The window period of all HIV testing varies and depends on the kind of sample that is tested.
The window period of HIV refers to the time after infection but before seroconversion. During this period, the markers of the infection, which is the p24 antigen, and antibodies are still absent or too few to be detected. During the window period, most tests do not always detect the virus.
The window period of all HIV testing varies and depends on the kind of sample that is tested. Most times, when the virus is detected during the window period, it is done using the blood plasma of the infected person. Whole blood got from a fingerprick and, oral fluids take longer to detect the virus in the window period.
The plasma is the colourless fluid portion of the blood, which is gotten after the whole blood is separated using laboratory equipment. The fingerprick blood is gotten by pricking the finger with a lancet while the oral fluid is gotten by taking a swab of the gums.
To get the right result for an HIV test, there are 2 questions you need to ask.
- How soon after exposure to the virus can the test detect its presence in the body?
- How soon after exposure can anyone be confident that a negative result is accurate?
UK guidelines on HIV window period
The UK guidelines state that a fourth-generation HIV test carried out on a blood plasma in a laboratory can detect HIV within four weeks of exposure in most individuals.
People who think that they might have been exposed to the virus within the past four weeks do not have to wait for long to get tested. They should not miss an opportunity to diagnose HIV infection, especially during the early stages of the infection when a person is highly infectious. People in this category would be offered a fourth-generation laboratory HIV test and are advised to repeat the test after four weeks of exposure.
The guidelines suggest that further testing is not necessary if the result of a fourth-generation test done after four weeks is negative, except the exposure is believed to carry a high-risk infection. Those with ongoing risk of infection are advised to carry out the test at regular intervals.
Duration of window periods for different HIV test
It is difficult to know precisely how long the window period last because the window period in individuals vary and research on this would require multiple blood samples over a few days and weeks of those who have been recently infected.
However, a recent study calculated the window period for some HIV testing assays. All the analyses were carried out using blood plasma samples because window periods are likely to be longer using fingerprick blood samples or oral fluid. These other samples give normal results when carrying out a rapid HIV test, point of care test, and using a self-testing device.
The research carried out confirms that the fourth-generation laboratory HIV test which detects both antibodies and p24 antigens can detect the virus between 1 – 3 weeks earlier than other HIV tests which detect only antibodies. The data from the research suggest that retesting after 90 days of possible exposure which is recommended by some countries’ guidelines may not be necessary.
- Fourth-generation laboratory test
The UK guideline recommends this test and, it requires a blood plasma or serum sample. The test can detect immunoglobulin G (IgG) antibodies, immunoglobulin M (IgM) antibodies, and p24 viral antigens (this is a protein present in the core of the virus. It is usually detected before antibodies.
Common fourth-generation laboratory test includes Abbott Architect HIV Ag/Ab, GS Combo Ag/Ab EIA, and Siemens Combo HIV Ag-Ab.
The median window period for this test is 18 days while the interquartile range is between 13 – 24 days. This indicates that half of all infections can be detected between 13 – 24 days after infection. With this test, about 99% of those infected with HIV will test positive within 44 days of exposure.
- Fourth-generation rapid test
Fourth-generation HIV rapid test is similar to the fourth generation laboratory test, but, the window period is likely to be several days longer, just like testing with fingerprick blood. The available fourth-generation rapid test includes Determine HIV-1/2 Ag/Ab Combo.
- Third generation rapid and point of care test
These tests detect immunoglobulin G (IgG) antibodies and immunoglobulin M (IgM) antibodies. Available third-generation rapid and point of care test include INSTI HIV-1/HIV-2 and Uni-Gold Recombigen HIV tests. Within 50 days of exposure, INSTI can detect about 99% of those infected with HIV.
The median window period is 26 days with an interquartile range of 22 to 31 days. This means that half of those infected with HIV can be detected between 22 – 31 days using INSTI. These estimates are exclusively for testing with plasma but, in practice, these tests are carried out using a fingerprick sample so, the window period is likely to be longer.
- Third generation laboratory tests
This test is no longer recommended because they do not detect p24 viral antigen, but, they only detect immunoglobulin G and immunoglobulin M antibodies. The window period is similar to INSTI third-generation rapid test but with a median of 23 days.
- Second generation test
Many rapid points of care tests are in this category. They detect only IgG but not IgM and p24 viral antigens which are more detectable in the early stages of the infection. The median window period for these tests is 31 days. Half of all infections can be detected 26 – 37 days after infection while, about 99% of infection can be detected within 57 days of infection. These estimates are solely for blood plasma as finger prick and oral fluid may have a longer window period.
Examples include OraQuick Advance Rapid HIV1/2, SURE CHECK HIV 1/2, Clearview HIV 1/2, and STAT-PACK.
Self-testing and self-sampling were not included in the study for HIV window periods although most self-testing devices are modified versions of rapid, point of care test kits that were made for use by health care professionals.
How accurate are these tests?
Early HIV tests are usually accurate, but, care should be taken while using fingerprick blood and oral fluids.
The tests may have an extended window period for those taking pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP) because they may have delayed antibody response.
If you need to test for HIV, you can book an appointment for an HIV test in London by calling our Walk-In Clinic today on 020 70968853.
970