What is the window period for HIV to show up on a test?

A friend had a sexual encounter with someone he had just met during a trip.
I am not sure the number of times or duration of their affair.

His trip was not more than a week and he had no contact with her since his return.

He was scared he might have got AIDs.Not that she was a patient,but he was paranoid and he was also feeling bad for cheating on his wife.

Within the next month or two he got himself tested for HIV.Tests came back negative.

Is it possible for the HIV not to show up in an early test?And does he have to get tested again?

Firm, I believe that it can show up for another 7 years. Or at least this is what I have heard.

I think the normal procedure is to go back for a second test in six months. This is because the tests involved testing for the body's antibodies against the disease. Antibodies are the immune particles the body produces to kill an infection. Sometimes they can take a while to be produced in large enough quantities to be detected -six to twelve weeks [1], hence why a test so soon after possible infection can prove to be a false negative. I have heard that this is rare, but not impossible.

There are also several different types of test with different efficacies. http://www.worldsbest-sexual-health-site.com/hiv-testing.html There is the screening test which can even give false positives and then a conformational test which is more specific and tends to confirm if the patient has contracted the disease.

[1] HIV Testing 101 - The Body (http://www.thebody.com/content/art31903.html)

Thanks bushg and templelane,

The links are great,will make sure he reads them.

I thought I had read something like this before.

Just to be safe I think I will recommend he get tested again.

Yes he definitely needs to be tested again. I'm surprised the clinic didn't tlak him through all this. Then again maybe they did but he was too nervous to listen properally. Good luck to him.