We tell people to test with the first void but not to give the first void for the real test. The first one of the day tends to be more concentrated because you weren't drinking fluids in your sleep so it should be the dirtiest one of the day. If you can pass that then the rest of the voids that day should be as concentrate or less than the first void. You wouldn't want to test an evening sample that might be a little diluted then go in the next day in the morning or early afternoon thinking you are clean when you weren't as clean as you thought.