When it comes to HIV testing, two of the most common questions we are asked online or in person are: “When can I test for HIV?” and “Are my results conclusive?”
There is a lot of confusing information out there, so this article will try to break down the timelines for accurate testing, and why this is the case.
These Questions Are Basically Asking – What is the HIV Window Period?
The HIV window period is the time between viral transmission to be able to get a conclusively accurate test result. This time period depends on the type of HIV testing performed – different tests have different window periods, with newer generation testing allowing for earlier and more accurate diagnosis of HIV than what was available in the past.
As testing facilities vary from country to country, it is important to recognize local health authority guidelines and speak to the doctor if you have any uncertainties on your test.
If you are within 10 days from a possible HIV exposure, you are still in an eclipse period. This is the period of time where no current testing can detect the virus. HIV Testing in this time period will have no diagnostic utility.
However, if you are within 3 days (72 hours) from a potential HIV infection, please see your doctor or contact us immediately for Post-Exposure Prophylaxis (PEP).
HIV Post-Exposure Prophylaxis (PEP) can reduce risk of HIV infection or prevent HIV infection.
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Also Read: When to get HIV Test after HIV PEP
So, What Types of HIV Tests are Available and How Do They Work?
For diagnostic purposes, most countries and guidelines now recommend first-line testing with the 4th Generation Antigen/Antibody test (also known as the Combo or Duo test). Other available testing includes 3rd Generation Antibody-only testing, HIV RNA/DNA PCR viral load and the HIV Pro-Viral DNA Test.
These tests are sometimes available as ‘point-of-care’ rapid tests, or alternatively can be performed in a laboratory with techniques such as ELISA or CMIA (don’t worry too much about the technicalities here). To understand the differences and rationale for each of these tests, we have to first understand what each of them is looking for.
The Core Structure of HIV is Made Up of a Protein (p24 Antigen)!
HIV is a type of retrovirus, and much of its core structure is made up of a protein known as p24 antigen – this is the type of antigen we look for with 4th generation HIV testing.
The p24 antigen usually becomes detectable from 10-14 days post-exposure, reaching a peak at around 3-4 weeks, and dropping to lower levels after 5-6 weeks once HIV antibodies start forming.
HIV antibodies are proteins formed by your own body in response to the virus and usually start forming as early as 14-17 days post-exposure, with 99.9% of patients having a detectable antibody response by 3 months.
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The HIV Pro-Viral DNA Test
The HIV Pro-Viral DNA test can be used in specific situations where there are challenges to getting an accurate HIV diagnosis with other available HIV tests including HIV Antibody tests (3rd Generation HIV test), HIV Antibody and Antigen tests (4th Generation HIV test) as well as HIV RNA PCR test.
It is especially useful in the following situations:
- Diagnosing HIV in newborns born to HIV +ve mothers
- Elite controllers with undetectable HIV viral load despite not being on anti-retroviral treatment
- Individual with sero-negative HIV infections i.e. People who get infected with HIV but do not develop anti-HIV antibodies : see FALSE NEGATIVE HIV ELISA TEST
It can be used for situations where the diagnosis of HIV is challenging, it has a lower false positive rate compared to the HIV RNA PCR test when used for diagnosis and it can be done 10 days post exposure.
HIV PCR RNA Test – 12 days or more post-exposure
As the HIV RNA PCR test is more sensitive, the window period is shorter and it can be used 12 days after exposure, as compared to 28 days for conventional Antibody/Antigen Testing.
HIV is a retrovirus. Retroviruses are a type of RNA virus that invades cells, in this case a kind of immune cells known as CD4 cells by injecting its genome directly into it. RNA is slightly different and less chemical complicated than DNA, hence the virus also has a special enzyme known as Reverse Transcriptase that will encode the HIV RNA into HIV DNA which is then merged with the host cell’s genome. Hence, if we can detect the presence of HIV RNA in the blood, we can be certain that there is HIV infection even without symptoms.
This is achieved through a technology known as polymerase chain reaction (PCR). In PCR, an automated device will process the patient’s sample with various chemicals. The polymerase enzyme and reaction will multiple any HIV RNA material into easily readable genetic information.
4th Generation HIV Test – 28 days or more post-exposure
The 4th generation Antigen/Antibody test has been shown to be conclusive from 28 days or more post-exposure. Remember, this test also checks for HIV antibody, so it will remain accurate for HIV diagnosis even after the p24 antigen levels drop, including years after initial infection.
3rd Generation HIV Test – 3 months or more post-exposure
The 3rd generation Antibody-only test is accepted as conclusive from 3 months or more post-exposure. If testing is done within these respective window periods, there is a chance of a false negative result.
For extremely rare cases where people do not mount an appropriate HIV-antibody response (known as seronegative infections), the 4th generation test will still be able to detect the p24 antigen, which is why it is considered the most accurate first-line test for HIV diagnosis.
You may also have heard of a test called the Western Blot; this is just another type of HIV antibody test that is usually used for confirmation testing, once a preliminary test has shown a positive result. As HIV is a serious and life-changing diagnosis, it’s important we get it right – most diagnostic algorithms require two separate tests showing a positive result before we confirm it to the patient. As the HIV window period may differ with these separate tests, it is important that your doctor chooses the right tests for you.
The last test type we’ll be looking at is the HIV RNA/DNA PCR test, also known as Nucleic Acid Amplification Testing (NAAT). This test has revolutionized HIV screening, especially for people at high risk. This test directly measures the amount of virus in a person’s blood and can detect conclusively from as early as 12 days post-exposure. It is also used to monitor viral load in people with a known HIV infection. Although this test is able to pick up HIV infection at the earliest stage, it is not widely available due largely to costs and turnaround time in Singapore. There is a slightly higher risk of false positive results with the HIV RNA/DNA PCR test.
In summary, here are the tests you can do at the appropriate time:
Day 1-3: Consider Post-Exposure Prophylaxis (PEP)
Day 1-9: Eclipse period (no HIV testing available)
Day 10: HIV Proviral DNA Test
Day 12 onwards: HIV RNA/DNA PCR testing (may have false positives)
Day 14 onwards: 4th Generation Combo testing can start to pick up the infection (may have a false negative)
Day 28 onwards: 4th Generation Combo testing is deemed conclusive
Day 90 onwards: 3rd Generation Ab-only testing is deemed conclusive
At the end of the day, it is important to speak to your doctor and inform them accurately about any potential exposure risks, so they can decide on which test is appropriate. And remember, 4th Generation Combo testing is the first line test recommended by most guidelines today. Ask for this test if you have any doubts!