Once Sherlock is able to find a vein he withdraws a little blood from the boy-no more than a few milliliters. He takes the needle out as carefully as he can manage and stands up to take the sample back over to the lab area.

While he's up he grabs a band aid and cotton ball before returning to Conan's side. He sticks the cotton ball on his arm where the blood was drawn from and puts the band aid over it.

"There, all done. Though you're probably gonna want to sit there for awhile."

And Sherlock is going to have to sit with him to make sure he doesn't experience any dangerous side effects.