Antibody only laboratory tests are no longer recommended for routine use in the UK. UK guidelines recommend the use of tests which detect both HIV antibodies and p24 antigen, otherwise known as “fourth-generation” tests.

An HIV antigen, known as p24, is a structural protein that makes up most of the HIV viral core. High levels of p24 are present in the blood during the short period between HIV infection and seroconversion, before fading away. A fourth-generation HIV test adds a technique for detecting p24 antigen to the traditional antibody test. Otherwise, the test is carried out in the same way, with blood samples at a laboratory.

Since p24 antigen is usually detectable a few days before HIV antibodies, the window period is somewhat reduced. Some people who have been HIV infected but have not yet seroconverted will have their infection diagnosed with this test.

It is hard to say exactly how long the window period for these tests lasts, as there are variations between individuals and it is a difficult topic to research (recently infected people would need to know exactly when they were exposed to HIV and then give multiple blood samples over the following days and weeks). Nonetheless, some experts believe that combined tests usually detect infection approximately 15 to 25 days after exposure, but occasionally this period will be a little longer. The UK guidelines say that, when this test is used, the majority of infections will be detected within one month.

Antibody/antigen laboratory tests are extremely accurate. In terms of sensitivity (correct identification of people with HIV), a Health Protection Agency evaluation found that nine out of the ten tests they evaluated had a sensitivity of 100%, while a French evaluation found that ten of twelve tests had a sensitivity of 100%. The lowest sensitivity was 99.8%.

Similar results were found for specificity, in other words, the ability of a test to correctly give an HIV-negative result. All tests checked by the Health Protection Agency evaluation had a specificity of 99.7% or above, and the French study found that all tests produced after the year 2000 had a specificity of 99.8% or above.