Re: anyone decline the PKU?
It's not how long after birth it's administered, but how long baby has been getting milk - not colostrum - that makes a difference.
Baby should be getting milk (again, not colostrum) for 24+ hours before the test.
For formula fed babies, everything is fine to do it 24 hours after the first bottle, kwim? But for breastfed babies, mom's milk wouldn't come in for the first couple days (or possibly even longer), so if the test is done in the first couple days it's actually useless.
They do it before baby leave the hospital, though, because then they cover their own arses by having done it, kwim?
For us, we have declined it done in the hospital with all the boys (didn't know this info with DD), and instead the nurses give us the test form (with the dots they fill in) and we come in later in the first week. The nurses haven't ever questioned us once we tell them we understand the test, and that baby should be on milk, and that we'll be watching for my milk to come in and will bring baby for the test in the day or 2 following that.