title: Kinetic behavior of urea is different from that of other water-soluble compounds: the case of the guanidino compounds.
abstract: BACKGROUND
  Although patients with renal failure retain a large variety of solutes, urea is virtually the only currently applied marker for adequacy of dialysis. Only a limited number of other compounds have up until now been investigated regarding their intradialytic kinetics. Scant data suggest that large solutes show a kinetic behavior that is different from urea. The question investigated in this study was whether other small water-soluble solutes, such as some guanidino compounds, show a kinetic behavior comparable or dissimilar to that of urea.
  
  
  METHODS
  This study included 7 stable conventional hemodialysis patients without native kidney function undergoing low flux polysulphone dialysis (F8 and F10HPS). Blood samples were collected from the inlet and outlet bloodlines immediately before the dialysis session, after 5, 15, 30, 120 minutes, and immediately after discontinuation of the session. Plasma concentrations of urea, creatinine (CTN), creatine (CT), guanidinosuccinic acid (GSA), guanidinoacetic acid (GAA), guanidine (G), and methylguanidine (MG) were used to calculate corresponding dialyzer clearances. A two-pool kinetic model was fitted to the measured plasma concentration profiles, resulting in the calculation of the perfused volume (V(1)), the total distribution volume (V(tot)), and the intercompartmental clearance (K(12)); solute generation and overall ultrafiltration were determined independently.
  
  
  RESULTS
  No significant differences were observed between V(1) and K(12) for urea (6.4 +/- 3.3 L and 822 +/- 345 mL/min, respectively) and for the guanidino compounds. However, with respect to V(tot), GSA was distributed in a smaller volume (30.6 +/- 4.2 L) compared to urea (42.7 +/- 6.0L) (P < 0.001), while CTN, CT, GAA, G, and MG showed significantly higher volumes (54.0 +/- 5.9 L, 98.0 +/- 52.3 L, 123.8 +/- 66.9 L, 89.7 +/- 21.4 L, 102.6 +/- 33.9 L, respectively; P= 0.004, = 0.033, = 0.003, < 0.001, = 0.001, respectively). These differences resulted in divergent effective solute removal: 67% (urea), 58% (CTN), 42% (CT), 76% (GSA), 37% (GAA), 43% (G), and 42% (MG).
  
  
  CONCLUSIONS
  The kinetics of the guanidino compounds under study are different from that of urea; hence, urea kinetics are not representative for the removal of other uremic solutes, even if they are small and water-soluble like urea.
authors.affiliation: Institute Biomedical Technology, Hydraulics Laboratory, Ghent University, Belgium. Sunny.Eloot@UGent.be
authors.forename: Sunny
authors.initials: S
authors.lastname: Eloot
authors.forename: An
authors.initials: A
authors.lastname: Torremans
authors.forename: Rita
authors.initials: R
authors.lastname: De Smet
authors.forename: Bart
authors.initials: B
authors.lastname: Marescau
authors.forename: Dirk
authors.initials: D
authors.lastname: De Wachter
authors.forename: Peter Paul
authors.initials: PP
authors.lastname: De Deyn
authors.forename: Norbert
authors.initials: N
authors.lastname: Lameire
authors.forename: Pascal
authors.initials: P
authors.lastname: Verdonck
authors.forename: Raymond
authors.initials: R
authors.lastname: Vanholder
mesh_terms: D000368:Aged; D003404:Creatinine / Q000097:blood; D005260:Female; D006146:Guanidines / Q000032:analysis* / Q000097:blood; D006801:Humans; D007676:Kidney Failure, Chronic / Q000097:blood / Q000628:therapy; D007700:Kinetics; D008297:Male; D008875:Middle Aged; D006435:Renal Dialysis; D012995:Solubility; D014508:Urea / Q000032:analysis* / Q000097:blood
chemical_list: D006146:Guanidines; D014508:Urea; D003404:Creatinine