title: A comparison of prognostic significance of strong ion gap (SIG) with other acid-base markers in the critically ill: a cohort study.
abstract: BACKGROUND
  This cohort study compared the prognostic significance of strong ion gap (SIG) with other acid-base markers in the critically ill.
  
  
  METHODS
  The relationships between SIG, lactate, anion gap (AG), anion gap albumin-corrected (AG-corrected), base excess or strong ion difference-effective (SIDe), all obtained within the first hour of intensive care unit (ICU) admission, and the hospital mortality of 6878 patients were analysed. The prognostic significance of each acid-base marker, both alone and in combination with the Admission Mortality Prediction Model (MPM0 III) predicted mortality, were assessed by the area under the receiver operating characteristic curve (AUROC).
  
  
  RESULTS
  Of the 6878 patients included in the study, 924 patients (13.4 %) died after ICU admission. Except for plasma chloride concentrations, all acid-base markers were significantly different between the survivors and non-survivors. SIG (with lactate: AUROC 0.631, confidence interval [CI] 0.611-0.652; without lactate: AUROC 0.521, 95 % CI 0.500-0.542) only had a modest ability to predict hospital mortality, and this was no better than using lactate concentration alone (AUROC 0.701, 95 % 0.682-0.721). Adding AG-corrected or SIG to a combination of lactate and MPM0 III predicted risks also did not substantially improve the latter's ability to differentiate between survivors and non-survivors. Arterial lactate concentrations explained about 11 % of the variability in the observed mortality, and it was more important than SIG (0.6 %) and SIDe (0.9 %) in predicting hospital mortality after adjusting for MPM0 III predicted risks. Lactate remained as the strongest predictor for mortality in a sensitivity multivariate analysis, allowing for non-linearity of all acid-base markers.
  
  
  CONCLUSIONS
  The prognostic significance of SIG was modest and inferior to arterial lactate concentration for the critically ill. Lactate concentration should always be considered regardless whether physiological, base excess or physical-chemical approach is used to interpret acid-base disturbances in critically ill patients.
authors.affiliation: Department of Intensive Care Medicine, Royal Perth Hospital, Wellington Street, Perth, WA 6000 Australia ; School of Population Health, University of Western Australia, Perth, Australia ; School of Veterinary and Life Sciences, Murdoch University, Perth, Australia.
authors.forename: Kwok M
authors.initials: KM
authors.lastname: Ho
authors.affiliation: School of Medicine and Pharmacology, University of Western Australia, Perth, Australia.
authors.forename: Norris S H
authors.initials: NS
authors.lastname: Lan
authors.affiliation: School of Nursing, Midwifery and Paramedicine, Curtin University, Perth, Australia.
authors.forename: Teresa A
authors.initials: TA
authors.lastname: Williams
authors.affiliation: Department of Intensive Care Medicine, Royal Perth Hospital, Wellington Street, Perth, WA 6000 Australia.
authors.forename: Yusra
authors.initials: Y
authors.lastname: Harahsheh
authors.affiliation: Department of Intensive Care Medicine, Royal Perth Hospital, Wellington Street, Perth, WA 6000 Australia.
authors.forename: Andrew R
authors.initials: AR
authors.lastname: Chapman
authors.affiliation: Department of Intensive Care Medicine, Royal Perth Hospital, Wellington Street, Perth, WA 6000 Australia ; School of Medicine and Pharmacology, University of Western Australia, Perth, Australia.
authors.forename: Geoffrey J
authors.initials: GJ
authors.lastname: Dobb
authors.affiliation: Critical Care Division, Department of Medicine and Physiology, Royal Victoria Hospital, McGill University Health Centre and McGill University, Montréal, Canada.
authors.forename: Sheldon
authors.initials: S
authors.lastname: Magder
keywords: Acidosis; Alkalosis; Anion gap; Outcomes; Strong ion difference