Anthropometry-based equations overestimate the urea distribution volume in hemodialysis patients.
BACKGROUND
Protein intake in hemodialysis patients can be estimated indirectly from the protein equivalent of total nitrogen appearance (PNA) during the interdialytic period. A reliable estimate of the patient's urea distribution volume (UDV) is required to assess protein intake from PNA values. UDV values are derived frequently from simple anthropometric equations.


METHODS
UDV values based on anthropometric methods were compared with UDV values determined by direct dialysate quantitation (DDQ) in 54 stable chronic hemodialysis patients. The anthropometric methods included the following: the Watson equations (WAT), a fixed proportion of postdialysis body weight, 58% for males and 55% for females (% body wt), and skinfold thickness measurements (SFT). Postdialysis blood samples were drawn at 15-minutes postdialysis.


RESULTS
UDV(WAT) and UDV(SFT) overestimated UDV(DDQ) by about 8 L [limits of agreement (LOA): 2.6 to 14.2 L] in males and about 6 L (LOA: -0.8 to 12.4 L) in females. The overestimation by UDV(%BW) was even larger: 10.5 L (LOA: 2.0 to 19.0 L) in males and 11.1 L (LOA: 2.1 to 20.1 L) in females. The difference between UDV(%BW) and UDV(DDQ) correlated with the percentage of body fat (r = 0.57) and body mass index (r = 0.48). In a subgroup of seven patients, UDV was also determined by dilution (DIL) of the stable isotope [(13)C]urea. UDV(WAT) and UDV(%BW) overestimated UDV(DIL) significantly. In contrast, UDV(DDQ) was significantly smaller than UDV(DIL), even after correction for incomplete postdialysis equilibration. PNA values calculated using the various UDV estimates were compared with dietary protein intake (DPI) assessed from food records. PNA(DDQ) (61 +/- 10 g/day) did not differ significantly from DPI (63 +/- 13 g/day), but the agreement in individual patients varied considerably (LOA, -24 to 20 g/day). Anthropometric-based PNA values overestimated DPI by 8 to 16 g/day.


CONCLUSIONS
Anthropometry-based equations overestimate UDV values in hemodialysis patients, leading to an overestimation of PNA values. Although PNA measurements by DDQ appear to be more reliable for assessing protein intake, PNA(DDQ) values should be interpreted with caution in individual hemodialysis patients.
Kloppenburg
W D
WD
Division of Nephrology, Department of Internal Medicine, Groningen University Institute for Drug Exploration, University Hospital, Groningen, The Netherlands. w.d.kloppenburg@int.azg.nl
Stegeman
C A
CA
de Jong
P E
PE
Huisman
R M
RM
D000328:Adult; D000368:Aged; D000886:Anthropometry; D015314:Dialysis Solutions / Q000737:chemistry; D015930:Diet Records; D004044:Dietary Proteins / Q000008:administration & dosage / Q000378:metabolism; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D009584:Nitrogen / Q000097:blood; D014508:Urea / Q000032:analysis / Q000378:metabolism*
D015314:Dialysis Solutions; D004044:Dietary Proteins; D014508:Urea; D009584:Nitrogen