Fractional Urea Clearance Estimates Using Two Anthropometric Formulas in Continuous Peritoneal Dialysis: Sex, Height, and Body Composition Differences

Abstract

Objective To compare estimates of urea volume (V) and KT/V obtained by the Watson and Hume anthropometric formulas, and to identify the similarities and differences between these estimates. Design Theoretical analysis applying wide variations in the determinants of anthropometric V (age, height, weight) in hypothetical women and men. Analysis of urea kinetic studies performed in patients on continuous peritoneal dialysis (CPD). Setting Four dialysis units in Albuquerque, two in Athens, and two in Thessaloniki. Participants Three hundred and two CPD patients who had 440 urea kinetic studies. Intervention Standard urea clearance was performed by 24-hour collections of urine and drained dialysate followed by blood sampling. V was estimated by both the Watson and Hume formulas. Main Outcome Measures Estimates of V and KT/V were compared separately in women and men by Student's t-test, linear regression, and limits of agreement (mean difference±2 SD). The agreement of the KT/V estimates was also tested by the kappa ratio using a value of 1.70 weekly as the lowest acceptable KT/V. Results The theoretical analysis indicated important disagreement only in extreme variations from the ordinary in height and, to a lesser extent, weight. Differences due to height variation were pronounced only in hypothetical women. CPD patient findings were as follows: in women, Watson V and weekly KT/V were 30.4±4.4 L and 2.10±0.61, respectively. Corresponding Hume estimates were 30.3±5.4 L and 2.12±0.66, respectively. Corresponding estimates for men were 40.5±5.7 L and 1.92±0.57 (Watson) plus 41.4±5.6 L and 1.88±0.57 (Hume), respectively. By linear regression, KT/VHume = -0.083 + 1.052 (KT/Vw8tson), r = 0.961 (women); and KT/VHume = -0.026 + 0.992 (KT/Vwatson), r = 0.985 (men). Limits of agreement were -1.41 L and 2.10 L for V, and -0.15 and 0.14 weekly for KT/V. In 94.3% of the cases, KT/Vw8tson and KT/VHume agreed (both >1.70 or both <1.70 weekly). Kappa ratio was 0.875 (excellent agreement). The concordant and discordant groups differed in height and degree of obesity, in agreement with the theoretical analysis. Conclusion The Watson and Hume formulas provide similar estimates of V and KT/V in CPD patients. Differences may be noted only if women's height or, to a lesser extent, both sexes’ weight is at a great variance with the ordinary values.