title: Body Composition Affects Urea Distribution Volume Estimated by Watson's Formula.
abstract: OBJECTIVE
  Dialysis machines use the Watson formula (Vwatson) to estimate the urea distribution volume (UDV) to calculate the online Kt/V for each dialysis session. However, the equation could give rise to inaccuracies. The present study analyzes whether body composition affects UDV estimated by Vwatson in comparison to bioimpedance spectroscopy (Vbis) as the reference method.
  
  
  METHODS
  This is a transversal study performed in the setting of a hemodialysis unit.
  
  
  METHODS
  Prevalent hemodialysis patients.
  
  
  METHODS
  The same day, UDV was measured using Vwatson and Vbis. We compared their results.
  
  
  METHODS
  Differences between UDV using Watson equation and Vbis.
  
  
  RESULTS
  We included 144 prevalent patients. Vwatson overestimated the volume with regard to Vbis (Vwatson - Vbis) by 2.5 L (1.8 L; P = .001). We found an excellent correlation between the 2 methods. A higher mean Vwatson - Vbis value was correlated to older age (P = .03), body mass index (P = .01), fat tissue index (P = .001), lower lean tissue index (P = .001), lower extracellular water (P = .01), and intracellular water (P = .001).
  
  
  CONCLUSIONS
  Body composition affects UDV estimated by Vwatson, thus modifying the result of Kt/V. In young patients who present more lean tissue and less fat tissue, Kt/V is underestimated with Vwatson.
authors.affiliation: Department of Nephrology, Hospital Gregorio Marañón, Madrid, Spain. Electronic address: vega.almudena@gmail.com.
authors.forename: Almudena
authors.initials: A
authors.lastname: Vega
authors.affiliation: Department of Nephrology, Hospital Gregorio Marañón, Madrid, Spain.
authors.forename: Caridad
authors.initials: C
authors.lastname: Ruiz
authors.affiliation: Department of Nephrology, Hospital Gregorio Marañón, Madrid, Spain.
authors.forename: Soraya
authors.initials: S
authors.lastname: Abad
authors.affiliation: Department of Nephrology, Hospital Gregorio Marañón, Madrid, Spain.
authors.forename: Borja
authors.initials: B
authors.lastname: Quiroga
authors.affiliation: Department of Nephrology, Hospital Gregorio Marañón, Madrid, Spain.
authors.forename: Kyra
authors.initials: K
authors.lastname: Velázquez
authors.affiliation: Department of Nephrology, Hospital Gregorio Marañón, Madrid, Spain.
authors.forename: Jara
authors.initials: J
authors.lastname: Ampuero
authors.affiliation: Department of Nephrology, Hospital Gregorio Marañón, Madrid, Spain.
authors.forename: Juan Manuel
authors.initials: JM
authors.lastname: López-Gómez
mesh_terms: D000273:Adipose Tissue / Q000378:metabolism; D000368:Aged; D000369:Aged, 80 and over; D001823:Body Composition; D015992:Body Mass Index; D001835:Body Weight; D003430:Cross-Sectional Studies; D017097:Electric Impedance; D005260:Female; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D006435:Renal Dialysis; D017582:Renal Replacement Therapy; D014508:Urea / Q000378:metabolism*
chemical_list: D014508:Urea