You can only rely on the pregnanct test result if you follow all of the instructions. Most instructions say to read the results after three minutes. Any result after that time should be discarded.
If you don't follow the instructions and you wait longer and read the results after the suggested time, the results are unreliable. Waiting longer than the suggested time can lead to a second line showing up which may not be hCG. That's an "evaporation line" which is not a positive test and it has nothing to do with hCG being present in the urine.
This is what I suggest:
1. Read the manufacturer's instructions carefully.
2. Wait the suggested amount of time and read the test results.
3. If there is a negative result after the suggested time, throw the test away to prevent any confusion.
4. Re-reading the test at a later time will confuse you.
5. If the test is only faintly positive after the suggested amount of time, chances are that you could be pregnant. The hCG in your urine may have been too low to give you a stronger positive result. If there is a faint line it's suggested that you repeat the test a couple of days later.