The plasma level of viral RNA, as determined by various techniques including target amplification assay by reverse transcriptase polymerase chain reaction and branched DNA technology with signal amplification. Because levels of detection vary with method, results of testing by different methods are not comparable.
Serial measurement of HIV viral load is a standard procedure to monitor the course of AIDS. Reported as the number of copies of viral RNA per mL of plasma, assessment of viral load provides important information about the number of lymphoid cells actively infected with HIV. This laboratory procedure has supplanted the CD4 count as an indicator of prognosis for people infected with HIV, in determining when to start antiretroviral therapy, and in measuring the response to therapy. Because the CD4 count is regarded as superior in determining the level of immune compromise and the risk of opportunistic infection, both tests are currently used. The U.S. Department of Health and Human Services advises starting antiretroviral therapy when plasma HIV RNA concentration exceeds 10,000–20,000 copies/mL. The International AIDS Society favors an action level of 30,000 copies/mL. When, as a result of treatment, the number of copies of viral RNA falls below the level that can be detected by standard methods, replication of HIV is considered to have been suppressed. In no case, however, has AIDS been cured, nor has viral proliferation remained arrested after cessation of antiretroviral therapy.
Reference: Stedman's Medical Dictionary