CD4 cell counts<7>

• Changes in CD4 cell count are helpful in showing whether or not HIV is damaging the immune system.

• CD4 cell counts are measured by testing the blood, so patients are asked to give a sample of blood when they visit their clinic.

• A normal CD4 cell count in a person without HIV varies between 400 and 1600 cells/mm³. Smokers tend to have higher counts than non-smokers, and women tend to have higher counts than men.

• Immediately after HIV infection your CD4 cell count is likely to fall to a level several hundred cells below the previous normal level.

• The average decline in CD4 cell count each year is around 40 cells/mm³. Every decrease of 100 cells/mm³ doubles the risk of developing AIDS.

• If you are HIV-positive, your CD4 cell count may go up and down for reasons which have nothing to do with HIV infection. Your CD4 cell count can be affected by the time of the day you are tested, by colds and flu, smoking, and in women, your monthly period.

• The CD4 cell count should be checked every few months to see whether it is stable or falling.

• If the CD4 cell count falls below 200 cells/mm³, the risk of developing Pneumocystis pneumonia (PCP) increases. Medicine to prevent you from getting PCP can be prescribed. This type of preventive medication is called prophylaxis.

• Some hospitals also recommend prophylaxis against Mycobacterium avium intracellulare, or MAI, if the CD4 cell count falls below 75 or 100 cells/mm³.

CD8 cell counts

• CD8 cell counts may give an indication of how active the immune system is against HIV.

Other markers

• Other markers which may be measured include neopterin and beta-2 microglobulin, but these are not as important as the CD4 count or viral load, and are very rarely measured except for research purposes.

Viral load

• Viral load tests are also known as viral load assays, PCR tests or bDNA tests.

• The tests detect the amount of HIV in your blood.

• Your viral load will be measured by testing your blood, so you will be asked to give a sample of blood when you visit your clinic.

• The amount of virus is counted in the form of `copies' per millilitre (ml) of blood. 10,000 copies per ml is low, and more than 100,000 copies per ml is high.

• Each test has a limit below which it cannot detect HIV's genetic material. This is known as the limit of detection, or undetectable viral load. This limit is currently 50 copies, but some experimental tests can detect HIV down to the level of 20 copies or 10 copies per ml.

• Viral load can go up briefly if you have an infection, or if you've just had a vaccination.

• Single viral load results don't matter as much as the trend in your viral load results. If your viral load goes up and stays up, this is worse than if it goes up and then goes down.

• Viral load changes are so huge that they are often counted as ten-fold, hundred-fold or thousand-fold changes. These changes are also described as log changes - 1 log is a ten-fold change, 2 log is a one hundred-fold change, and so on.

Drug resistance tests

• If HIV develops resistance to the drugs you are taking, there are two types of tests which can be used to work out which drugs are no longer working.

• A genotypic resistance test looks for changes in the genes, and will also give clues about which other drugs might work in the future. This test may not be able to show how high the level of resistance might be.

• A phenotypic resistance test will show how much of the drug is needed to control HIV. As HIV develops resistance to a drug, the amount needed to control it will go up. This test won't show which other drugs will definitely work against HIV.

• Both these tests are still experimental.

Viral load changes after starting treatment

• Changes in viral load after you start treatment are the best predictor of whether or not treatment will delay the development of AIDS.

• The length of time that viral load is kept below 5000 copies/ml on treatment is clearly related to the length of time for which disease progression can be delayed.

• Lowering viral load to beneath 400 copies/ml results in a reduced risk of disease progression compared with lowering viral load to between 400 and 5000 copies/ml.

• A reduction below 5000 copies/ml is better than a viral load reduction which doesn't take you below 5,000 copies. So although a reduction in viral load below 50 copies/ml is currently recommended as the ideal outcome of treatment, a reduction to somewhere between 400 and 5000 copies/ml will nevertheless have a very good long-term clinical benefit.

• These viral load changes appear to have the same impact on your future risk of developing AIDS-related illness whether or not you've taken any anti-HIV treatments before.