Table 3

 Illustration of the advantages and disadvantages of different techniques for measuring body composition in clinical practice. The patient is a 10 year old boy with chronic renal failure on peritoneal dialysis. Which measurements might be useful in terms of baseline assessment and monitoring of body composition? In this child, measurement of lean mass would be potentially useful as a guide for basing nutritional requirements or drug doses, but measurements may be complicated by oedema or variation in FFM hydration or both

*Multifrequency BIA could provide informative data on hydration in the future.
BIA, bioelectric impedance analysis; BMI, body mass index; DXA, dual energy x ray absorptiometry; FFM, fat-free mass; FM, fat mass; TBW, total body water.
Conclusions: In practical terms, the techniques most useful in this patient are BMI ± skinfold thickness measurement for monitoring fat mass and distribution. If available, DXA could be used for assessing changes in (limb) lean mass, which might be more relevant for nutritional requirements and drug doses. This technique will be of greater use in practice once reference data are available.
Simple measurements
Skinfold thicknessUseful index of regional fatnessPoorer accuracy and precision in obese/oedematous individuals
BMIUseful as a simple baseline and longitudinal measurement of relative weightWill not allow assessment of fat and lean masses and changes in fat which may be more relevant in terms of nutritional/drug requirements and metabolic risk
Waist circumferenceUseful baseline and longitudinal measurement. Centile charts are available, and central fatness is of greater relevance to metabolic riskValue may be limited in this child who is on peritoneal dialysis
Predictive measurements
Skinfold thickness equations predicting FM and FFMPoor precision and accuracy—any error will be to magnified by the use of prediction equations not derived from a comparable population
BIACould provide information on the direction of longitudinal changes in lean massPoor accuracy in absolute terms. Changes in body weight, the relative proportions of trunk and limbs, or FFM hydration will introduce further errors*
Two-component techniques
DXACould be used to measure regional (limb) lean mass which could provide information on which to base nutritional requirements or drug dosesLimited use for measuring baseline fat mass or longitudinal changes in fat mass, since measurements are known to be biased by body size (thickness). In this child, skinfold thicknesses may already provide information on regional fatness
Densitometry (BodPod)Could provide longitudinal data on both lean and fat mass since its accuracy is less likely to be affected by changes in fatnessVariations in the hydration (and therefore density) of FFM in this child will invalidate assumptions of constant density
Deuterium dilutionCould estimate whole body lean mass if hydration known, and could potentially aid calculation of dialysis fluidsResults for both TBW and FFM may be affected by variation in hydration in this child