C H A N G E I N T O T A L B O D Y W A T E R A S A P R E D I C T I V E T O O L F O R G R O W T H H O R M O N E T R E A T M E N T R E S P O N S E Discussion The present study evaluates the use of changes in body composition after 6 weeks for the prediction of the effect of rhGH treatment on growth after one year. In an earlier study of Hoos et al.13 a change in body composition proved to be valuable for the distinction between good and poor responders to rhGH treatment in 28 children with GHD. In this study the same cut off value17 for growth response was taken and the connection with the change in body composition was evaluated in a larger cohort of GHD children. At the same time this was done in children born SGA without spontaneous catch up growth. Conform the study of Hoos et al.13 we used the Deuterium dilution method to esti-‐ mate the TBW. Because in this technique the Deuterium as well as the collected urine could be transported by mail, it was easy possible to conduct a nationwide study. The TBW was used instead of the FFM because the hydration level of the FFM, which in adults is assumed to be 73%, is age-‐dependent and not reliable enough in children. To interpret the TBW in patients who differ in height, the re-‐ sults are corrected for height as TBW/height2, in accordance with the Body Mass Index (BMI). The TBW was not corrected for weight, because height was the dis-‐ criminating factor for good and poor responders and accepted as a more constant factor over a period of 6 weeks. The change in TBW correctly predicted the growth response in 75% of the GHD patients (Figure 1, quadrant B and D). To use this method as a predictor of the growth response, the positive and negative predictive values are most important. If the child has a change in TBW of > 0.7 l/m2, the chance to have a good growth reac-‐ tion is 73% (positive predictive value). The negative predictive value 75% suggests that if a child shows a low increase in TBW/height2, the expectation is that this child will not grow more than 0.7 SDS in one year. At the same time 8% will have a false negative test result. Whether the change in body composition of < 0.7 l/m2 is a reason to stop treatment with rhGH is under discussion, because the cut off value of 0.7 SDS is based on the mean response value in literature, but is in fact arbitrary. The reason for the different cut off value in both studies is probably based on the difference in the amount of children included. In the study of Hoos, only 28 GHD children are involved. In our study 88 GHD children are involved. In SGA children the relation of a change in body composition with the growth reac-‐ tion is less clearly demonstrated, although they have a comparable growth reaction compared to the GHD children (mean growth response of 0.832 SDS versus 0.874 SDS). In SGA children a change in TBW of > 0.7 l/m2 is strongly predictive for a growth response of more than 0.7 SDS in one year (Figure 2 quadrant B, positive 51
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