CD4 and Viral Loads

The CD4 count and viral load are the two measures which are most commonly used to determine how HIV may be impacting on the immune system and how much HIV is in the body.


This measure is an indication of the strength of the immune system.


CD4 cells are an extremely important part of our immune system, HIV attacks these cells which can affect the number of them in the body.  Doctors will test a cubic ml of blood, and use this to determine the CD4 count i.e. the number of CD4 cells in the sample.  This will then allow the doctor to see how healthy a person’s immune system is. The higher the CD4 count, the healthier the immune system is.


It is important to acknowledge that your CD4 count will have natural variations and these do not mean that you necessarily have a stronger or weaker immune system.


It has been said that a more reliable figure to give an indication of the immune system is the CD4 percentage.  Your HIV team will have both your CD4 count and your CD4 percentage and if you ask they will be more than happy to tell you your results.



This measure indicates how much HIV there is per cubic millilitre of blood.


The lower this number, the less HIV is in the system and the less damage it can cause.  The aim of putting people on treatment is to get the viral load count to undetectable levels.  This means that the amount of HIV in the system is at such low levels that ill-health is unlikely due to the HIV and the risk of passing on HIV is in effect close to zero.


Viral loads will vary between individuals, but the vast majority of people will quickly achieve an undetectable viral load once treatment has started.



This is mainly only achieved when taking HIV medication.  The term undetectable viral load can be misleading to some people.  Although it is called undetectable, there is still HIV present in the body.  It is called undetectable as the devices used to determine the viral load cannot detect HIV if there are fewer than 40 to 50 copies of HIV per cubic millilitre of blood.


Having an undetectable viral load is the aim of treatments and means that:


  • treatments you are taking are working
  • HIV is present in very low levels
  • HIV is causing less damage to the body
  • the likelihood of passing on HIV is in effect close to zero



If someone is not on treatment, the viral load and the CD4 count may be factors that determine when the doctors think it is appropriate to start doing so.  However, current UK guidelines state that anyone living with HIV can start treatment when they feel that the time is right.  You can talk to your HIV consultant about this.

Living with HIV? Want to talk to us?
Call 0161 274 4499 or email: