The nurse from the hospital told me I can mix formula with breast milk at times to ensure LO is getting enough. She has been getting plenty of breast milk so far (doctor had us test by offering a bottle of either formula or pumped milk after feeding, and she would not take it). However, now it appears she is going through a growth spurt, and for 5-6 hours a night she seems to want to nurse EVERY HOUR! I am now concerned she is not getting enough, as wanting to eat more frequently has to be drying me out some. So tonight I made a bottle of 3oz breast milk and 1 oz formula (this will also allow DH to do a late night feeding, so maybe I can sleep a little). My question is about storage. The formula is the pre-made similac (from sample bottles). It has already been mixed and was in the fridge before the hour cut-off time for room temp. If LO does not take all of it in 1 sitting (I doubt she will take the entire 4 oz, but I'd rather have too much than not enough), can it go back in the fridge until the next feeding? The similac says it does not have to be refridgerated before opening, but the sample bottles do not address storing after this. Thanks!

Re: Formula with breast milk at night
You cannot reuse formula. It is pretty normal for your LO to want to nurse every hour at this age. She is actually going to help your supply increase which will ensure a successful BFing experience.
If I were you I'd just nurse her.
Unless this is all about the extra sleep... you should not supplement! The baby is cluster feeding to tell your body to make more milk & it's a good thing that she's eating so frequently. If you start to supplement now your body isn't going to know how much your LO really needs and may not ever make enough.
If you really want to give a bottle, then I would just give her straight breast milk. If you mix formula in, then you cannot reuse it. If you give her straight breast milk, then you can reuse the same bottle within 4 hours of the time you start to give it to her. If she's really hungry, then formula will not make her full for longer so you might as well just do breast milk.
Also, just a side note, losing 8 oz in the hospital is not a crazy amount unless she was like 3 lbs at birth. My DS lost almost an entire pound in the first week of life. I hate the fact that more nurses and pediatricians are not supporting breast feeding and telling mothers they need to supplement because their LO is not gaining enough. Don't they realize it makes mothers feel like sh*t because you are new at this and hormonal and they are basically telling you that you suck as a mother? Ugh! This has become one of my pet peeves. Keep breast feeding and your LO WILL gain weight. And PP are correct, her nursing very often will increase your supply. I understand it can be frustrating, though. Keep up the good work! I promise it will get better!