Fourth-generation tests are advanced HIV tests that look for HIV antibodies as well as p24 antigens. When a person contracts HIV, the immune system starts to produce HIV antibodies. This is the body’s method of fighting the virus. The immune system produces these antibodies in response to p24 antigens.
These antigens are a part of the virus, and research indicates that they tend to appear within 2 weeks of HIV entering the body. The right time to take this test is around 4 weeks after exposure. Most people will develop detectable antibodies (proteins made by the immune system to fight the HIV virus) within this time frame, but in rare cases, it may take up to 3 months for them to develop. This test is sensitive to P24 antigens, so it is useful for diagnosing HIV infections when antibody levels are still initially low. However, it is recommended to do retesting after three months have passed since potential exposure for conclusive results.