I work in a Level II NICU that takes 30+ weeks.  The younger the gestation, the greater the chance that they will be on TPN (total parenteral nutrition) before 48hr old.  I don't know if your states forms are similar to Iowa's, but there is a box to check when the screen is done as to what feeding the baby was getting primarily.  We mark NPO and/or TPN if done before starting enteral/trophic feeds.  This alerts the lab that feedings have not been started and they adjust the screening criterial.  We have had a few false positive results that we retest for (usually because of the TPN and on the early gestation babies).  We actually try to get the screen done before starting TPN if possible.  The usual culprit for positive is CAH (congential adrenal hyperplasia).  As far as I can recall, all of our repeats have come back negative (when done a week after TPN is stopped.) It is very rare if one of our "normal nursery" babies ever has to have a repeat done. 
Brenda Enfield, RN IBCLC
Cedar Rapids, IA 

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