74 Chapter 4 Endocrine Parameters IGF-1 was sampled from umbilical cord blood at birth and from venipuncture or capillary puncture every other week between birth and 36 weeks PMA. A chemiluminescence immunoassay (LIAISON®, DiaSorin, Saluggia, Italy) was used to analyze IGF-1 (intra-assay percent coefficient of variation (% CV): 8%, inter-assay % CV: 7%). The number of samples per week PMA is depicted below (Table 2). Table 2. Sample size for IGF-1 analyses per postmenstrual age. PMA 24 25 26 27 28 29 30 31 32 33 34 35 36 N Total 2 3 7 17 21 33 30 34 25 33 25 26 29 N Postnatal 0 0 1 3 6 12 16 22 25 33 25 26 29 N Total reflects the total number of samples taken. N Postnatal reflects the number of samples excluding umbilical cord blood. Potential Confounders The following comorbidities were assessed as potential confounders in the association between nutrient intake and IGF-1: • Bronchopulmonary dysplasia (BPD); defined as having had a need for supplemental oxygen for at least 28 days at 36 weeks PMA or discharge home (whichever came first) (17). • Necrotizing enterocolitis (NEC); classified according to the Modified Bell’s staging criteria (18). • Late-onset sepsis (LOS), defined as sepsis occurring 72 h after birth with a positive blood culture or a full course of antibiotic treatment (19). • Retinopathy of prematurity (ROP), classified according to the International Classification for Retinopathy of Prematurity (20). • Intraventricular hemorrhage (IVH), classified according to the Papile grading system (21). • Patent ductus arteriosus (PDA), which was defined as hemodynamically significant if treatment was prescribed (22). In addition, gender, gestational age at birth, postmenstrual age at the time of blood sampling, weight and weight SD score were assessed as potential confounders. Statistical Analysis The change in IGF-1 over time was predicted for each individual using mixed models. The associations between nutrient intake, IGF-1, and potential confounders were assessed with regression analyses. Analyses were conducted using IBM® SPSS® Statistics 26 for Windows (IBM Corp., Armonk, NY, USA). Two-sided statistical significance was assumed at p-values less than 0.05.
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