HIV attacks immune system cells called CD4 cells. HIV takes over these cells and turns them into virus factories that produce thousands of copies of HIV. As the virus grows, it damages or kills CD4 cells, weakening your immune system.

Viral load is the amount of HIV in your bloodstream. The higher the amount of HIV, the greater the risk of your immune system being damaged.

Viral load can be measured by two different lab tests: a polymerase chain reaction (PCR) test or a branched DNA (bDNA) test. Both tests are accurate, but each has a different way to measure the amount of virus. It is best to stick with the same kind of test and not switch between the two or it will be difficult to compare results over time.

Viral load results are reported as copies of HIV in one milliliter of blood. The lower the number, the less virus there is in your blood. Numbers can range from about one million copies to as few as 50 copies. If you have less than 50 copies, your health care provider may tell you that your results are “undetectable.”

Being undetectable is the best result because it means your virus is under control. However, undetectable does not mean that you have been cured of HIV; it just means that there is not enough virus for the test to measure (below 50 copies).

Viral load tests are an important tool to:

• Monitor HIV progression
  While CD4 cell counts measure how healthy your immune system is today, viral load tests can help figure out whether you’re at risk for more immune damage in the near future. A viral load test tells you how active HIV is in your body. When compared over time, viral load results show if the amount of HIV in your bloodstream is higher or lower than it was before. The higher your viral load, the more active HIV is and the more likely you are to lose CD4 cells in the future.
• Measure how well HIV drugs are working
  HIV drugs prevent the virus from making copies of itself (reproducing). When a combination of HIV drugs (regimen) is working, the viral load usually goes down within weeks of starting the drugs. If your viral load goes up while taking HIV drugs, you and your health care provider should take another viral load measurement. If that result is the same or higher, your drugs may not be working as well as they should. You should talk to your health care provider about switching drugs. You should also tell him or her if you are having any problems taking your HIV drugs on schedule. If you do not take your drugs correctly, it can cause problems that may lead to an increased viral load.

One goal of HIV treatment is to keep viral load levels as low as possible for as long as possible. This gives you the best chance of staying healthy. With effective HIV treatment regimens, viral load can be reduced to undetectable in many people. This is a great result. It means that your HIV medications are working and you should continue to take them. HIV is still in your body. If you stop taking your HIV drugs, the virus usually starts reproducing and your viral load will increase.

It is also important to know that even with an undetectable viral load you might still infect someone else with HIV if you share needles or have unprotected sex.

Viral load levels used to be used with CD4 cell counts to determine when you need treatment. Now guidelines put out by the US government recommend using CD4 cell count as the main way to determine when someone should start HIV treatment.  However, the guidelines state that having a high viral (100,000 or more) is a reason to start treatment sooner rather than waiting.

The US treatment guidelines also provide recommendations on when to have viral load tests:

If you are not on HIV treatment, have a viral load test

• When you start receiving HIV care
• Repeat test every three to six months

When you start treatment, have a viral load test

• When starting and two to eight weeks after treatment was started
• Repeat test every one to two months until viral load becomes undetectable

Once you are on a stable treatment regimen and undetectable, have a viral load test

• Every three to six months

When you change treatment, have a viral load test

• When changing and two to eight weeks after treatment was changed
• Repeat test every one to two months until viral load becomes undetectable

If your regimen is working, your viral load should become undetectable within six months of starting treatment. If this does not happen or if your viral load becomes detectable on stable therapy and keeps increasing, it can signal that your regimen isn’t controlling HIV as well as it should. You and your health care provider should consider all possible reasons (problems with drug absorption, adherence, or drug resistance) and take steps to correct the problem, including additional testing and considering changing drug treatments.

Our understanding of viral load has grown since 1996, when the first viral load test was approved and began to be widely used. Most early clinical trials that studied the role of viral load looked primarily at groups of men. Women were not enrolled in enough numbers in these trials for anyone to know whether there were sex-based differences in viral load.

Since 1996, a number of studies have compared viral load levels between groups of men and women. Some of these studies have found sex differences in viral load. At similar CD4 cell counts, women tend to have lower viral load levels than men. The differences seem greatest during the early course of HIV infection.

When women and men with the same viral loads are compared, women generally progress faster. In other words, women may develop AIDS at lower viral load levels than men with similar CD4 counts.

This may be a reason for women to start treatment earlier than men. However, evidence is not strong enough for it to be included in the US treatment guidelines and recommendations for starting HIV treatment are the same for men and women.

Because the current US treatment guidelines base decisions for starting treatment more on CD4 cell count than on viral load, this probably does not impact treatment for many women. However, women should be aware that a viral load considered moderate in men may actually be high for them.

If you are thinking about starting or switching treatment, it is important to take into account your viral load, CD4 cell count, other labs results, and how you are feeling. Talk to your health care provider about the best treatment plan for you. The good news is that the differences in viral load do not seem to affect how well women respond to treatment.

Researchers are looking into whether reproductive hormones, contraceptives, and immune cells influence viral loads in women. One study found that certain cells in women produced more of an immune-signaling protein than in men. This protein can lead to immune system over activation which could cause faster progression to AIDS. This may explain the sex differences in disease progression at similar viral loads. Further research is needed to check whether differences in disease progression are linked to sex hormones.

It has also been noted that viral load and the level of certain HIV drugs go up and down during the course of a single menstrual cycle. This could have an impact on drug dosing and timing of viral load tests in women.

Understanding more about sex differences in viral load will lead to better care for HIV+ women. In the meantime, following the US treatment guidelines for viral load testing is an important way for you and your health care provider to check your HIV infection and how you are responding to HIV treatment.