HIV Tests - Testing for HIV/AIDS Infection
There are a number of methods to test for HIV using blood, saliva or urine samples. However, the most accurate and the most common way is by testing a person’s blood. There are three biological markers that can be tested to diagnose an HIV infection:
- Testing for HIV Antibodies (the body’s immune response to an HIV infection);
- Testing for the p24 Antigen (a protein that is produced during the early phase of an HIV infection);
- Testing for the HIV virus itself (that causes AIDS)
1. HIV Test for Antibody
This is the most common and standard test to detect an HIV infection. It can be done with blood, saliva or urine. However, the techniques used for testing affect the window period significantly. With the modern ELISA technique, HIV antibodies can be detected as soon as 3-4 weeks after infection. While a rapid test for HIV is available (results in 10 minutes) it may take up to 3 months after the infection for the antibody level to be detected in the blood. Therefore many health services may still quote three months as the window period for an HIV test.
2. HIV Test for p24 Antigen Test
The p24 antigen test measures the p24 protein that is produced during the early phase of an HIV infection. This is detectable earlier than HIV antibodies due to the initial burst of virus replication and is associated with high levels of the virus during which the individual is highly infectious. P24 is usually detectable 2 weeks after infection and a positive result is highly specific for an early infection with specificity in the range of 99.7 – 99.9%. However, the level of p24 may decline after 4 weeks of infection due to the production of antibodies.
3. HIV Test for P24 Antigen and Antibody Test (Combo Test)
The 4th generation HIV test detects both p24 antigens and antibodies thus reducing the window period for an HIV test to 2 weeks after exposure. This improves the accuracy of screening and reduces the anxiety of a 3 month wait. The results of “combo” test can be available within 24 hours from the time a blood sample is taken. There is information on the web regarding the various testing times for HIV.
The “combo” test has been used in Europe for over a decade and was approved by the United States Federal Drug Administration (FDA) in 2010. It is also being used by Blood Banks to screen donor blood. Due to the different types of test kits being used most international guidelines (eg the British HIV Association (BHIVA), suggest 4th generation HIV tests be undertaken 1 month after exposure. Some international guidelines such as the WHO European STD Board still suggest a repeat test 3 months after exposure.
At Hong Kong Sexual Health Centre, our in-house laboratory (Neo-Lab) is equipped with two HIV combo testing systems that are FDA and CE certified. Beckman Coulter Access 2 Analyzer using Bio-Rad GS HIV Combo Ag/Ab EIA kit is highly sensitive in detecting HIV P24 and antibody in both chronic and primary infection. The bioMerieux Vidas HIV panel can further confirm and differentiate P24 antigen and antibody independently.
4. HIV RNA Test
The HIV RNA test is sometimes called the HIV PCR test and can detect an infection within 1 week of exposure. However it requiresspecialised equipment and personnel to perform the test and may take about 2-3 days for the results to become available. During actual infection, within 2-3 weeks, the viral load of HIV virus can reach over 10 million copies/ml of blood. HIV RNA test can detect HIV virus less than 100 copies/ml of blood. So, it is highly sensitive in detection of early infection.
HIV RNA test (7 days Window Period)
Ultrasensitive HIV RNA (5 days Window Period)
All the above tests vary in terms of cost, turnaround time and most importantly, the window period, ie the time from infection to the time when it can be detected. The following table summarises the differences:
|Antibody test||P24+Antibody||HIV RNA|
|Window Period||Up to 3 months||2 weeks||5-7 days|
|Reporting time||10 minutes||24 hours||24 hours|
HIV Tests are accurate?
Modern HIV tests are very accurate. Our health care professionals will determine which test is the most appropriate for you depending on when you were exposed. As the tests are designed for screening purposes, they are highly sensitive in detecting the infection. However, a positive result does not always indicate infection. False positives can happen in 1-3 cases for every 1000 tests performed. To confirm a positive result, further tests will be performed on the blood sample for verification.
The window periods specified apply in most standard situations, however, it can be affected by the use of antiviral medications, immune-suppressants or immunological conditions of individuals.