Understanding HIV-Related Lab Tests II: Viral Load, Resistance and Tropism

Table of Contents

• Lab Tests Are Important Tools
• Viral Load Test
• Resistance Test
• Tropism Test
• Hypersensitivity Test for Ziagen
• Bottom Line

Lab Tests Are Important Tools

Having regular lab tests is a necessary part of caring for your health. If you are living with HIV (HIV+), lab tests are especially important tools that help you and your health care provider keep track of how you are doing in the following areas:

Immune System Status

• How HIV is affecting your immune system
• If you need to start or switch HIV drugs
• Example of lab test: CD4 count (see our info sheet on Understanding CD4 Cells and CD4 Cell Tests)

Overall Health

• How your body's organ systems are functioning
• If you have side effects from the drugs
• If you are having problems that are not related to HIV
• Examples of lab tests: complete blood count, blood chemistry tests, blood fat and sugar tests (see our info sheet on Understanding Lab Tests I: Complete Blood Count and Blood Chemistry)

HIV Illness

• How HIV is responding to the drugs you are taking
• Which drugs to use
• The specific characteristics of your virus
• Examples of lab tests: viral load test, resistance test, tropism test (explained below)

Viral Load Test

Viral load is the amount (number of copies) of HIV in your bloodstream. Viral load can be measured by several different lab tests: a polymerase chain reaction (PCR) test, a branched DNA (bDNA) test, or a nucleic acid sequence-based assay (NASBA). All these 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 among them 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 fewer than 50 copies, your health care provider may tell you that your results are "undetectable."

Being undetectable is a great result because it means your virus is under control. However, undetectable does not mean that you have been cured of HIV or that you cannot pass it to others. It just means that there is not enough HIV for the test to measure. It is also important to know that labs that test viral load have different cut-offs below which they cannot detect HIV. For example: you could have 35 copies of HIV in your blood, and in lab #1, which cannot detect any HIV below 50 copies, your viral load would be considered 'undetectable.' However, in lab #2, which cannot detect any HIV below 20 copies, your viral load be considered detectable.

Viral load tests are an important tool to:

• Check HIV progression
  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 greater the chances that your immune system is being damaged.
• Measure how well HIV drugs are working
  HIV drugs work by preventing the virus from making copies of itself (reproducing). When a combination of HIV drugs (your drug 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, your drugs may not be working as well as they should.

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 drugs are working and you are doing a great job taking them. However, HIV is still in your body. If you stop taking your HIV drugs, the virus usually starts reproducing and your viral load will go up.

When Should You Get a Viral Load Test?

• 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 drug regimen is working, your viral load should become undetectable within six months of starting treatment. If this does not happen, if your viral load stays detectable on stable therapy, or if your viral load keeps increasing, it can signal that your regimen is not controlling HIV as well as it should. It is important that you and your health care provider discuss all possible reasons (e.g., problems with your body's ability to absorb the drug, problems sticking to your regimen, drug resistance) and take steps to correct the problem. These steps may include additional testing and considering changing HIV drugs.