcriptase (RT) activity [Malmsten A, Shao XW, Sjödahl S, Fredriksson EL, Pettersson I, Leitner T, Källander CFR, Sandström E, Gronowitz JS: Improved HIV-1 viral load determination based on reverse transcriptase activity recovered from human plasma. J. Med. Virology 76:347-359, 2005.] and can therefore detect all types and subtypes of HIV. The technology does not require sophisticated laboratories and can therefore make viral load testing more accessible in all parts of the world. [Greengrass VL, Turnball SP, Hocking J, Dunne AL, Tachedjian G, Corrigan GE, Crowe SM. Evaluation of a low cost reverse transcriptase assay for plasma HIV-1 viral load monitoring. Curr HIV Res 3(2):183-190, 2005.] [ Jennings C, Fiscus SA, Crowe SM, Danilovic AD, Morack RJ, Scianna S, Cachafeiro A, Brambilla DJ, Schupbach J, Stevens W, Respess R, Varnier OE, Corrigan GE, Gronowitz JS, Ussery MA, Bremer JW. Comparison of two human immunodeficiency virus (HIV) RNA surrogate assays to the standard HIV RNA assay. J Clin Microbiol. 2005 Dec;43(12):5950-6.]

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.

References