Treatment training manual

2.14 How CD4 and viral load are related

Although the CD4 and viral load tests measure different things, the pattern of results between each test is usually related.

  • When viral load is low, CD4 counts will be high.
  • When CD4 counts are low, viral load will be high.

A few weeks after infection, HIV viral load is very high, and the CD4 count drops.

Then as the immune system brings viral load down, CD4 counts go back up again.

There is sometimes a time lag between viral load and CD4 changes:

  • After starting HIV treatment (ART) viral load drops quickly. The CD4 count only increases slowly (often over several months).
  • If treatment fails and the viral load level starts to rebound, the CD4 count may take a while before it starts to fall.
  • As viral load gets higher, the CD4 count will nearly always start to fall within a few weeks.

The figure below shows how the CD4 count and viral load curves fit together.

CD4 count and viral load without ART

CD4 count and viral load without treatment

After infection, viral load levels are very high. Then your body fights back and it drops to much lower levels. Over time though, usually over several years, viral load increases again. Viral load continues to rise and the CD4 count continues to fall.

When the CD4 count is very low, the immune system is no longer strong enough to fight off infections. This causes serious illnesses. Some of these infections can be fatal.

Without ART, for nearly everyone, HIV is likely to be fatal.

Effect of ARVs on CD4 count and viral load

CD4 count and viral load with treatment

After infection, viral load levels are very high, but then your body fights back and it drops to much lower levels. Over time though, usually over several years, the levels of virus increase again. It is usually high (around 50,000-100,000 copies/mL) by the time that your CD4 count drops to around 350 cells/mm3.

After starting treatment, viral load falls quickly and CD4 counts rise slowly.

If ART brings viral load down to less than 50 copies/mL, then treatment can last for many years.

Last updated: 1 January 2016.