It doesn't work that way, Doc. The kidneys provide a steady drip to the bladder whether you're peeing or not. Only the urine that has collected in the bladder comes out the penis. The urine in the beginning, middle, and end of the stream comes from the same bag. If you've diluted then there's probably a few cc's produced while you're peeing but it would have the same chemical makeup as the urine produced 5 minutes before that's already sitting in the bladder. I'm probably misunderstanding what you're saying. I do that a lot. Or maybe I don't understand physiology as well as I think I do. Maybe that's the reason I can't see why it would make a difference at what point in your stream the specimen is collected from.
I suck sometimes at explaining physiology and anatomy. I know the processes but can't explain every detail so people tend to not believe my explanations. I'm pretty sure about this midstream thing though. It doesn't hurt though to try and catch mid stream.