Calculation of unmeasured anions fails to assist outcome prediction in patients in an adult intensive care unit

Introduction

The anion gap (AG) and base excess (BE) are used to identify the presence of a metabolic acidosis. A method of analysis using physico-chemical principals has been developed by Stewart and refined by Fencl. This technique has recently been demonstrated to detect unmeasured anions, due to metabolic derangement, more readily than the traditional markers of AG and BE. This method may be a more sensitive indicator of patients with circulatory inadequacy or organ dysfunction. The Fencl-Stewart method of assessing unmeasured anions, resulting in the strong ion gap (SIG), has been shown to be more strongly associated with mortality in paediatric patients than BE or AG [1]. This study examines the predictive value of these measurements in an adult population of critically ill patients.

Methods

100 consecutive patients admitted to an adult intensive care unit had electrolyte and blood gas analysis performed on admission. The AG, SIG were calculated on admission and the base excess measured (BE) by blood gas analyser. APACHE II data and 28 day mortality were recorded.

Results

100 mixed medical and surgical patients (48 and 52 respectively) with a mean age 60.5 years (range 18-97) and mean APACHE II score of 20.4 (range 5-40) were enrolled into the study. Twenty-eight day mortality was 31%. Logistic regression analysis showed that the APACHE II score was the best predictor of outcome (OR 1.17 95% CI 1.07-1.26 P < 0.001). Predictions did not significantly improve when either BE, AG, SIG or lactate were included. From ROC analysis, the best of the acid base variables for prediction of mortality was the BE (area under curve 0.70) followed by AG (AUC 0.66), lactate (AUC 0.66) and then SIG (AUC 0.57).

Conclusion

These data indicate that SIG does not have a useful predictive value in the adult patients in our ICU practice. Further studies are required to determine whether the application of SIG differs between adult and paediatric ICU patients.