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Knowing about HIV Antibody Test

Window Period

Window period is the period between the moment a person get infected with HIV and that when the antibody/antigen test shows a positive result. The window period for HIV testing is 3 months. During this period, the virus replicates rapidly in the infected person who becomes highly infectious and can transmit the virus to other people even though his HIV antibody/ antigen test remains negative.

One of the testing technologies commonly used nowadays - enzyme-linked immunosorbent assays (ELISAs) – can detect HIV antibody in a sample. After initial exposure, HIV antibody takes some time to develop in human body before reaching a detectable level; this period varies in different individual and can be as long as 3 months.

In the past few years, HIV testing technology has gone through multiple stages of development with newer generations offering higher sensitivity, higher specificity and shorter window periods. For example, the fourth generation of HIV test kit can detect both HIV-1 and HIV-2 antibody, as well as p24 antigen. The window period can reduce to 2 to 3 weeks. Therefore, combining the ELISAs and p24 antigen detection assays, as in the 4th generation of HIV test kits, can shorten the window period significantly.

Nowadays, 2nd,3rd and 4th generation test kits are commonly used by professionals or AIDS organisations that provide testing service. However, for self test kits, only 2nd and 3rd generations are available on the Hong Kong market as at December 2016. A shorter window period only means that the infection can be picked up earlier, a negative result still cannot exclude infection if the user has recent exposure in the previous 3 months. Users must take note of this and repeat the test or consult a doctor whenever in doubt.

Generation of HIV test kits Technology used Window period
1st ELISA At the earliest 5 weeks
2nd ELISA Down to at the earliest 4 weeks
3rd ELISA Down to at the earliest 3 weeks
4th ELISA + p24 antigen assay Down to at the earliest 2 weeks