Thank you for your question. The BioSURE HIV Self Test is extremely reliable. HIV tests are classified as high risk medical devices and, as such, are subjected to very severe scrutiny before they can be approved for sale. This process of CE Marking is regulated by the EMA (European Medicines Authority) and the MHRA (Medicines and Healthcare products Regulatory Authority). There is a Common Technical Specification for HIV tests and the test has, at least, to meet these. The BioSURE HIV Self Test exceeds these requirements. For tests of this nature there is a "window period". This is the time between infection and the first time that the test will reliably detect the infection. This is largely governed by an individual's response to the infection. For the BioSURE HIV Self Test this "window period" is 3 months. Unfortunately, no test will be 100% accurate. If you are HIV positive, the BioSURE HIV Self Test will detect the infection 999 times in every 1,000 cases. For people who do not have HIV and use the BioSURE HIV Self Test, the test will be negative 997 times in every 1,000 tests. So, if you have taken two tests, both with negative results, after the "window period", the chances of them both being wrong is around 1 in a million.