Pitfalls in the assessment of body composition in survivors of acute lymphoblastic leukaemia

Abstract

Background: Body fat mass (FM) and fat free mass (FFM) in childhood are often estimated by conversion of a measured variable into compartmental body composition using constants or regression equations that have been previously derived in healthy individuals. Application of such constants or equations to children with disease states may lead to inappropriate conclusions since the "normal" relationships may become altered.

Aims and methods: To test this hypothesis by taking measurements of body composition using dual energy x ray absorptiometry (DEXA) as a "gold standard" method and calculating hydration and body potassium constants using isotopic water dilution and whole body potassium counting. Measurements of bioelectrical impedance (BIA) by two different analysers (RJL and Holtain) were also performed to allow comparison with body water measurements.

Results: Measurements were performed in 35 children treated for acute lymphoblastic leukaemia (ALL) and compared to those in 21 children treated for a variety of other malignancies and 32 healthy sibling controls. The mean hydration and potassium content of FFM was significantly reduced in the ALL group compared to both other malignancies and controls. Application of equations derived from controls for the measurement of FFM derived from bioelectrical impedance led to an underestimation of 1.15 kg when compared to that derived from DEXA in children treated for ALL but not in other malignancies. For all groups combined, BIA was significantly different in the two analysers.

Conclusion: Care needs to be taken in the application of equations derived from the normal population to body composition measurement in children treated for ALL.

Figure 1

Difference between impedance values from the RJL and Holtain analysers against the mean impedance for the two methods. r = 0.78, p<0.001. Dashed lines represent limits of agreement.³³

Figure 2

Relationship between total body water measured by isotopic water dilution and the impedance index (height²/impedance) for the three groups combined. Squares represent measurements from Holtain, r = 0.98, p<0.001. Circles represent measurements from RJL, r = 0.98, p<0.001.