Sources of Variation in Estimates of Lean Body Mass by Creatinine Kinetics and by Methods Based on Body Water or Body Mass Index in Patients on Continuous Peritoneal Dialysis

Objective

We identified factors that account for differences between lean body mass computed from creatinine kinetics (LBM_cr) and from either body water (LBM_V) or body mass index (LBM_BMI) in patients on continuous peritoneal dialysis (CPD).

Design

We compared the LBM_cr and LBM_V or LBM_BMI in hypothetical subjects and actual CPD patients.

Patients

We studied 439 CPD patients in Albuquerque, Pittsburgh, and Toronto, with 925 clearance studies.

Intervention

Creatinine production was estimated using formulas derived in CPD patients. Body water (V) was estimated from anthropometric formulas. We calculated LBM_BMI from a formula that estimates body composition based on body mass index. In hypothetical subjects, LBM values were calculated by varying the determinants of body composition (gender, diabetic status, age, weight, and height) one at a time, while the other determinants were kept constant. In actual CPD patients, multiple linear regression and logistic regression were used to identify factors associated with differences in the estimates of LBM (LBM_cr<LBM_V, or LBM_cr<LBM_BMI).

Main Outcome Measure

We sought predictors of the differences LBM_V – LBM_cr and LBM_BMI – LBM_cr.

Results

Both LBM_V (regardless of formula used to estimate V) and LBM_BMI exceeded LBM_cr in hypothetical subjects with average body compositions. The sources of differences between LBM estimates in this group involved differences in the coefficients assigned to gender, age, height, weight, presence or absence of diabetes, and serum creatinine concentration. In CPD patients, mean LBM_V or LBM_BMI exceeded mean LBM_cr by 6.2 to 6.9 kg. For example, the LBM_V obtained from one anthropometric formula was 50.4±10.4 kg and the LBM_cr was 44.1±13.6 kg (P < .001), whereas among the 925 clearance studies, only 216 (23.3%) had LBM_cr>LBM_V. The differences in determinants of body composition between groups with high versus low LBM_cr were similar in hypothetical and actual CPD patients. Multivariate analysis in actual CPD patients identified serum creatinine, height, age, gender, weight, and body mass index as predictors of the differences LBM_V−LBM_cr and LBM_BMI−LBM_cr.

Conclusions

Overhydration is not the sole factor accounting for the differences between LBM_cr and either LBM_V or LBM_BMI in CPD patients. These differences also stem from the coefficients assigned to major determinants of body composition by the formulas estimating LBM.