Hi Naomi. Although the BioSURE HIV self test is extremely stable (result doesn't really change over time) we have a read time of an hour, this means that the test should not be read after an hour as there is a small possibility the result might change. I must say in all the tests I have performed, my result has never changed however sometimes the blood and buffer solution does settle right at the bottom of the strip. The negative tests you have performed are conclusive and there is no need to re-test. I hope this makes sense and thank you for choosing BioSure. Kindest regards Francesca Bard, BioSure (UK) Ltd