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

• Viral load can be used to:
• Predict disease progression.
• Assess the risk of opportunistic infections when CD4 is less than 200 cells/mm³.
• Assess prognosis by use of the viral ‘set point.’
• Gauge likelihood of HIV transmission.
• Evaluate the time to begin antiretroviral therapy.
• Measure therapeutic response to therapy.

Viral load is used as a 'marker of disease progression'. It 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 is continues to replicate in the presence of that drug. By suppressing viral replication as far as possible, the emergence of resistant mutants is delayed, 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 done usually only for research purposes.