Almost all infants born <29 weeks gestational age develop apnea of prematurity and are treated with caffeine. Type of diet and disease states may be significant contributors of variability in caffeine metabolism and pharmacokinetics (PK) in this population. This prospective, observational, open-label, opportunistic PK study will compare the population PK of caffeine between infants fed formula and infants fed exclusively breast milk; compare the activities of caffeine metabolizing enzymes between infants fed formula and infants fed exclusively breast milk; and determine the effect of hypoxia, hypotension, and infection on caffeine PK and metabolism in premature infants.

This study will use a consecutive patient sampling approach. All eligible participants admitted at each site will be approached. Caffeine and feedings will be administered per standard of care. Manipulations of caffeine dosing or feeding regimens will not be a part of this protocol. To minimize the amount of blood sampling, the investigators will use a sparse sampling methodology. Urine will be collected for analysis of caffeine and caffeine metabolite concentrations. Caffeine pharmacokinetics will be described by population pharmacokinetic analysis. Urinary concentrations of caffeine and its metabolites will be used to calculate metabolic ratios as markers of enzyme activity.

Inclusion Criteria:

• Written informed consent from parent(s) or legal guardian(s)
• <29 weeks birth gestational age
• Postnatal age ≤15 days
• Receiving caffeine (intravenous or oral) per standard of care for prevention or treatment of apnea of prematurity

Exclusion Criteria:

• Known major congenital or chromosomal anomaly

Premature infants given caffeine for the prevention or treatment of apnea of prematurity.

Will consider on a case-by-case basis (de-identified data only)