Why are you told not to have sex the night before a testosterone level test? Does it increase or decrease the result? Just curious.

No idea. From what I read it has a minimal effect. I suppose it depends on why you're being tested. If you get a chance you should ask your doctor that. He would be the best one to talk to. Since testosterone is not a usual test chances are you will be called to discuss the results. Just put this on the list of questions to ask him.