Oral Amoxicillin and Cephalexin PK/PD in Neonates

The goal of this study is to figure out the best doses for two oral antibiotics (called amoxicillin and cephalexin) when they are used to treat infections in newborns and young infants. In order to do this, the study plans to enroll two groups of young infants who are admitted to the Children's Hospital Colorado. In the first group, infants already receiving one of these two antibiotics will be enrolled. Over a few days in the hospital, up to 5 blood samples will be collected from each infant to measure how much of the drug is in their body at different points in time after a dose. In the second group, infants who are already receiving an IV antibiotic and who are eating normally or receiving oral medications will be enrolled. These infants will receive one dose by mouth of either amoxicillin or cephalexin, in addition to the IV antibiotic already being used to treat their infection. After the oral antibiotic dose,blood will be drawn every few hours to measure how much of the drug is still in their body. Blood levels of the antibiotics will be used to calculate how much and how often both antibiotics would need to be given to treat a variety of infections that are common in infants. The study will calculate if using a single blood level can predict whether or not an infant might be at risk of the antibiotic not working well enough for them.