Risk Factors for Iron Deficiency in Very Preterm Infants at 4-6 Months Corrected Age

Background: Iron is an essential micronutrient, especially in infants and young children and is required for erythropoiesis and development of the central nervous system. However, iron deficiency (ID) is the most common micronutrient deficiency worldwide. ID and iron deficiency anemia (IDA) have been associated with poor neurodevelopmental and behavioural outcomes later in life. Preterm infants are particularly at risk of developing ID in early life due to lower iron stores at birth, accelerated growth in the first weeks of life and multiple phlebotomies while in hospital. Therefore, international recommendations suggest prophylactic iron therapy of 2-4 mg/kg/day starting at 2-6 weeks of age until at least 6-12 months in preterm and low birth weight infants. This prophylactic iron supplementation has been shown to be effective at reducing the incidence of ID and IDA. However, the published work mainly involves moderate to late preterm infants and the research is lacking on iron status after discharge in very preterm infants (VPI, <31 weeks gestational age). Based on our previous work, 32% of the VPIs were iron deficient at 4-6 months corrected age despite this early supplementation. Since the development of ID may have permanent detrimental effects on the developing brain of these high-risk preterm infants, a knowledge of risk factors for ID is also important to identify strategies focused on its prevention.

Objective: To investigate the risk factors associated with development of ID

Methods: A retrospective cohort study was conducted at the IWK Health Centre using a population based provincial Perinatal Follow-Up Program database. All live-born VPIs born in Nova Scotia between 2005-2018 were included. Patients with congenital malformations, chromosomal anomalies, or who died prior to outcome assessment were excluded. As a standard of care, all these infants were started on prophylactic iron supplements (2-3 mg/kg/day) at 2-4 weeks of chronological age. Iron dosage was regularly adjusted during the hospital stay as guided by serum ferritin levels. At discharge, it was recommended to continue iron prophylaxis until 9-12 months corrected age. All these infants underwent a blood test during their first neonatal follow-up visit at 4-6 months corrected age to check for hemoglobin, reticulocyte count and serum ferritin. ID was defined as serum ferritin <20g/l or <12g/l at 4 and 6 months respectively. A univariate analysis was performed by using a series of single variable logistic regression models to identify the factors associated with presence of ID. Factors with a p-value < 0.20 in the univariate analysis were entered into a multivariable risk model for occurrence of ID using a backwards selection procedure. Variables with a p-value < 0.05 were retained.

Results: Of 411 infants included in the study, 32.1% (n=132) had ID. The prevalence of ID decreased over time (37.6% in 2005-2011 vs 25.8% in 2012-2018 cohort). Table 1 compares the antenatal and neonatal characteristics of the ID and non-ID groups. Table 2 compares sociodemographic variables and clinical variables at the time of follow up of the two groups. Independent risk factors for ID were: gestational age (<27 weeks to >27 weeks ) (OR:1.7 (1.0-2.9), p=0.04) and gestational hypertension (OR: 2.1(1.2-3.7), p=0.009). Independent factors protective for ID were: mixed feeding (breast milk and formula compared to formula alone) (OR: 0.5 (0.2-0.9), p=0.021) and iron supplementation at follow-up (OR:0.5 (0.3-0.9), p=0.02).

Conclusion(s): Despite prophylactic iron supplementation, one-third of VPIs had ID at 4-6 months corrected age. Gestational hypertension in mother and gestational age < 27 weeks were independent risk factors for ID. In addition, despite adjusting for iron supplementation at follow-up, the formula feeding group was more likely to have ID compared to the mixed feeding group. This may be because of the sub-therapeutic iron intake in the formula fed infants. It is often thought that formula milk may have sufficient iron to meet the demands of growing infants and thus, they are less likely to receive higher doses of supplemental iron beyond what is contained in the formula. However, this may not be true since the iron present in formula may not have the same bioavailability as breast milk. Future prospective studies are required to further validate these observations. Nonetheless, the study identified important areas to mitigate ID in VPIs.