title: Sodium removal and sodium concentration during peritoneal dialysis: effects of three methods of sodium measurement.
abstract: BACKGROUND
  Sodium removal (NaR) may have a major impact on the survival of peritoneal dialysis patients. The dialysate/plasma sodium concentration ratio (D/P(Na)) is an indirect index of transcellular water transport by aquaporin channels, and thus of ultrafiltration. Sodium concentration can be assessed by means of flame photometry (F), and direct (D-ISE) or indirect ion-selective electrodes (I-ISE), but these methods have different properties. I-ISE is being used increasingly in clinical laboratories. The aim of this study was to evaluate NaR and D/P(Na) using the three different measurement methods.
  
  
  METHODS
  We performed peritoneal equilibration tests (PETs) in 44 peritoneal dialysis patients and calculated the NaR. We also calculated D/P(Na) during the test; plasma and dialysate sodium concentrations were measured by F, D-ISE and I-ISE.
  
  
  RESULTS
  NaR was lower (P<0.001) with D-ISE (69+/-29 mmol) than with F (81+/-29 mmol) or I-ISE (79+/-28 mmol). D/P(Na) was also lower at baseline (0.92+/-0.02 vs 0.95+/-0.02 and 0.95+/-0.02; P<0.001), after 60 min (0.87+/-0.03 vs 0.90+/-0.03 and 0.90+/-0.03; P<0.001) and at the end of PET (0.88+/-0.04 vs 0.92+/-0.04 and 0.92+/-0.04; P<0.001) when measured by D-ISE in comparison with F and I-ISE, respectively.
  
  
  CONCLUSIONS
  NaR and D/P(Na) were lower when measured by the D-ISE method compared with the F and I-ISE methods. NaR and D/P(Na) were similar when measured by F or I-ISE. I-ISE can be used reliably in the evaluation of NaR and D/P(Na) in everyday clinical practice of peritoneal dialysis.
authors.affiliation: Department of Nephrology and Dialysis, A Manzoni Hospital, Lecco, Italy. v.lamilia@ospedale.lecco.it
authors.forename: Vincenzo
authors.initials: V
authors.lastname: La Milia
authors.forename: Salvatore
authors.initials: S
authors.lastname: Di Filippo
authors.forename: Monica
authors.initials: M
authors.lastname: Crepaldi
authors.forename: Simeone
authors.initials: S
authors.lastname: Andrulli
authors.forename: Lucia
authors.initials: L
authors.lastname: Del Vecchio
authors.forename: Pietro
authors.initials: P
authors.lastname: Scaravilli
authors.forename: Giovambattista
authors.initials: G
authors.lastname: Virga
authors.forename: Francesco
authors.initials: F
authors.lastname: Locatelli
mesh_terms: D019963:Clinical Chemistry Tests / Q000379:methods; D005260:Female; D017583:Hemodiafiltration / Q000379:methods; D006801:Humans; D008297:Male; D008875:Middle Aged; D010530:Peritoneal Dialysis; D012964:Sodium / Q000032:analysis / Q000378:metabolism*
chemical_list: D012964:Sodium