title: Accuracy of base excess--an in vitro evaluation of the Van Slyke equation.
abstract: OBJECTIVE
  To evaluate the precision, bias and CO2 invariance of base excess as determined by the Van Slyke equation over a wide P(CO2) range at normal and low hemoglobin concentrations.
  
  
  METHODS
  Prospective in vitro study.
  
  
  METHODS
  University research laboratory.
  
  
  METHODS
  Normal human blood, both undiluted and diluted with plasma.
  
  
  METHODS
  Two experiments were conducted. In the first, blood unmodified or after adding HCl or sodium bicarbonate was rendered hypercarbic (P(CO2) >70 torr) by gas equilibration. Rapid Pco2 reduction in > or =10 steps to a final P(CO2) < or =20 torr was then performed. In the second experiment, blood unmodified or diluted to a hemoglobin concentration of approximately 4 G% was mixed anaerobically (9:1, vol:vol) with varying concentrations of lactic acid in saline (0-250 mmol/L).
  
  
  RESULTS
  In the first experiment, blood gas analysis at each step during the progressive P(CO2) reduction revealed that base excess remained nearly constant (SD all specimens < or =0.6 mmol/L) whereas P(CO2) changed by >80 torr. In the second experiment, simultaneous blood gas and plasma lactate analyses showed that changes in base excess correlated closely with changes in both plasma and whole blood lactate concentrations (r2 > or = 0.91) despite concurrent P(CO2) elevations as great as 200 torr. Quantification by base excess of change in whole blood lactate concentration was precise with slight negative bias (mean negative bias, 1.1+/-1.9 mmol/L) in both diluted and undiluted blood. There was significant underestimation of change in plasma lactate concentration in undiluted blood, presumably because base excess is a whole blood variable.
  
  
  CONCLUSIONS
  Base excess calculated using the Van Slyke equation accurately quantifies metabolic (nonrespiratory) acid-base status in blood in vitro. This accuracy is little affected by large simultaneous alterations in P(CO2), or by very low hemoglobin concentrations similar to that used to calculate standard base excess.
authors.affiliation: Division of Anesthesiology and Intensive Care, Royal Brisbane Hospital, Queensland, Australia.
authors.forename: T J
authors.initials: TJ
authors.lastname: Morgan
authors.forename: C
authors.initials: C
authors.lastname: Clark
authors.forename: Z H
authors.initials: ZH
authors.lastname: Endre
mesh_terms: D000136:Acid-Base Equilibrium; D002245:Carbon Dioxide / Q000097:blood*; D006801:Humans; D019344:Lactic Acid / Q000097:blood*; D008962:Models, Theoretical; D015203:Reproducibility of Results
chemical_list: D002245:Carbon Dioxide; D019344:Lactic Acid