Question

What is the window period in fourth generation tests?

14 July 2010. Related: All topics, HIV testing.

Hello,

How do doctors claim conclusive results at 28 days while test manufacturers state: ‘4th generation tests shorten window period 1 to 2 weeks’.

So, in my opinion, it should be conclusive at 10-11 days, not on the 28th day.

What is your opinion? What am I missing?

Best,

Ali

Answer

Answer: Charlotte Walker

Thank you for your question.

The fourth generation tests look for both antibodies AND antigens. Antigens are proteins on the surface of the HIV particle. Antibodies are produced by the immune system in response to the HIV antigens.

Antigens are able to be detected usually about two weeks after infection. HIV tests look for p24 (protein 24) which is is detectable an average or 14 days after infection. However, testing for p24 is not accurate enough on it’s own to be the basis of an HIV test.

Antibodies are not usually produced until 3-4 weeks after an infection depending on the speed and strength of the individual’s immune system. About 95% of people produce either antibodies or p24 by day 28.

Individual response to HIV can vary considerably. There are odd cases where responses are much quicker and other cases where everything takes much longer.

In order to be effective as a test it needs to be accurate at picking up genuine infections and also accurate at ruling out infections when they are not present.

These two factors are complicated and guidelines are usually conservative to allow for the extremes.