A bit about my background
I have been involved in HIV care since 2005. I was running one of the designated Anonymous HIV Testing Sites in Singapore. I am also part of the Singapore HIV PrEP Taskforce and am one of the authors of the Singapore HIV PrEP prescribing guidelines. I also am one of the authors of the Blueprint to end AIDS and HIV transmission in Singapore. I have written multiple articles on HIV for magazines and other media. And have been interviewed on the topic.
In the course of my career so far I have conducted literally tens of thousands of HIV tests. Although the majority of patients breathe a sigh of relief once they know they are in the clear, a significant minority of patients always have lingering doubts about the accuracy of their tests.
So let’s look at exactly how accurate are HIV tests.
First the basics of HIV
It is important to know some basic science on HIV anatomy and immune function in order to understand how HIV testing works. Please note that I have tried to make this as basic as possible just to illustrate some core concepts of HIV testing. I apologize to immunologists for any and all technical inaccuracies.
The anatomy of the HIV virus
The HIV virus is simply made up of a strand of genetic material (RNA – RiboNucleic Acid) encapsulated in a protein shell (viral envelope). There are lots of little bits of protein (antigens) on this shell each given fancy names like P24, GP120, GP41, P31, GP41 etc etc
This is important to know because tests for HIV either tests for the presence of HIV genetic material (RNA) or the presence of a particular part of the viral envelope namely the P24 antigen or the presence of antibodies to the various antigens on the viral envelope e.g. GP120, GP41 etc
How the HIV virus infects CD4 cells in the body
CD4 cells are special immune cells in our bodies that the HIV virus targets. The HIV virus binds onto a particular protein known as a CCR5 receptor on the CD4 cells. The virus then injects its RNA genetic material into the cell. The RNA is converted into DNA (Deoxy-RiboNucleic Acid) in the cell via a process known as reverse transcription. The HIV DNA can now integrate itself into the cell’s DNA. The virus then hijacks the cell’s systems. Once this is done, the HIV DNA uses the cell’s resources to make millions of copies of strands of HIV RNA (hence the HIV DNA is known as Pro-Viral DNA). The HIV RNA then bursts out of the cell and go on to infect other cells.
How the immune system works
Our immune system recognizes the HIV virus as a foreign invader. It will send special cells to swallow the HIV virus, chew it up and present bits of it to the rest of the immune system so that they can recognize the HIV virus and fight it. The bits that are presented to the rest of the immune system are the protein bits found on the HIV envelope. Remember P24, GP120 and the rest? I’m sure you do.
Special cells in the immune system then produce antibodies to attack these specific bits of protein (antigens). The first types of antibodies produced are called IgM antibodies. They are big and clunky. After a few weeks, the body starts to produce IgG antibodies which are the smaller sleeker ones that we are so familiar with.
Antibodies operate on what is known as the “lock and key” theory. The immune system produces antibodies specific to these antigens. So for the P24 antigen there will be an anti-P24 antibody. For the GP120 antigen, there will be an anti-GP120 antibody. And so on. In other words, antibodies are specific to a particular antigen. So if, for example, antibodies to the GP120 antigen is detected in the blood, then it is reasonable to conclude the GP120 antigen is present in the blood and therefore HIV is present. Testing for antibodies is like looking for the smoke and not the fire.
Different Types of Tests for HIV
There are 3 kinds of tests available to detect the HIV virus in the body.
- Nucleic Acid Amplification Tests
- HIV Antigen Tests
- HIV Antibody Tests
Nucleic Acid Amplification Tests (a.k.a HIV RNA PCR, HIV Viral Load, HIV Pro-Viral DNA PCR)
These tests all look for the genetic material of the HIV virus. The more common test detects HIV RNA which, as you recall, is the genetic material found within the HIV virus. There are also tests to detect the HIV Pro-Viral DNA (the DNA that is used to make more HIV RNA). But this test is less commonly used. It is used mainly to detect a HIV infection in a newborn.
HIV Antigen Tests (a.k.a P24 Antigen test)
The only antigen that I am aware can be tested for commercially is the P24 antigen. It is frequently done together with an antibody test. Therefore called the Combo or Duo test. Simple because the test Combines 2 (Duo) tests into 1.
HIV Antibody tests
This is the most common test for HIV. Please note that when Doctors say “HIV antibodies” we are not talking about 1 particular antibody. We are talking about a constellation of different antibodies directed at different parts of the HIV envelope. There is now no need to differentiate which antibodies are present. Most HIV antibody tests will test for a variety of HIV envelope antibodies. The presence of any one (or more) will make the test reactive i.e. turn positive to indicate a HIV infection.
There is a test called the HIV Western Blot that can actually separate out different antibodies to different parts of the HIV virus (i.e. antibodies to GP120, antibodies to P41 etc etc). This was commonly done because older generation HIV antibody tests were not so accurate and could give false positive results. Nowadays, HIV antibody tests are much more specific. Both the WHO and US CDC no longer require using the Western Blot as a confirmatory test for HIV infection in their latest HIV testing algorithms. Although some countries are still using the Western Blot as the final confirmatory test.
We obviously cannot see antibodies with our naked eyes. So there are several lab techniques to help us determine if antibodies are present or not. The most common technique is using a color change. Some use light and others use radiation. These various lab techniques have fancy names like ELISA or ECLIA. There is really no need to get too much into this. Whichever technique the machine in the lab uses are equally accurate as long as it is calibrated right.
Generations of HIV antibody tests
1st Generation HIV Antibody tests
The first generation HIV antibody tests placed pieces of HIV antigen on a surface. The patient’s blood is then allowed to flow over it. The theory is that if there are HIV antibodies, they will stick onto the HIV antigens. Another solution is then applied that contains antibodies that will stick onto the tails of the HIV antibodies. These HIV anti-antibodies contain an enzyme that will cause a color change, or a radioactive substance of a light emitting substance that can be picked up by a detector. The problem with the 1st generation HIV tests was that the bed of antigen was not very pure. Therefore, sometimes, even non-HIV antibodies will get stuck causing a false positive reaction. This was why these tests were always followed up with a Western Blot test as a confirmatory test. The other problem with this test was that since the antigens were presented on a flat bed, only small IgG antibodies can get trapped. The bigger IgM antibodies which are produced first, are too big to get trapped. Therefore the testing window period for these tests were long.
2nd Generation HIV Antibody Tests
These tests were very similar to the 1st generation HIV antibody tests. The only difference was that the antigen bed was made purer. This reduced the number of false positive tests. The antigen mix was also changed to improve the detection of HIV-1 Group O and HIV-2 antibodies.
3rd Generation HIV Antibody Tests
There was a huge leap in technology from the 2nd generation tests to the 3rd generation tests. The 3rd generation HIV tests used what is now known as the sandwich technique. The same bed of HIV antigen was used. However, HIV antibodies were then placed on this bed. Another layer of HIV antigens were then washed over this. This resulted in a much purer layer of HIV antigens therefore greatly reducing the number of false positives. Also, now that the antigens were sticking up instead of stuck to a flat surface, the larger IgM antibodies could now stick to the antigens and detected. This therefore also reduced the length of the window period.
4th Generation HIV Antibody Tests
This is a misnomer. There was actually no technological improvement from the 3rd Generation HIV tests to the 4th Generation HIV tests. The so called 4th Generation HIV tests are simply tests that combine the 3rd Generation HIV tests with the P24 antigen test onto one single test strip. That is why the 4th Generation HIV test is also known as the Combo Test or the Duo Test.
HIV Testing Window Period
The HIV testing window period refers to the time from which a person is infected with HIV to when tests for HIV can detect the infection. This is different from the HIV Symptoms window period which refers to the time a person is infected with HIV to when the person develops symptoms of infection.
The first 10 days after a person is infected with HIV is called the Eclipse Period. During this time not a single test for HIV we have currently is accurate. In other words, it is a complete waste of time and resources to conduct any sort of HIV test within this period. If your exposure was less than 72 hours ago, see your Doctor about HIV PEP. This is a course of medication that could significantly reduce your risk of getting a chronic HIV infection.
From Day 10 after infection, the HIV RNA PCR test starts to become useful. The amount of HIV virus rises extremely rapidly in the blood and can easily be detected with this test. The problem with the HIV RNA Test is that it can give a false positive result. Although in my career so far after doing thousands of such tests, I have yet to see a false positive results. Nevertheless, in scientific reports, this has been reported. If the HIV RNA PCR test reports fewer than 5000 viral copies per ml, it is likely a false positive. However, I too understand this can cause extreme anxiety.
The P24 antigen then becomes detectable between 14 to 20 days after infection. Which means the 4th Generation HIV tests or Combo Test or Duo test can now be used.
From 20 to 23 days after infection, IgM antibodies start to become detectable. Which means both 3rd and 4th Generation HIV tests can now be used,
From 28 to 48 days after infection, IgG antibodies can be detected. Which means 1st and 2nd Generation HIV tests can be used.
From these numbers, you can now see why most HIV testing guidelines would recommend using the 4th Generation HIV test at 1 month post exposure and the 3rd Generation HIV test at 3 months post exposure. This is simply to build in enough of a buffer period so that we can be absolutely sure these tests are accurate.
What are the causes of a False Negative HIV Antibody test?
Many patients are worried that the tests they have done are inaccurate.
By far the commonest cause of a False Negative HIV Antibody test is the window period. If the wrong test was done at the wrong time, the test may not be accurate. That is why I always recommend people to consult with a Doctor familiar with HIV screening. The Doctor will be able to recommend the most accurate test based on your unique situation. Or maybe even tell you not to test at all.
All the other causes of a False Negative HIV Antibody test are extremely rare. They are:
Hypogammaglobinaemia or Agammaglobinaemia
These are extremely rare genetic conditions which makes a person unable to produce antibodies.
There was a single reported case in the New England Journal of Medicine in 2005.
Sero-Reversion in end-stage AIDS
HIV destroys the immune system and causes AIDS. In very late stage AIDS, the immune system can be too weak to even produce antibodies. This can lead to a false negative HIV antibody test.
After a patient is placed on treatment, the immune system usually recovers to a point where it can start producing antibodies again and the HIV antibody tests will then turn positive.
Sero-Negative HIV infections
Some people infected with HIV just do not produce antibodies against the HIV. Scientists and Doctors still do not know why this happens. A current theory is that there is some mutation in the immune system that just makes it not react to a HIV infection.
Such patients usually progress to AIDS rather rapidly. Although several of such cases have been described, they are still extremely rare. Occurring in about 1 in a million patients with HIV.
Home testing or see a Doctor?
Home HIV tests are convenient and private. However there are a few things you should take note of:
- Always make sure you get your test kits from a reliable source.
- Always read the instructions carefully before use. And follow the instructions exactly.
- Always see your Doctor if you are in any way unsure of the results.
You should also be emotionally and mentally prepared for whatever the outcome of the test may be. It would be ideal if you have a friend or a family member with you to provide support. If your test really is positive please do not panic. See your Doctor to have these results confirmed.
What happens if my test is positive for HIV?
Your Doctor will do another test to confirm the result of the first test. If you are truly infected with HIV do not panic. Do not worry. People live long and productive lives with HIV. It is a chronic disease that can be easily managed simply with tablets. It is no longer a death sentence. See a Doctor familiar with HIV treatment to advise you. If you need someone to talk to you can click here to contact Oogachaga. They are a non-profit organization that provides counselling and support anonymously.
What happens if my test is clear for HIV?
If your results is clear for HIV please do remember to continue to stay safe. Always practice safe sex unless you are in a mutually monogamous relationship with another HIV –ve person.
Talk to your Doctor to find out if HIV PrEP is right for you.
In conclusion
As you can see, HIV screening is an extremely complex science. As long as this article is, it barely scratched the surface of the multiple variables that must be considered when deciding which HIV test to do and how accurate it is for the individual patient.
Honestly, the best advice I can give is for you to see a Doctor familiar with HIV testing. Be honest and upfront with the Doctor on your exposure and your concerns. Listen to the Doctor’s advice and do the necessary tests. Repeat the tests if your Doctor advises it. Most importantly if and when your Doctor tells you that you are in the clear, be relieved, thankful and forget about it. And of course continue to keep yourself safe.
Have a question on HIV testing? Please put it in the comments section below and I’ll get back to you as soon as I can.
Hi, Im from Philipphines, I just want to ask, What is the Accuracy of SD BIOLINE 3.0 HIV 1/2(IaG,IgG,IgM) test given after 6-7 weeks of possible exposure?
Hi Cris. In my opinion, as good as 100% accurate. I am assuming the test was stored and conducted according to manufacturer guidelines. That said, I understand most health authorities will require another confirmatory test at 3 months post exposure. But honestly I think you can stop worrying.
Doc, How long HIV will survive outside the body in temperature like philipphines we have a normal temperature of 38-41°©
That is an impossible question to answer. There are too many variables like the material of the surface, humidity levels, presence of other chemicals etc. What you are probably more interested in is whether or not you can get infected by HIV on a surface. That has never been scientifically reported. You cannot get infected with HIV by touching a contaminated surface like public toilets etc
This is the information about the SD BIOLINE. Do you think doc that my test conducted by this test after 6-7 weekd will be accurate?? https://www.who.int › hiv-rdtsPDF
Web results
Part 5
I sent the wrong link! I will I cannot send the link here.. i will send it to via gmail doc.
You sent me a document describing the technical specifications of the SD Bioline test. What exactly is your concern?
Yess! Based on your analysis doc of that test kit, Can it give an accurate result of 6 weeks? thanks!
Reply docc plss! thanksss!
Hi Cris. This does not change my initial answer to you. In a nutshell you do not need to worry but if your local health guidelines recommend another test in 3 months you should get that done.