Limitations

For the study to be adequately powered to determine iron status based on a difference in mean haemoglobin of at least 5 g/L, 76 preterm infants needed to be recruited. As only 61 infants were included in the final data analysis, the study may lack power to accurately determine a difference in iron status between preterm infants living in Auckland at four months after discharge. There is little available information on the nutritional status of preterm infants after discharge, and determination of the sample size was difficult. The power calculation was based on just one study investigating the iron status of low birth weight infants in New Zealand, and therefore may not be completely accurate (Thom et al., 2003). Our study does however provide valuable insight into the post-discharge iron status and feeding and supplementation practices of preterm infants living in New Zealand and is a basis for larger studies to be conducted.

Another limitation of this study was that medical records from other hospitals could not be accessed. Seventeen infants were transferred to hospitals other than Auckland City Hospital for continued care prior to discharge home. In addition, infants may have been readmitted to hospitals other than Auckland City Hospital after initial discharge. Information about erythrocyte transfusions, iron supplementation and other procedures which may have affected infant iron status at these other hospitals could not be collected. Having access to medical records from Counties Manukau District Health Board and Waitemata District Health Board would have been beneficial and allowed for more accurate determination of factors which may affect iron status at four months after discharge. Conducting a multi-centre study could also provide a solution to this problem.

While it was a strength of this study that the cohort consisted mostly of moderate to late preterm infants, as only 23% of the infants were born at less than 32 weeks gestation, the study was inadequately powered to perform comparisons between the iron status of infants born with differing degrees of prematurity. As more moderate to late preterm infants are born each year compared to very and extremely preterm infants (592 compared to 228) it is not surprising fewer infants born before 32

weeks gestation were recruited (Pot et al., 2012). As there was only a short period of time for recruitment, it made it difficult to recruit a large enough sample size of infants born before 32 weeks gestation. Running the study over a longer period of time, focusing on a longer recruitment period, would have meant that more infants born very and extremely preterm could have been recruited into this study, ensuring a larger overall sample size and enabling sub-group comparisons to be made.

Although the aim of this study was to present a situational analysis of preterm infant iron status at four months after discharge, infants who were discharged more than five months prior to appointment were also included in statistical analysis (n=3). This highlights the difficulties associated with conducting home visits as part of scientific research as appointment dates needed to be convenient for the families involved in the study.

To determine the iron status of each infant, capillary blood samples were taken via a heel prick. This method of collection was chosen in preference to venous blood collection due to limitations in resources and to avoid subjecting the infants to unnecessary invasive procedures. The literature however shows that the concentration of iron biomarkers determined using capillary samples is less precise than the corresponding concentrations from venous blood samples due to the fact that interstitial fluid may dilute capillary samples (Gibson, 2005). The results of this study may therefore not accurately reflect the real prevalence of ID and IDA in preterm infants; although the effect of biomarker dilution is likely to be small.

Finally, there were limitations with regards to determining iron status at four months after discharge. In order for the iron status of an individual to be assessed, at least two biomarkers must be reported; one reflecting iron stores and one reflecting red blood cell indices (Lynch, 2010). Despite using different methods to determine iron status, including a point-of-care device for detecting anaemia, a full set of results were not available for all infants in this study. Iron status could only be described for 48 infants in this study, with three infants only having biomarkers reflecting their iron stores and ten infants having only haemoglobin concentrations. This means that there

may have been infants who were classified as having ID whereas in fact they actually had IDA. In addition, it is possible that the infants who only had haemoglobin concentrations may have had ID which could not have been detected, therefore underestimating the rate of ID in this group.

Despite the limitations of this study it is clear that current post-discharge iron supplementation protocols should be addressed. This study has shown that preterm infants who are discharged home from hospital without iron supplements have an increased risk of developing ID and IDA within the first four months after discharge. This is of concern as a suboptimal iron status early in life has been linked with gastrointestinal disturbances, immune and thyroid dysfunction, and temperature instability (Collard, 2009; Rao & Georgieff, 2009). The most devastating effects of ID and IDA in preterm infants are on neurodevelopment and cognition, with ID and IDA associated with poorer recognition memory, altered motor movements and learning difficulties later in life (Georgieff & Innis, 2005; Siddappa et al., 2004; Tamura et al., 2002). Routine iron supplementation for all preterm infants regardless of their gestational age or mode of feeding (but with the exception of those receiving multiple erythrocyte transfusions) appears to be a safe and effective way to reduce the risk of ID and IDA at four months after discharge.