You will find exact information about this in the instructions for the purchased express test. In most cases, they are sensitive to a certain concentration of hCG - above 25 mIU / ml. This level of the hormone in the urine is determined already on the first day of delay. After a few days, the concentration of hCG will increase significantly, and the test will be much more accurate in diagnosing pregnancy.
There are rapid tests that determine pregnancy at an earlier date. They are sensitive to hCG at a concentration of 10 mIU / ml. Such tests can be used to diagnose pregnancy 2-3 days before the date on which the critical days were supposed to begin.