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The Measurement of Total Body Potassium in Patients on Peritoneal Dialysis

♦ Objectives To assess the validity of measuring total body potassium (TBK) to estimate fat-free mass (FFM) and body cell mass (BCM) in patients on peritoneal dialysis (PD). ♦ Methods We studied 29 patients on PD (14 men, 15 women) and 30 controls (15 men, 15 women). We calculated TBK by using a whole-body counter to measure 1.46 MeV gamma-ray emissions from naturally occurring 40K. We measured total body water (TBW) by deuterium oxide dilution, and extracellular water (ECW) from bromide dilution. These measurements allowed us to estimate intracellular water (ICW), fat-free mass dilution (FFMDilution), and body cell mass dilution (BCMDilution). ♦ Results The FFMTBK in male PD patients (55.7 ± 7.0 kg) did not differ from that in male controls (57.0 ± 10.9 kg). The FFMTBK in female PD patients (38.4 ± 6.8 kg) was less than that in female controls (44.7 ± 4.5, p < 0.01). The FFMDilution did not differ from the FFMTBK. Correlation of FFMTBK and FFMDilution was r = 0.90, p < 0.0001 for all subjects; r = 0.90, p < 0.0001 for PD patients; and r = 0.90, p < 0.0001 for controls. Bland–Altman comparison of FFMDilution with FFMTBK in individuals showed bias 0.6 kg, range –8.5 kg to 9.7 kg for the whole group; bias 1.4 kg, range –7.9 kg to 10.7 kg for PD patients; and bias –0.2 kg, range –9.0 kg to 8.6 kg for controls. The BCMTBK in male PD patients (30.1 ± 4.5 kg) did not differ from that in male controls (31.9 ± 6.2 kg). The BCMTBK in female PD patients (19.0 ± 4.4 kg) was less than that in female controls (23.1 ± 2.9 kg, p < 0.01). The BCMDilution results did not differ from those for the BCMTBK. Correlation of BCMTBK and BCMDilution was r = 0.90, p < 0.0001 for all subjects; r = 0.87, p < 0.0001 for PD patients; and r = 0.93, p < 0.0001 for controls. Bland–Altman comparison of BCMDilution with BCMTBK in individuals showed bias 0.1 kg, range –5.9 kg to 6.1 kg for the whole group; bias 0.0 kg, range –6.9 kg to 6.9 kg for PD patients; and bias 0.1 kg, range –5.0 kg to 5.2 kg for controls. The [K+]ICW did not differ between PD patients and controls (148.0 ± 25.1 mmol/L vs 148.1 ± 14.3 mmol/L, p = nonsignificant). ♦ Conclusions Total body potassium is a valid, noninvasive technique for measuring FFM and BCM in PD patients. In our PD patient group, depletion of FFM and BCM as compared with controls was identified in the women but not in the men.