I work on an OB unit, and it is my job to perform the metabolic screen (which many people call the PKU) before the babies go home.
Not only do we have to mark down what the baby is eating (breastmilk, formula, both, or IV nutrition), but also how many hours old and how many grams the baby weighs.
We are not allowed to do the test before 24hrs, and we try to do it as close to discharge as possible.
A previous poster is right, the heel prick they do tests for MANY metabolic disorders. It's often called the PKU, simply because that is the first one they started testing for, and they have added on since then like 50+ diseases, but the name "PKU" has seem to stuck.
Waiting can be a dangerous game, because the sooner the disease is caught, the better the outcome. Babies don't always (in fact, almost never) display symptoms to many of these diseases, and if you're too late you're too late. You can only prevent the damage from happening, not reverse it once it's happened.
Also, it's interesting to hear someone say that they bleed easier the older they are. I have actually experienced the opposite. The ones I do at barely 24hrs bleed like a stuck hog, the ones that are a few days old I often have to poke multiple times. The younger ones are also more sleepy and don't even seem to notice me doing the test, the older ones pitch quite the fit though.
But maybe if they are a week old they'd bleed easier, the babies I do it on are never that old so I don't know.
This is an interesting topic. I'm having a homebirth and am still trying to decide when I am going to have the test done.