adssf

When a doctor asks for midstream urine, it means that he needs a sample for bacteriological culture (e.g. when he suspects a bladder infection). Healthy urine is sterile, as long as it is in the bladder. But some bacteria are present in the outer part of the urethra, and are picked up by your first squirts as you pee. By having you flush out those bacteria, and keeping a midstream sample only, the doctor will avoid the risk of false positives (growth of germs from the urethra, which could be mistaken for germs from within the bladder).
But for our purpose these germs are absolutely harmless, so imho there is absolutely no point in drinking your midstream only.