HIV testing | Differences between lab tests and rapid tests? How does RNA testing work?

Human Immunodeficiency Virus (HIV) usually does not show any symptoms, making it difficult to confirm the infection with symptoms. Luckily, many HIV tests are available on the market. In this article, we will go over differences between lab tests and rapid tests. Early detection of HIV infection means early start to an effective treatment.

What are the differences between HIV tests?

There are typically three types of HIV testing: HIV RNA test, P24 antigen/antibody test and rapid antibody test.

HIV RNA early detection test

HIV RNA early detection test detects in the blood, the presence of viral RNA, the genetic materials of HIV. When HIV enters our bodies, HIV replicates rapidly by attacking certain immune cells. The HIV RNA also increases in amount. When HIV RNA reaches a detectable level in blood, healthcare professionals could determine HIV infection with HIV RNA early detection. Among three types of tests, HIV RNA early detection has the shortest window period.

P24 antigen/antibody test

P24 antigen/antibody test detects whether HIV P24 antigen and antibody are present in the blood sample. Antigen is a molecule on the virus surface, while antibody is produced by our immune system with a certain antigen as target. When HIV enters our bodies, HIV replicates rapidly. When an abundant amount of HIV is present in the blood, the P24 antigen reaches a detectable level. This makes it possible to determine HIV infection with a P24 antigen test.

Yet, as our immune system produces HIV antibodies against P24, the blood level of P24 antigen decreases after acute infection, gradually becoming undetectable in blood. Therefore, P24 antigen/antibody test also detects the presence of HIV antibody in the blood to determine HIV infection. The window period of P24 antigen/antibody is a bit longer than that of HIV RNA early detection.

Rapid antibody test

Rapid antibody tests usually require blood or saliva samples, for detecting HIV antibodies. When HIV enters our bodies, our immune system gradually produces antibodies against HIV to protect ourselves. Therefore, rapid antibody tests can determine HIV infection by detecting HIV antibodies. Since it takes time for HIV antibody production, it shows up later than HIV RNA and P24 antigen. Thus, the window period for rapid antibody tests is the longest among three types.

Why does the window period of HIV RNA test the shortest?

Window period is the duration between HIV infection and HIV testing detection of HIV.

HIV RNA early detection has a window period of 7-10 days, the shortest among the three types of HIV testing. That means it can detect HIV the earliest. Since HIV replicates rapidly after entry, it first needs to replicate their own RNA before encoding in our own DNA and packaging a new HIV with viral proteins. Therefore, the blood level of HIV RNA rises much faster than that of HIV themselves. This explains why the window period is shorter than that of the P24 antigen/antibody test.

Is the result of HIV RNA early detection accurate?

HIV RNA early detection has a sensitivity of more than 99%, indicating over 99% of HIV infected individuals could obtain a positive result with the test. Yet, this only applies to people who are not taking HIV prevention medicine.

Since HIV RNA early detection test determines HIV infection by detecting HIV RNA in the blood, the results’ accuracy would be impacted if HIV prevention medicine (such as PrEP and PEP) is taken. This is because these pills suppress HIV ability to self-replicate and lower the HIV RNA blood level.

What are the differences between qualitative and quantitative HIV RNA early detection?

HIV RNA early detection is available in two kinds: qualitative and quantitative:

• Qualitative: Capable of confirming the presence of HIV in the blood, yet unable to determine the amount present
• Quantitative: Capable of confirming the presence of HIV in the blood, and the HIV RNA level present

If I am worried of an HIV infection, what can I do?

If you are worried of an HIV infection after unprotected sex, different things can be done based on the timing:

• Within 72 hours after sex: Consult the doctor on prescription PEP (Post-Exposure Prophylaxis). The risk of HIV infection can be lowered for people completing the 28-day regimen as instructed by doctors.
• 7-10 days after sex: Carry out an HIV RNA early detection.
• 14 days after sex: Carry out a P24 antigen/antibody test.
• 90 days after sex: Carry out a rapid antibody test.

If you are infected with HIV, go to a doctor and manage HIV infection with treatment. Taken medicine as instructed can suppress HIV self-replication, preventing the progression into AIDS and prolonging life. Lowering the viral load to an undetectable level means HIV is untrasmittable through sexual intercourses.

HIV infection cannot be diagnosed with symptoms alone. If you have unprotected sex or suspect to be infected with HIV, choose the test most suitable to your needs.

MY PLACE is established by Hong Kong charity organization “AIDS Concern” to provide professional services for sexual health checkup. The services include traditional blood test for HIV, HIV RNA early detection (quantitative), and self-test kits, in addition to mental health and emotional support counselling services. Click here for more information, or donate to support our services.