Think of HIV as a car speeding towards an accident, which is the point at which you become ill. The viral load tells you how fast the car is speeding and the CD4 count shows how many miles of road are left before the accident.

Viral load tests measure the number of HIV particles in a sample of your blood. The result of a measurement is given as the number of ‘copies’ of HIV per cubic centimetre of blood. There are several different viral load tests in use at present. Each test uses slightly different technology to measure the number of HIV particles, but all tests are equally reliable. It is important, however, that you are always tested with the same type of viral load test.

Continuous monitoring of viral load levels along with other indicators help you to decide whether to start anti-HIV treatment. If the treatment is successful, your viral load levels will fall. The goal of anti-HIV therapy is to reduce viral load levels so that they are undetectable. Even if your viral load does become undetectable, this does not mean that the Virus has been eradicated from your body, but that its level has been greatly reduced and disease progression slowed down or stopped.

There are currently two levels of viral load testing - ordinary and ultrasensitive. The new ultrasensitive viral load tests have the ability to probe lower levels of virus (20 or 50 copies as compared to the conventional 400 or 500 copies). This gives a more accurate picture of viral activity.

Your blood should be tested regularly (about every 3 months) to check both CD4 cells and viral load. If there are any sudden changes, unexpected results or 'blips' a second test should be taken much sooner. If you start or change anti-HIV therapy you should get an extra test within a month to see how it is working. The results of your tests should normally be available within 2 - 3 weeks.

The moment you start taking the drug for which a Mutation has made the virus resistant, the mutant is better suited to survive in the new drug-containing environment and it will quickly become the dominant strain as the non-resistant virus is killed off.  This is the reason that HIV specialists believe that all virus replication must be suppressed and that viral load levels should be kept as low as possible for as long as possible.

When HIV faces a combination of anti-HIV drugs, to which it would need to make many mutations to become resistant, the odds against the virus making such a successful sequence of changes are strongly stacked against it.  But even if one drug in a combination loses its power to suppress HIV, the continued success of the whole combination can be threatened.  It is possible to directly measure the Resistance characteristics of the HIV in your body using a resistance test.