title: Unmeasured anions in critically ill patients: can they predict mortality?
abstract: OBJECTIVE
  To determine whether base excess, base excess caused by unmeasured anions, and anion gap can predict lactate in adult critically ill patients, and also to determine whether acid-base variables can predict mortality in these patients.
  
  
  METHODS
  Retrospective study.
  
  
  METHODS
  Adult intensive care unit of tertiary hospital.
  
  
  METHODS
  Three hundred adult critically ill patients admitted to the intensive care unit.
  
  
  METHODS
  Retrieval of admission biochemical data from computerized records, quantitative biophysical analysis of data with the Stewart-Figge methodology, and statistical analysis.
  
  
  RESULTS
  We measured plasma Na+, K+, Mg2+, Cl-, HCO3-, phosphate, ionized Ca2+, albumin, lactate, and arterial pH and Paco2. All three variables (base excess, base excess caused by unmeasured anions, anion gap) were significantly correlated with lactate (r2 =.21, p <.0001; r2 =.30, p <.0001; and r2 =.31. p <.0001, respectively). Logistic regression analysis showed that the area under the receiver operating characteristic (AUROC) curves had moderate to high accuracy for the prediction of a lactate concentration >5 mmol/L: AUROC curves, 0.86 (95% confidence interval [CI], 0.78-0.94), 0.86 (95% CI, 0.78-0.93), and 0.85 (95% CI, 0.77-0.92), respectively. Logistic regression analysis showed that hospital mortality rate correlated significantly with Acute Physiology and Chronic Health Evaluation (APACHE) II score, anion gap corrected (anion gap corrected by albumin), age, lactate, anion gap, chloride, base excess caused by unmeasured anions, strong ion gap, sodium, bicarbonate, strong ion difference effective, and base excess. However, except for APACHE II score, AUROC curves for mortality prediction were relatively small: 0.78 (95% CI, 0.72-0.84) for APACHE II, 0.66 (95% CI, 0.59-0.73) for lactate, 0.64 (95% CI, 0.57-0.71) for base excess caused by unmeasured anions, and 0.63 (95% CI, 0.56-0.70) for strong ion gap.
  
  
  CONCLUSIONS
  Base excess, base excess caused by unmeasured anions, and anion gap are good predictors of hyperlactatemia (>5 mmol/L). Acid-base variables and, specifically, "unmeasured anions" (anion gap, anion gap corrected, base excess caused by unmeasured anions, strong ion gap), irrespective of the methods used to calculate them, are not accurate predictors of hospital mortality rate in critically ill patients.
authors.affiliation: Department of Intensive Care, Austin and Repatriation Medical Centre, Melbourne, Australia.
authors.forename: Jens
authors.initials: J
authors.lastname: Rocktaeschel
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Morimatsu
authors.forename: Shigehiko
authors.initials: S
authors.lastname: Uchino
authors.forename: Rinaldo
authors.initials: R
authors.lastname: Bellomo
mesh_terms: D018806:APACHE; D000136:Acid-Base Equilibrium; D000140:Acidosis, Lactic / Q000209:etiology* / Q000401:mortality* / Q000473:pathology; D000368:Aged; D001639:Bicarbonates / Q000097:blood; D001784:Blood Gas Analysis / Q000379:methods*; D002118:Calcium / Q000097:blood; D002245:Carbon Dioxide / Q000097:blood; D002712:Chlorides / Q000097:blood; D003627:Data Interpretation, Statistical; D005260:Female; D017052:Hospital Mortality; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007362:Intensive Care Units; D019344:Lactic Acid / Q000097:blood; D008274:Magnesium / Q000097:blood; D008297:Male; D008499:Medical Records; D008875:Middle Aged; D010710:Phosphates / Q000097:blood; D011188:Potassium / Q000097:blood; D011237:Predictive Value of Tests; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D012709:Serum Albumin; D012964:Sodium / Q000097:blood; D014739:Victoria / Q000453:epidemiology
chemical_list: D001639:Bicarbonates; D002712:Chlorides; D010710:Phosphates; D012709:Serum Albumin; D002245:Carbon Dioxide; D019344:Lactic Acid; D012964:Sodium; D008274:Magnesium; D011188:Potassium; D002118:Calcium