Virus Load

Updated February, 2009

Chinese Version

Mark Holodniy, M.D

Associate Professor of Medicine

Director of the HIV Clinical Program and AIDS Research Center at Veterans Affairs Medical Center, Palo Alto.

Division of Infectious Diseases and Geographic  Medicine

Stanford University,

Stanford, CA

               During acute HIV infection, plasma HIV RNA levels range from 10⁵ to 10⁷ copies/ml and drop about 1- 2 logs after seroconversion as equilibrium develops between virus replication and immunologic containment. In a chronically infected untreated individual, on the order of 10³-10⁶ virions/ml are present in plasma; whereas the concentration of virus in lymph nodes is usually two-to-three orders of magnitude higher. This steady-state level is highly variable among patients and is strongly predictive of disease progression (19, 21, 35).

               Although plasma HIV RNA levels may be up to 2-fold lower in women than in men during the early stages of HIV infection (1, 10, 31), the levels increase slowly over time (about 0.1 log₁₀ per year) in both women and men and are comparable in patients of either sex with CD4 counts <100 cells/mm³ (1, 31). Although some clinicians believe that the plasma HIV RNA threshold for starting antiretroviral therapy should be lower for women than for men (10), this is not standard practice.

               Acute infections, vaccinations, or even tuberculin skin testing may cause transient increases in plasma HIV RNA levels of up to 0.5 log₁₀ copies/ml in untreated patients or in treated patients with poorly controlled virus replication (11, 18, 22, 28, 32).

Viral Load Monitoring During Treatment

               Within 1-2 weeks of starting highly active antiretroviral therapy (HAART) in a previously untreated patient, plasma HIV RNA levels decrease to about 1% of their pre-treatment levels. A viral load of <50 copies/ml can usually be achieved within 16 weeks. Based on observations such as these, criteria have been established for viral load thresholds that define treatment failure and indicate a change in regimen. For example, patients who have not achieved a 0.5 to 0.75 log₁₀ copies/ml reduction in viral load by 4 weeks, a 1 log₁₀ copies/ml reduction by 8 weeks, or an undetectable viral load (<50 copies/ml) by 20-24 weeks should be considered for a treatment modification or change.

               The nadir viral load level, which depends on a patient's baseline viral load and the potency of the HAART regimen, often predicts the duration of a patient's response to treatment (15). Indeed, previously untreated patients who maintain a viral load <50 copies/ml for six or more months are at extremely low risk of developing virologic rebound provided they do not discontinue their medications (24). In contrast, patients who do not achieve a viral load of <50 copies/ml are more likely to demonstrate virologic failure within the first year (25).

               In patients discontinuing therapy, viral load generally return to the baseline pretreatment level upon discontinuation of HAART (5, 6, 13) (8).

Plasma HIV Viral Load Testing

               Plasma HIV RNA levels are used to determine the likelihood of disease progression, to make decisions about starting antiretroviral therapy, and to monitor response to therapy. Quantitative plasma HIV RNA assays have replaced p24 antigen assays and qualitative proviral DNA assays as the most sensitive means for diagnosing acute HIV infection in seroconverting individuals and for screening blood donations. In clinical trials, plasma HIV RNA levels are also used to assess the efficacy of new antiretroviral drugs and drug treatment regimens. The indications for plasma HIV RNA testing are summarized in Table 1.

Commercially Available Quantitative Assays

               There are three commercial FDA approved assays for quantifying plasma HIV-1 RNA: (i) AMPLICOR HIV Monitor Test Version 1.5 (Roche Diagnostics Systems) is a quantitative PCR assay, (ii) Versant HIV-1 RNA 3.0 Assay (Bayer Diagnostics) is a branched DNA (bDNA) signal amplification assay, (iii) Nuclisens HIV RNA QT (BioMerieux) uses a non-PCR-based method of amplification (NASBA; nucleic acid sequence based amplification). The standard format of the AMPLICOR assay has a linear range of 400 – 750,000 copies/ml; the ultrasensitive format has a linear range of 50 – 75,000 copies/ml. The bDNA assay has a linear range of 50 – 500,000 copies/ml. The Nuclisens assay has a linear range of 200 – 5,000,000 copies/ml.

               All three assays have been shown to be accurate and reproducible in multiple comparative studies (4, 17, 26, 29). Nonetheless, the results of these assays may differ by as much as 0.5 logs₁₀ and clinicians should not test samples from the same patient with different assays (4, 30). Plasma HIV RNA assay variability may also be caused by both technical and biological factors. Technical variability, defined as the standard deviation of testing the same sample multiple times is about 0.2 log₁₀ copies/ml (3, 20). Biological variability, defined as the standard deviation of testing plasma samples obtained over brief periods of time from patients with stable treatment (or no treatment) is about 0.3 log₁₀ copies/ml (2, 3, 7).

               All three assays have improved ability to quantify non-B subtypes relative to earlier versions (9, 23, 33). But whether they are as accurate for quantifying non-B viruses compared with subtype B viruses is not known.

               For analysis with any of the commercially available assays, plasma should be collected in tubes containing the anticoagulants EDTA or ACD (acid citrate dextrose) but not heparin (12, 16, 27). Due to the instability of viral RNA in whole blood, plasma should be separated within four to six hours of collection and should be separated and frozen at –20°C to –70°C if it is not tested immediately (14).

FDA: FDA Approves First Nucleic Acid Test to Screen for Additional Types of HIV In Donated Blood and Tissues. December, 2008. http://www.fda.gov/bbs/topics/NEWS/2008/NEW01936.html

Tables and Figures

Table 1. Indications for Plasma HIV RNA Testing

Footnote:  Adopted from Table 2 of the DHHS guidelines (34). *The IAS-USA guidelines recommend checking plasma HIV RNA levels every 2-3 months in this situation. (36)

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