Updated 26 June 2014

Viral tests

Viral tests are used to diagnose very early HIV infection. There are two types of test available to detect HIV itself, the PCR test and the p24 test.


Viral tests are used to diagnose very early HIV infection. There are two types of test available to detect HIV itself, the PCR test and the p24 test.

In the first four to six weeks after a person is infected with HIV, he or she may test negative if an antibody test (ELISA or rapid test) is used. (See the window period.)

So, if there is reason to believe that you may have become infected with HIV within the last six weeks, you should be tested with a test that directly detects the virus itself.

A person who shows signs of a primary HIV illness (seroconversion illness), but has a negative result from an HIV antibody test, should also have a test that directly detects the virus itself.

Unfortunately, the PCR test and the p24 test are extremely expensive, and are not always available in remote areas of the country.

1. The PCR test
The PCR test detects the genetic material of the virus in the blood.

This test is very accurate and is the best test to diagnose early HIV infection. The PCR test can detect the virus from about 14 days after infection.

The PCR or the (qualitative) polymerase chain reaction technique is especially useful in cases where early diagnosis is required such as for:

  • Post-exposure prophylaxis purposes – (that is in cases of accidents in hospitals)
  • For diagnosing babies born to HIV infected mothers
  • For use as a post-rape test before beginning anti-retroviral therapy.

A variation of the PCR test (quantitative) is also used after diagnosis for treatment purposes. This test is used to establish the number of viral RNA particles in the blood. This is also called the HIV viral load test. The viral load is usually a reliable indicator of the infected individual’s prognosis (outlook), and it is also used to measure an individual’s response to antiretroviral therapy.

2. The p24 test
The p24 test detects one of the proteins that makes up the core of the virus.

This test is less sensitive than the PCR test and will fail to detect the virus in some cases of early HIV infection.

The p24 test can detect the virus about one week before antibodies can be detected with an antibody test, typically three to four weeks after infection.

The HIV p24 antigen tests are usually only used to test newborn babies, because the test is usually only effective in the early phase of infection, and again in the final phases of aids.


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Dr Sindisiwe van Zyl qualified at the University of Pretoria in 2005. She is a patients' rights activist and loves using social media to teach about HIV. She is in private practice in Johannesburg.

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