An important limitation to current IGF-I assay procedures is the potential for intra-patient variability in assay results, at least as has been shown in healthy subjects [17]. For example, in one study, IGF-I levels in healthy subjects age 50–90 years varied up to 30% between two visits made in a 2-weeks period, resulting in a change in IGF-I quartile in 40% of the subjects[32]. In this study, the intra-patient variability exceeded the assay imprecision, suggesting biological within-subject variability of IGF-I contributed to the differences over time in healthy subjects [17, 32]. However, it was stated that variability in IGF-I may have in part been due to handling, storage, or shipping [32].