Hi Curious1, False negative results are most common when users test inside of the window period or if the user is/has been on PrEP or PEP within the last 12 weeks. The BioSURE HIV Self Test has a built in sample control line. This essentially means that if the test is faulty or the user does not perform the test correctly, the control line will not appear and it will be very clear to the end user that the test result is invalid. The clinical specificity of the BioSURE HIV Self Test is 99.9%. This means that on average 999 out of 1000 negative results will be correct. There is always a chance that a user may receive a false result but the risk is minimised when a test is performed outside of the window period; which is 12 weeks (84 days). I hope this helps. Kindest regards, Becky.