Most STIs cannot be detected immediately after you have caught them. If you know have been in contact with someone with an STI, or if you have any symptoms, you should test straight away. However, you should be aware that most STIs reliably show in tests within four weeks of exposure, and HIV, syphilis or hepatitis may take twelve weeks to show up in tests. Therefore if you take the test before this window period is up and you have a negative result, you should take the test again after the window period.