156 Chapter 7 Discussion Numerous studies have addressed how to measure body composition in preterm infants.17-35 Indeed there is an urgent need to monitor body composition in this population given the less favorable body composition in infancy and childhood as well as the increased risk of adverse cardiometabolic outcomes in later life.1-7 Nevertheless, till date, there is no consensus onwhichmethod should preferentially be used to assess body composition in preterm infants. Reference methods The studies included in this review used ADP, DXA, isotope dilution and MRI as reference methods. In our opinion, ADP, DXA and isotope dilution are acceptable reference methods. ADP, DXA and isotope dilution have been validated against chemical carcass analysis in piglets. 36-40 The body composition of piglets are considered to be comparable to the body composition of preterm infants. 37 Therefore, in practice, these methods are accepted as accurate measures. Nevertheless, we should take into consideration that there is variation within and between these methods. For example different types of software are used to analyze DXA. It has been reported that pediatric and infant software rely on different assumptions and yield varying results.41 Furthermore, as discussed below (under the accuracy of ADP), the statistical agreement between ADP and isotope dilution may be interpreted as poor.31,32 Therefore, we believe that some reservation is needed when comparing different reference methods. In contrast to ADP, DXA and isotope dilution, MRI has not been validated against chemical carcass analysis in subjects comparable to the neonatal population, but it has been found accurate in adult human cadaver and animal studies.42,43 Over the years MRI has been increasingly used to measure body composition in the neonatal population as well.30,44-46 However, it is yet to be widely implemented and we find it preliminary to use MRI as a reference method in comparative studies with preterm infants. In our opinion, due to the lack of a gold standard and the difference between reference methods, some reservation is needed when drawing conclusions on the validation studies included in this review. Assessment of validity To assess whether two methods agree, Bland-Altman analyses are an accepted and widely implemented method. 47 Agreement should be based on a maximum difference between the two methods that is clinically acceptable.47-50 Nevertheless, this so-called predefined clinical agreement limit was omitted in all the studies included in this review. Hence, the interpretation of these studies is limited.
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