It makes it much more difficult, which is why you should decline to take those tests. If you're driving was awful, if you were running into parked cars and you can't stand upright, and then they find a whole bunch of some drug in your system, that's going to be their argument. But, most cases aren't like that. In most cases, the person's driving is okay. Their actions are okay. But they will use these little tests where it is very subjective for the officer to say what they saw on the test, what they measured in the test, and they can use that against you. Declining to take the test is always a good idea.