title: Comparison of point-of-care versus central laboratory measurement of electrolyte concentrations on calculations of the anion gap and the strong ion difference.
abstract: BACKGROUND
  Clinicians calculate the anion gap (AG) and the strong ion difference (SID) to make acid-base diagnoses. The technology used is assumed to have limited impact. The authors hypothesized that different measurement technologies markedly affect AG and SID values.
  
  
  METHODS
  SID and AG were calculated using values from the point-of-care blood gas and electrolyte analyzer and the central hospital laboratory automated blood biochemistry analyzer. Simultaneously measured plasma sodium, potassium, and chloride concentrations were also compared.
  
  
  RESULTS
  Mean values for central laboratory and point-of-care plasma sodium concentration were significantly different (140.4 +/- 5.6 vs. 138.3 +/- 5.9 mm; P < 0.0001), as were those for plasma chloride concentration (102.4 +/- 6.5 vs. 103.4 +/- 6.0 mm; P < 0.0001) but not potassium. Mean AG values calculated with the two different measurement techniques differed significantly (17.6 +/- 6.2 mEq/l for central laboratory vs. 14.5 +/- 6.0 mEq/l for point-of-care blood gas analyzer; P < 0.0001). Using the Stewart-Figge methodology, SID values also differed significantly (43.7 +/- 4.8 vs. 40.7 +/- 5.6 mEq/l; P < 0.0001), with mean difference of 3.1 mEq/l (95% limits of agreement, -3.4, 9.5 mEq/l). For 83 patients (27.6%), differences in AG values were as high as 5 mEq/l or more, and for 46% of patients whose AG value was outside the reference range with one technology, a value within normal limits was recorded with the other.
  
  
  CONCLUSIONS
  Results with two different measurement technologies differed significantly for plasma sodium and chloride concentrations. These differences significantly affected the calculated AG and SID values and might lead clinicians to different assessments of acid-base and electrolyte status.
authors.affiliation: Department of Intensive Care, Austin, and Repatriation Medical Centre, Heidelberg, Victoria, Australia.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Morimatsu
authors.forename: Jens
authors.initials: J
authors.lastname: Rocktäschel
authors.forename: Rinaldo
authors.initials: R
authors.lastname: Bellomo
authors.forename: Shigehiko
authors.initials: S
authors.lastname: Uchino
authors.forename: Donna
authors.initials: D
authors.lastname: Goldsmith
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Gutteridge
mesh_terms: D002712:Chlorides / Q000097:blood; D004573:Electrolytes / Q000097:blood*; D006801:Humans; D007753:Laboratories / Q000592:standards*; D019095:Point-of-Care Systems / Q000592:standards*; D011785:Quality Assurance, Health Care; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012964:Sodium / Q000097:blood
chemical_list: D002712:Chlorides; D004573:Electrolytes; D012964:Sodium