The window period refers to the time between HIV infection and when a test will yield an accurate result for a certain percentage of samples. Different types of tests have varying window periods:

• 4th Generation Antibody/Antigen (Ab/Ag) Tests: 45 days to detect 99% of samples.
• 3rd Generation Antibody-Only Tests: 60 days to detect 99% of samples.
• Point-of-Care Tests (self-sampling, self-testing, and rapid tests): 90 days to detect 99% of samples.

During the window period, an individual can be highly infectious while still testing negative for HIV.

UK guidelines recommend a window period of six weeks for a 4th generation antigen/antibody test (BHIVA/BASHH 2020). By this time, 99% of infections will be detected. Testing after only four weeks will detect 95% of infections.

A negative result at six weeks with a 4th generation test does not require confirmation. However, a negative result from testing earlier—such as after four weeks—should be confirmed with a second test later on.

Time to develop antibodies

The image above illustrates the range of times it can take for individuals to respond to HIV infection.

The earliest marker is the HIV viral load, which can be detected in the first weeks after infection, typically from 1 to 6 weeks post-exposure. A high viral load is often associated with seroconversion symptoms.

The first HIV protein (antigen) that can be measured is p24, detectable from 1 to 8 weeks after exposure. However, viral load and p24 tests are not reliable for diagnosing early HIV if the results are negative.

HIV antibody responses can be detected as early as two weeks in some individuals, with over 99.9% of people showing an antibody response by 12 weeks. An antibody test at 4 weeks can detect 95% of infections.

While antibody testing at 4 weeks can provide a good indication of your HIV status, a test at 12 weeks post-exposure is necessary to confirm that you are HIV negative.

Timeline for HIV infection, immune responses and window period for tests

Both these graphics need to be updated to also show the six week time point.

HIV infection progresses through several stages before it can develop into AIDS (Acquired Immunodeficiency Syndrome). These stages are:

1. Acute HIV Infection

• Description: This is the phase where HIV is present in the body but antibodies have not yet developed. It is also known as the "window period," during which HIV or antigens can be detected, but antibody tests may be negative or uncertain.
• Timing: Usually occurs 2-6 weeks after exposure to the virus.
• Symptoms: During this period, the immune system is actively combating the virus, and symptoms are similar to those of other viral infections (like the flu). Symptoms can include headache, fatigue, muscle aches, sore throat, swollen lymph nodes, red rash (often on the trunk), and fever. These symptoms typically last 1-2 weeks and then resolve completely.
• Detection: About half of patients in this phase will experience a rash, often leading them to seek dermatological care. The viral load is high, making individuals very infectious. Early diagnosis and behavioral intervention are crucial for public health.

2. Chronic HIV Infection (Clinical Latency Stage)

• Description: After the acute symptoms disappear, the disease moves into the asymptomatic or clinical latency phase. Some people may not exhibit symptoms from the first stage and may be unaware of their infection, but they can still transmit the virus to others.
• Timing: This stage can last for 10 years or more.
• Impact: Untreated HIV continues to destroy CD4 cells and weaken the immune system, leading to a decline in CD4 cell count and increased susceptibility to other infections.

3. AIDS (Acquired Immunodeficiency Syndrome)

• Description: This is the final and most severe stage of HIV infection, marked by severe immune system damage.
• Symptoms: Complicated clinical symptoms may include persistent fatigue, swollen lymph nodes in the neck or groin, prolonged fever lasting over 10 days, night sweats, unexplained weight loss, purple spots on the skin that do not disappear, shortness of breath, chronic diarrhea, fungal infections in the mouth, throat, or genital areas, unexplained bruising or bleeding, and other opportunistic infections.
• Duration: The duration of the asymptomatic phase can vary widely, from as short as 2 years to as long as 20 years. The length depends on the route of transmission. For example, blood-borne infections (mainly through illegal blood transfusions or shared needles) typically lead to an AIDS diagnosis in 4-5 years, while sexual transmission may take 11-13 years. If an asymptomatic phase lasts 13 years, the person may be considered a long-term survivor. Once the immune cells can no longer combat HIV, it signifies the final stage of HIV infection, leading to AIDS. Individuals with AIDS are highly susceptible to other infections, and common illnesses like pneumonia can become uncontrollable, leading to death within 6 to 24 months.

HIV Testing Window Period

• Historical Context: Traditionally, the window period for HIV testing was considered to be 3 months, based on the testing methods available in the 1980s.
• Advancements: As understanding of HIV and testing technology has improved, the window period has been shortened. Advances include improvements in antigen and antibody detection technologies, such as the transition from first-generation mixed antigens to second-generation recombinant antigens, third-generation chimeric antigens, and fourth-generation tests that combine HIV nucleic acid testing with antigen detection.
• Current Estimates: By 1996, the WHO revised the window period to 2-3 weeks. Different countries have updated their estimates based on these advancements, with some adjusting the window period to 3-6 weeks.
• In China: As of 2014, the window period for HIV testing has been shortened to less than 2 weeks with fourth-generation tests. However, third-generation tests, which are more commonly used, have a window period of 3-6 weeks. Therefore, in China, a negative result 3 weeks after high-risk behavior can generally rule out HIV, and a result after 6 weeks can almost completely exclude it.