1 Renal Unit, Leeds General Infirmary, Leeds, 2 Centre for Bone and Body Composition Research, University of Leeds, Leeds, 3 MRCHuman Nutrition Research, Cambridge and 4 Academic Unit of Medical Physics, University of Leeds, Leeds, UK
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Abstract |
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Methods. We compared the equations of Chertow (TBWCher), Chumlea (TBWChum), Hume and Weyers (TBWHW), Johansson (TBWJ), Lee (TBWL), Watson (TBWW) and TBW as 58% of body weight (TBW0.58Wt) with TBWD in 31 peritoneal dialysis (PD) patients and 32 controls. Estimates were compared with TBWD using Bland and Altman comparison. Extracellular water (ECW) was also estimated by sodium bromide dilution.
Results. In PD patients, mean TBWD was 35.04 (SD 7.84) l. Estimates were greater for TBWCher, TBWChum, TBWHW, TBWJ and TBW0.58Wt. Mean TBWL and TBWW did not differ from TBWD. Ninety-five percent limits of agreement (LOA) compared with TBWD (as a percentage of the mean) were similar for all of the different equations in PD patients (between ±15.4 and ±17.3%) except TBW0.58Wt, which was far greater (±26.4%). In controls, mean TBWD was 37.03 (SD 6.63) l. Estimates were greater for TBWCher, TBWChum, TBWHW, TBWJ and TBW0.58Wt. Mean TBWL and TBWW did not differ from TBWD. Ninety-five percent LOA compared with TBWD (as a percentage of the mean) were similar for all equations in the controls, and closer than in PD patients (between ±9.1 and ±11.5%) except TBW0.58Wt, which was again far greater than the other equations (±28.1%). TBWHW TBWD correlated with mean TBW (r=-0.412, P<0.05 in PD and r=-0.383, P<0.05 in controls). TBWW TBWD (r=-0.539, P<0.005) correlated with mean TBW in PD. TBW0.58Wt TBWD correlated with body mass index (BMI) (r=0.624, P<0.0001 in PD and r=0.829, P<0.0001 in controls) and ECW/TBW (r=0.406, P<0.05 in PD and r=0.411, P<0.02 in controls).
Conclusions. Predictive equations were less accurate in PD than controls. TBW0.58Wt was most inaccurate, with systematic overestimation of TBW with increasing BMI and ECW/TBW. There were no differences in LOA with TBWD for the other equations within each group.
Keywords: anthropometric equations; body composition; dialysis adequacy; peritoneal dialysis; total body water
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Introduction |
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The most commonly used equations for estimating TBW for calculation of Kt/V are those of Watson et al. [1] and sometimes Hume and Weyers [2]. Most crudely, TBW may be estimated as a fixed proportion of body weight (e.g. 58% of body weight). Because of the substantial potential inaccuracies seen in individual subjects with these methods, newer predictive equations derived in varying contemporary subject groups comprising healthy [3] and CRF patients [46] have been developed. The aims of this study were to evaluate these varying predictive equations in a group of patients on PD and a group of healthy control by comparison with TBW measured by deuterium oxide dilution (TBWD).
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Subjects and methods |
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Extracellular water (ECW) was estimated by bromide dilution. Patients ingested a dose of 1.55 g/kg body weight of 3.22% sodium bromide solution (25 mmol Br-/ml of solution). Bromide was measured in plasma pre-dose and at 4 and 6 h post-dose. ECW was calculated by assuming complete equilibration of bromide throughout the extracellular space and the derived value corrected by factors for the fraction of water in plasma (0.94), GibbsDonnan effect (0.95) and intracellular concentration of bromide (0.90).
TBW was estimated by the anthropometric equations of Chertow et al. [5] (TBWCher), Chumlea et al. [3] (TBWChum), Hume and Weyers [2] (TBWHW), Johansson et al. [4] (TBWJ), Lee et al. [6] (TBWL), Watson et al. [1] (TBWW) and as 58% of body weight (TBW0.58Wt). Full details of the equations are given in the Appendix.
Differences of means between groups were calculated by paired or unpaired t-tests as appropriate, relationships between variables by the Pearson correlation coefficient and comparison of measurement techniques by the Bland and Altman method [7].
The study was approved by the local research ethics committee and all subjects gave written consent to participate in the study.
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Results |
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Estimates of TBW were all very strongly correlated with TBWD, but much less so for TBW0.58Wt than for other equations (Table 4). Correlations of the intermethod differences were performed with mean values of TBW to determine any systematic error related to size of the TBW compartment, and with body mass index (BMI) and the ECW/TBW ratio to determine any relationship between intermethod differences with body fatness or hydration status (Tables 5
and 6
). There were negative correlations of TBWHW TBWD and TBWW TBWD, suggesting progressive underestimation of TBW by these equations in the PD group with increase in the TBW volume. A similar relationship was found only for TBWHW TBWD in the controls. TBW0.58Wt TBWD was correlated with BMI and ECW/TBW in both the PD and control groups, suggesting progressive overestimation of TBW by TBW0.58Wt with increasing fatness and extracellular fluid volume expansion. The only other significant correlations were found in the control group, with TBWCher TBWD and TBWL TBWD correlated with BMI and TBWHW TBWD correlated with ECW/TBW.
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Discussion |
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We, and others, have previously demonstrated significant differences between anthropometric equations for TBW estimation and gold-standard dilution techniques in PD [4,1719]. In relation to the modest variability in clearances achieved with PD, differences in Kt/V that would be considered clinically very significant can arise from different methods of calculating V with the same actual total clearance (Kt) [17,18]. This has led to the development of further equations derived in more contemporary populations, including equations derived in specific renal disease populations. The characteristics of the various equations, including terms utilised, populations they were derived in and reference method for estimating TBW, are summarised in Table 7.
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Mean values for anthropometric estimates of TBW all tended to be higher than TBWD. This difference was significant in both PD and control groups for TBWHW, TBWJ, TBWChum, TBWCher and TBW0.58Wt, but not for TBWW or TBWL. Whilst this difference was modest for TBWHW, TBWJ and TBWChum, it was most marked for TBWCher and TBW0.58Wt. Previous studies have shown TBWW to be similar to [6], modestly increased [17] or modestly decreased [18] in PD patients compared with dilution methods. Arkouche et al. [19] found TBWW to produce higher values of TBW than estimates by H218O dilution but not TBWD [19]. Lee et al. [6] also found no significant difference between mean TBWW and TBWHW compared with BIA in haemodialysis patients. Tzamaloukas et al. [20] have shown TBWCher to exceed TBWW by 3.5 l in PD patients and Lee et al. [6] have shown TBWCher to overestimate TBW estimated by BIA by 4 l in haemodialysis patients and healthy controls. These are comparable with the overestimate we observed of TBWCher compared with TBWW and TBWD. It is of significance that TBWCher was derived by measurements of TBW by BIA in a large population of haemodialysis patients, with BIA being performed prior to a dialysis session. At this time, TBW will have been at its highest in most patients, prior to fluid removal by dialysis. Thus, this equation may overestimate TBW when applied to subjects where TBW is likely to represent a lower proportion of body weight. Also, differences in body composition (e.g. related to body fat and racial differences) between the North American population in which TBWCher was derived and our European population may be important. The use of bioelectrical impedance, the results of which may differ significantly in individuals from reference methods [1719], as a reference method rather than isotope dilution may also have contributed to the differences observed.
The Bland and Altman comparisons show very significant errors in estimating TBW in individual subjects. The ranges of these differences are consistently greater in the PD group than healthy control subjects. The magnitude of the range of 95% LOA for TBWW were of very similar magnitude to those described in previous works by ourselves [17], Johansson et al. [4] and Arkouche et al. [19]. Even greater ranges were reported by the study of Wong et al. [18], which found larger ranges for individual agreements in obese compared with non-obese subgroups, demonstrating the importance of variable body fatness. In the two-compartment model of body composition, fat is anhydrous, whereas FFM has water as its dominant component, so variation in ratio of body fat and FFM would be expected to have a major impact on prediction of TBW, with overestimation of TBW in obesity. This would be most significant in TBW0.58Wt, which makes no account for body fatness. Hence, the discrepancy seen with this estimate and the strong systematic error related to BMI are entirely to be expected. Differences in body fat will also be a major factor underlying effects of gender. In particular, greater body fat in women will explain the greater overestimate with TBW0.58Wt in females. Increasing body fat is associated with ECW expansion [13], which explains the relationship of TBW0.58Wt TBWD with ECW/TBW in both PD and control groups. Interestingly, we did not identify major differences in hydration in PD subjects compared with controls, suggesting that overhydration is not an inevitable feature in these patients. The presence of subjects with more abnormal hydration would have increased the degree of error of estimation observed. It is also of note that the ratio of ECW/TBW was higher than traditionally quoted in the literature.
Thus, variability in body composition between individuals cannot be sufficiently accounted for by anthropometric equations to allow precise estimation of TBW. This is especially marked in patients with conditions such as advanced renal failure and dialysis, which affect body composition. The factors included in these equations may partly account for the variability of body fat (which is anhydrous and thus an important determinant of the proportion of body weight accounted for by TBW) but are unlikely to be sensitive to variable hydration. Variations in the performance of these equations when applied in our study populations may relate to the differences in methodology and subject characteristics in the original derivation of the equations.
This major limitation in estimating TBW must be considered in future decisions about the most appropriate methods of describing dialysis adequacy and solute clearances in individual patients. Whilst it seems logical to normalise clearances to measures reflecting individual patient size, such normalisation will produce a degree of arbitrary variability. Thus, Kt/V may be useful for observing changes within an individual or comparing groups. However, caution must be applied in interpreting individual clearances normalised in this way, for example when comparing values to targets for clearance as significant amounts of variability may reflect the normalisation procedure rather than biological effects. Given the large experience with the Watson equation, none of the alternatives provide a sufficient benefit to justify change (at least in patients with similar characteristics to our study group). TBW estimated as a fixed proportion of body weight produces unacceptably inaccurate values with systematic error related to fatness and hydration and should be abandoned from clinical practice.
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Appendix |
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Chertow et al. [5]
TBW=-(0.07493713xage)-(1.01767792xgender)+(0.12703384xheight)-(0.04012056xweight)+(0.57894981xdiabetes)-(0.00067247xweight2)-(0.03486146xagexgender)+(0.11262857xgenderxweight)+(0.00104135xagexweight)+(0.00186104xheightxweight)
Where for gender male=1; diabetes=1
Chumlea et al. [3] (equations for white subjects)
Male: TBW=23.04-(0.03xage)+(0.50xweight)-(0.62xBMI)
Female: TBW=-10.50-(0.01xage)+(0.20xweight)+(0.18xheight)
Hume and Weyers [2]
Male: TBW=(0.194786xheight)+(0.296785xweight)-14.012934
Female: TBW=(0.34454xheight)+(0.183809xweight)-35.270121
Johansson et al. [4]
Male: TBW=-10.759-(0.078xage)+(0.312xweight)+(0.192xheight)
Female: TBW=-29.994-(0.0004xage)+(0.214xweight)+(0.294xheight)
Lee et al. [6]
Male: TBW=-28.3497+(0.243057xheight)+(0.366248xweight)
Female: TBW=-26.6224+(0.262513xheight)+(0.232948xweight)
Watson et al. [1]
Male: TBW=2.447-(0.09156xage)+(0.1074xheight)+(0.3362xweight)
Female: TBW=-2.097+(0.1069xheight)+(0.2466xweight)
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Acknowledgments |
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Notes |
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References |
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