We usually measure viral load in blood. Several other important places in the body have barriers that limit both HIV and HIV drugs from moving freely. These are called compartments or sanctuary sites.

Viral load in these compartments can differ from viral load in your blood.

These compartments include the genital tract, the CSF (cerebral spinal fluid – the fluid that circulates around the brain and spinal column), and the brain itself.

• HIV can develop independently in these compartments. Some drugs get into these compartments better than others.
• Resistance can be different in different compartments. It will usually develop in one compartment but can then travel to other sites.
• Viral load levels can be different in each compartment.
• People who keep their viral load undetectable (below 40 or 50 copies/mL) in their blood have a very high chance of having undetectable viral load in their CSF.
• About 10% of people with an undetectable viral load in their blood have a detectable viral load in other compartments, including genital fluids. This is why using a condom is still important even when your blood viral load is undetectable.

This makes HIV a very complicated illness.

Current research suggests that the direction of infection is lymph → blood → compartments. But because HIV can develop independently in a compartment, there is also a concern that infection can travel in both directions. For example if resistance developed in a compartment site this might then cause resistance in the rest of your body.

In practice, because blood is used for most tests, you are unlikely to know exactly what is going on in other compartments.

Reducing HIV in blood and lymph nodes seems to stop HIV related complications in other compartments (eg in the brain) even if HIV drugs don’t penetrate these parts of the body.

This is a complex area of research because testing viral load accurately in compartments other than the blood is difficult.