Bibian van der Voorn

124 CHAPTER 9 METHODS All subjects were originally included in a nutritional RCT (‘Study Towards the Effect of Postdischarge nutrition’ [STEP-1]) that compared the effects of postdischarge formula, term formula, and human milk on growth and body composition of very preterm (gestational age ≤32 wks) and/or very low birth weight (≤1,500 g) infants, as described previously 15 . Exclusion criteria were congenital malformations or other conditions known to affect growth or body composition. At term age, infants fed formula were randomized to receive either a protein- and mineral-enriched postdischarge formula or term formula between term age and 6 mo. corrected age. Corrected age is the age of the preterm born child calculated from the term date (i.e., 40 weeks gestation), and not from birth. This correction is usually maintained until the corrected age of 24 months. At the age of 8 y, parents from the STEP-study participants were asked to participate in the follow-up study, STEP-2. Exclusion criteria were incomplete follow-up, severe physical impairment or other conditions known to affect growth or body composition. DATA COLLECTION For STEP-1, the following data were extracted from medical records: birth weight, birth length, gestational age and gender. The neonatal therapeutic intervention scoring system (NTISS), an indicator for neonatal illness severity and mortality risk 16 , was calculated, and parents were asked to report their ethnicity, which was categorized as Caucasian or non-Caucasian. At term age, 3 mo. and 6 mo. corrected age, weight was measured with a digital scale to the nearest 1.0 g, and length with a length board to the nearest 0.1 cm. SD-scores (SDS’s), which quantify the deviation from a reference population, were calculated for all auxological parameters. At birth and at term age, this was done by the use of neonatal anthropometric charts, adjusting for sex and gestational age 17 . At 3 and 6 mo. corrected age, this was done by the use of postnatal growth curves, adjusting for sex and (corrected) age 18 . At term age, 3 mo. and 6 mo. corrected age, fasting venous blood samples were collected. Mean fasting duration was recorded as the interval between blood sampling and the last feed before blood sampling. Mean fasting duration was 3.4 ± 0.7h at term age, 3.6 ± 0.7h at 3 mo. corrected age, and 3.5 ± 0.7h at 6 mo. corrected age.