What do CD4 and viral load numbers mean?

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If you’ve tested positive for HIV, it’s important to get to grips with these two important terms:

  • CD4 count
  • Viral load

The CD4 count
CD4 cells are white blood cells that fight viral and bacterial infections. They’re also known as T-cells, or T helper cells.

HIV targets and infiltrates the CD4 cells in the human body, tricking the cells into making more copies of HIV. It’s this mechanism that makes HIV such a tricky virus to control.

HIV targets and infiltrates the CD4 cells in the human body. The CD4 cells replicate in response to the presence of HIV and, as a result, more copies of HIV-infected CD4 cells are made.

In other words, HIV tricks the cells into making more copies of HIV. It’s this mechanism that makes HIV such a tricky virus to control.

Your CD4 count is indicated in cells per cubic millilitre (mm3) and is measured by taking a blood sample. A normal CD4 count is between 800 and 1,500 cells per mm3, although this varies between individuals.

If your CD4 count is decreasing, it means that your immune system is becoming less effective at fighting off infections. CD4 cell counts are the best predictors of the risk of opportunistic diseases in people with HIV/AIDS.

If you’re HIV-positive, you should start antiretroviral treatment (ART) regardless of your CD4 count. Generally speaking, a CD4 count below 500 cells per mm3 in someone who is HIV-positive indicates immune suppression and vulnerability to opportunistic infections. Once the CD4 drops to below 350 cells per mm3, the risk is even higher. 

Here is a quick guide to CD4 counts:

  • Between 500 and 1500 cells per mm3 = range of normal CD4 count
  • Above 350 cells per mm3 = HIV treatment is advisable
  • Below 350 cells per mm3 = HIV treatment is recommended

Note that your doctor may give your CD4 results as a percentage:

  • Above 29% = similar to a CD4 count of above 500 cells per mm3
  • Around 22% = similar to CD4 count of 350 cells per mm3
  • Below 14% = similar to a CD4 count of below 200 cells per mm3

What is ‘viral load’?
The CD4 count is one measure of HIV progression; the other is viral load.

This refers to the amount or concentration of virus in the blood and is an indication of how sick an HIV-infected person is. The higher the viral load, the more progressive the HIV disease.  

If you’ve recently been infected with HIV, your viral load is likely to be high. This will decrease over time, as your treatment kicks in. 

The viral load can be “counted” by doing a blood test such as the polymerase chain reaction (PCR) test, which measures the number of HIV copies in your blood. The bDNA (branched DNA) and NASBA (nucleic acid sequence-based amplification) methods are also used to test viral load. The result is expressed as the number of copies of HIV per millilitre of blood. 

If you’re on ART, your viral load should be lower than detectable limits. If the viral load is detectable, it means that the treatment isn’t working well enough. This could either be because of non-adherence (i.e. you’re not taking your medication exactly as prescribed) or because the virus has developed resistance to the drugs. 

Here’s a quick guide to viral load count in South Africa (note that different countries use different cut-off counts):

  • Over 1,000 copies/ml = high
  • Less than 50 – 1,000 copies/ml = no change to treatment is needed but adherence issues should be addressed 
  • Below 50 copies/ml = “undetectable” or suppressed (i.e. the virus is still present, but your treatment is successfully controlling it)

Note that tests made by different manufacturers might give slightly different results. This is why your doctor or clinic nurse should use the same type of test every time.

The ‘dance’ between viral load, CD4 count and disease progression
Disease progression, or the extent to which an HIV-infected person gets sick with opportunistic diseases and infections, will depend on the viral load as well as the CD4 cell count.

The higher the viral load, the lower the CD4 cell count – and the higher the risk of infections.

Opportunistic infections associated with advanced disease are more likely to occur once your CD4 count falls below 350 cells per mm3. The types and numbers of infections may also increase as your CD4 count drops to below 200 cells per mm3.

This is known as the AIDS phase, and the viral load is likely to be at significantly higher levels. The progression to the final phase of AIDS (and death) will be much faster.

Conversely, a lower viral load goes hand in hand with a higher CD4 cell count, because fewer viruses in the blood give the immune system a chance to build up its resources again. If you’re HIV-positive, and have a low viral load and a high CD4 count, you can remain healthy for years because your immune system is strong enough to fight off infections.

If you’re HIV-positive, it’s important to get your CD4 and viral load tests done regularly. These results will give you essential information about the effect HIV is having on your body. The aim of HIV treatment is to have a very low or undetectable viral load and a high CD4 count. 

The Universal Test and Treat (UTT) policy
Previously in South Africa, treatment started when an HIV-positive individual’s CD4 count dropped to 200 or less or when certain opportunistic infections became evident – whichever came first.

The CD4 count for treatment eligibility was later changed to 350 cells per mm3 or less, and then later on to 500 cells per mm3 or less. Then, in September 2016, a Universal Test and Treat (UTT) policy was introduced for all HIV-positive people. 

This means that all HIV-positive children, adolescents and adults, regardless of CD4 count, are offered ART, prioritising those with a CD4 count of 350 cells per mm3 or less.

Once treatment is started, there’s normally an increase in CD4 count and a corresponding decrease in viral load.

Reviewed by Dr Pooja Balani, MBBS (UK). Medical Technical Advisor at the Southern African HIV Clinicians Society. March 2018.

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