Charles Farthing (Chief of Medicine, AIDS Healthcare Foundation) gives expert video advice on: Why do I need to take lab tests if I have HIV? and more...
Why do I need to take lab tests if I have HIV?
If you have HIV, it's very important to have disease monitoring, to have lab tests every three months or, at the most, every four months. The reason for that is, even if a patient is doing well and the virus is very inactive in their body, and the viral load is very low in the blood, the virus can mutate and change and become more aggressive. You never know when that's going to occur. We need to watch it, because if it does start to act badly, then treatment may need to be started. If someone has HIV and they're on anti-retroviral medication, the reason for the repeat tests every three or four months is to make sure that the medication is still working and doesn't need to be changed, and also to make sure that there's no toxicities from the drugs to the liver or kidneys for example, as time goes on. Those toxicities are rare, but if you don't monitor them then you'll miss them and disadvantage some patients.
What lab tests do I need to take if I have HIV?
The HIV lab tests that the doctor does to determine what sort of treatment you need are the CD4 count, which is also known as the T cell Count, the T lymphocyte count or the helper cell count, and the HIV viral load, which is tested by PCR and is sometimes called the HIV PCR. Those two tests enable the doctor to know what to do. They tell us different things. The CD4 count tells us how advanced the disease is, how much immune system has been destroyed. If it's low, that's bad. The HIV viral load tells us how active the virus is. If it is very high, the virus disease is very active. If it's very low, it's very inactive. If the disease is very active, it means that the disease is moving fast and the CD4 count will probably fall over time quite quickly. If the disease is very inactive, the CD4 count may remain stable for a very long time.
What is a 'viral load'?
A viral load simply means the quantity of virus in the blood, the load of virus in the blood. And it is explained in terms of number of copies of virus per millilitre or cc of blood. So a viral load of a thousand copies of virus per cc of blood or per ml is a low viral load. A viral load of greater than 10,000 copies of virus per cc of blood is a high viral load. It can be much higher. A patient who has just been infected could have a viral load of 6 million copies of virus per cc of blood. A patient who is on therapy should have an undetectable viral load. And that is usually expressed as less than 50 copies of virus per cc of blood. The reason it's not read down to zero is that the assays are not very accurate between zero and fifty as to how many copies of virus there actually is. So, usually undetectable is read as less than fifty copies of virus per cc of blood.
What are the different types of viral load tests?
There are a number of different what we call “assays”--different ways of testing the HIV viral load. There is the “PCR” viral load test, which counts the number of copies of the virus, but there's also what they call the “BDNA” viral load test. The viral load tests measure the same thing. They measure the number of viral RNA molecules in the bloodstream, but they do it in slightly different ways.
What is an 'undetectable' HIV viral load?
If you have an undetectable viral load, it doesn't necessarily mean you've got zero copies of HIV in your blood, it means you have less than 50 copies of the virus, and sometimes that means a viral load of zero, and sometimes it might mean a viral load of 20 or 30. The viral load test is usually not read out as 20 or 30 because the accuracy of that determination isn't good enough to give an actual number. Some laboratories report viral load undetectable, which means zero. Other laboratories report just less than 50.
What is a 'CD4 count'?
The CD4 count is a count of the number of CD4 cells in the blood. It's usually expressed per cubic millimetre of blood, but it's really the same as per CC. The CD4 cell is a specialised type of lymphocyte which is a specialized type of white cell in the blood that is the one that gets infected by HIV sometimes called the T-cell or T-helper cell. It's all referring to same thing. So, the CD4 count in a normal individual is between something like 500 and 1500 per cubic millimetre of blood. In someone who has HIV infection, it's often reduced, and we use the cut-off for HIV as 200. If it's below 200 per cubic millimetre of blood, we say the patient has Aids.
How does my CD4 count determine my HIV treatment?
What we use to determine whether or not to treat a patient most is the CD4 count. If it's low, the patient definitely needs treatment. If it's high, they don't absolutely need treatment. The usual cutoff for treatment, if there's no symptoms, is a CD4 count of about 35. Below that, we recommend anti-retroviral therapy. Above that, we're happy with watch and wait so long as there are no symptoms of HIV disease and the person is perfectly well. This is the current guidelines. It may change in the future. As drugs get better and better, and less and less toxic, we may recommend treatment for all people with HIV. Which, as an infectious disease physician, that's what I think would be best. But at the moment, the guideline generally is to start therapy before the immune system is majorly destroyed but when it's being affected significantly and that is around about a CD4 count of 35. Most of us do not use the viral load as a guide to when to start therapy but if it's very high, we know we have to watch that patient more carefully because the CD4 count is more likely to fall quickly.
How does a CD4 count test my immune functions?
The CD4 count tests your immune functions in one specific way. The CD4 test tests the immune system in the part of the system that HIV destroys. So the CD4 test is not a full battery of immune system testing but it is that part of the immune system that you have to watch if someone has HIV infection.
What is a 'complete blood count' or 'CBC'?
A CBC, complete blood count, sometimes called a full blood count, is a count of all the different cells in the blood. The CBC is a very common blood test, done everywhere, not just in people with HIV. A CBC test measures the haemoglobin, how many red cells in the blood, and how many white cells of different sorts.
Why do I need a CBC count if I have HIV?
When we do a CD4 count, we need a CBC test, because we need the white count and the differential to be able to calculate the CD4 number. So when you get a CD4 test, you always have a CBC done.
What is a 'chem-screen' test?
"Chem-screen" is an analysis of various different levels of different chemicals in the blood hence chem-screen. And what the usual chem-screen is looking at is the concentration of salt in the blood. The chem-screen test also includes the liver-function tests which is certain levels of liver enzymes, and the kidney function tests which is the blood level of creatinine. So a chem-screen has multiple results that tell us whether the blood glucose and the kidneys and the liver are functioning well.
How often do I need to take a 'chem-screen' test?
If you are not on any treatment, a chem-screen test doesn't need to be done very often, maybe yearly, just as you would in any individual having a routine health screening. But if you're on any antiretroviral therapy, we recommend a chem-screen three or four times a year to check that there's no toxicities from the medication.