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Human Immunodeficiency Virus Viral Load

Acute or primary HIV-1 infection is defined as the period from initial exposure and viral infection to development of an HIV-1 antibody response. During this time, patients often experience a mononucleosis-like viral syndrome. Patients with acute HIV-1 infection typically have viral loads between 250, 000 and 9.5 million copies/mL. Patients whose HIV viral load is greater than 100,000 copies/mL within six months of seroconversion are ten-fold more likely to progress to AIDS within five years than those with lower viral loads.

The CD4 cell count is the major determinant of initiating antiretroviral therapy. Most experts recommend starting therapy when the CD4 cell count is between 200 and 350/uL (JAMA 2002; 288:222-35). However, individuals with higher CD4 counts who have a viral load is above 50,000 to 100,000 copies/mL need to be closely monitored because the CD4 count decreases more rapidly in untreated persons. Initiation of antiretroviral therapy may be considered in individuals with a viral load above 50,000 to 100,000 copies/mL

In the asymptomatic individual, CD4 cell counts and viral load are measured to assess antiretroviral therapy. Viral load is usually measured at 4, 8 to 12, and 16 to 24 weeks. A decrease in viral load indicates a reduction in virus replication. Effective antiretroviral therapy typically reduces viral load by more than 1 log10 (10 fold or 90%) within 8 weeks of treatment. Failure to attain a 90% reduction by 4 weeks of therapy suggests poor patient compliance, viral resistance or inadequate dosage. Because of assay imprecision, a 30 to 50% (0.2 to 0.3 log) reduction in RNA copies/mL is the minimum change that should be considered to be a significant change in viral load.

Measurement of HIV-1 viral load is a critical tool for monitoring antiviral treatment response in infected individuals. Two sequential measurements of viral load below the lower limit of detection (50 copies/mL) indicate viral suppression. Once viral suppression has been achieved, viral load is usually measured every 8 to 12 weeks. More frequent monitoring may be appropriate for individuals with intercurrent illness, change in therapy, or questionable medication compliance. Frequent monitoring often reveals small increases in viral load in the range of 50 to 400 copies/mL. These small isolated increases do not indicate subsequent treatment failure.

Quantitation of viral load may also be helpful in predicting perinatal transmission of HIV-1 disease. Perinatal transmission is likely when maternal plasma HIV RNA exceeds 50,000 copies per mL, but is unlikely with levels below 20,000 copies per mL.

In summary, the clinical indications for HIV viral load testing include:

  • Diagnosis of HIV-1 infection in individuals with acute or early HIV-1 infection.
  • Diagnosis of HIV-1 infection in infants of less than18 months of age born to infected mothers
  • Determine viral load in HIV-1-infected individuals:
    • To establish a baseline before initiating antiretroviral therapy
    • To monitor disease progression while on or off antiretroviral therapy
    • To investigate possible HIV-1 drug resistance
    • To determine noncompliance with antiretroviral therapy

Assays used to quantitate viral load in the United States are most accurate and sensitive for subtype B, the predominant strain in industrialized countries. Subtype B is far less prevalent in developing countries and dissemination of non-B subtypes is anticipated. Several studies have reported the failure of conventional assays to accurately detect and quantitate non-B subtypes. New assays recently cleared by the FDA for diagnostic use target the ltr region in addition to the gag region that was targeted previously. Specificity for both targets provides more stable detection and quantitation for all subtypes.

In addition, the new assays offer improved sensitivity. HIV-1 viral load can now be quantitated down to 20 copies/mL, versus the previous lower limit of 50 copies/mL. Results are expressed as HIV-1 RNA copies per mL of plasma. Reference value is undetected.

Specimen requirement is one lavender top (EDTA) tube of blood. Specimen stability is 5 days for plasma that is stored refrigerated and 35 days for plasma stored frozen.


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