Improved use of the {13C} octanoic acid breath test as intraindividual parameter to study the effect of a prokinetic drug on gastric empytying in preterm infants with oral feeding intolerance / Kulik W, van Weissenbruch MM, Menelik N, Cranendonk A, Kneepkens CM, Lafeber HN J Chromatogr B Biomed Sci Appl 2001 Jan;750(1): 147-53