A comparison of prognostic significance of strong ion gap (SIG) with other acid-base markers in the critically ill: a cohort study

Kwok M Ho; Norris S H Lan; Teresa A Williams; Yusra Harahsheh; Andrew R Chapman; Geoffrey J Dobb; Sheldon Magder

doi:10.1186/s40560-016-0166-z

A comparison of prognostic significance of strong ion gap (SIG) with other acid-base markers in the critically ill: a cohort study

J Intensive Care. 2016 Jun 29:4:43. doi: 10.1186/s40560-016-0166-z. eCollection 2016.

Authors

Kwok M Ho¹, Norris S H Lan², Teresa A Williams³, Yusra Harahsheh⁴, Andrew R Chapman⁴, Geoffrey J Dobb⁵, Sheldon Magder⁶

Affiliations

¹ 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.
² School of Medicine and Pharmacology, University of Western Australia, Perth, Australia.
³ School of Nursing, Midwifery and Paramedicine, Curtin University, Perth, Australia.
⁴ Department of Intensive Care Medicine, Royal Perth Hospital, Wellington Street, Perth, WA 6000 Australia.
⁵ 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.
⁶ Critical Care Division, Department of Medicine and Physiology, Royal Victoria Hospital, McGill University Health Centre and McGill University, Montréal, Canada.

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.

Keywords: Acidosis; Alkalosis; Anion gap; Outcomes; Strong ion difference.