Uses of viral load testing

Viral load refers to the amount of HIV circulating in the blood plasma (the fluid between blood cells). Viral load is sometimes referred to as HIV RNA. The number indicates the number of viral copies per millilitre of blood (written as copies/ml). Viral load can range from below 50 to over one million copies/ml. The test is performed using a sample of blood taken from a vein in the arm. Viral load is used to:

• Predict disease progression.
• Assess the risk of opportunistic infections when CD4 is less than 200 cells/mm³.
• Assess prognosis in early stage infection by use of the viral ‘set point’.
• Gauge likelihood of HIV transmission.
• Evaluate the time to begin antiretroviral therapy.
• Measure response to therapy.

Viral load is used as a surrogate marker of disease progression because there is a significant association between decreased viral load and clinical outcome. Viral load indicates how much virus is available to damage the immune system, but does not measure immune function. If HIV infection is left untreated, generally the viral load will steadily increase over time. Higher levels of HIV indicate that the virus is causing considerable damage to the immune system. A rise in viral load is often followed by a decrease in CD4 count and subsequent illness.

One of the most important reasons to suppress viral load as far as possible is to minimise the risk of HIV developing resistance to the drugs being taken. HIV can only develop resistance to a drug if it continues to replicate in the presence of that drug. Suppressing viral replication as far as possible delays the emergence of resistance, prolonging the effectiveness of therapy.

Viral load tests may be used to diagnose acute HIV infection prior to the emergence of antibodies to HIV, but there is a high rate of false positives (2 to 9%). In general, viral load testing is not an appropriate diagnostic test for HIV infection.

Special viral load tests may be done to determine how much HIV is inside blood cells, the brain, the central nervous system, genital fluids (such as semen), and in lymph tissue. These tests are usually done for research purposes only.