The Difference Between Critical Care Initiation Anion Gap and Prehospital Admission Anion Gap is Predictive of Mortality in Critical Illness*

OBJECTIVE: We hypothesized that the delta anion gap defined as difference between critical care initiation standard anion gap and prehospital admission standard anion gap is associated with all cause mortality in the critically ill. DESIGN: Observational cohort study. SETTING: Two hundred nine medical and surgical intensive care beds in two hospitals in Boston, MA. PATIENTS: Eighteen thousand nine hundred eighty-five patients, age ≥18 yrs, who received critical care between 1997 and 2007. MEASUREMENTS: The exposure of interest was delta anion gap and categorized a priori as <0, 0-5, 5-10, and >10 mEq/L. Logistic regression examined death by days 30, 90, and 365 postcritical care initiation and in-hospital mortality. Adjusted odds ratios were estimated by multivariable logistic regression models. The discrimination of delta anion gap for 30-day mortality was evaluated using receiver operator characteristic curves performed for a subset of patients with all laboratory data required to analyze the data via physical chemical principles (n = 664). INTERVENTIONS: None. RESULTS: Delta anion gap was a particularly strong predictor of 30-day mortality with a significant risk gradient across delta anion gap quartiles following multivariable adjustment: delta anion gap <0 mEq/L odds ratio 0.75 (95% confidence interval 0.67-0.81; p < 0.0001); delta anion gap 5-10 mEq/L odds ratio 1.56 (95% confidence interval 1.35-1.81; p < 0.0001); delta anion gap >10 mEq/L odds ratio 2.18 (95% confidence interval 1.76-2.71; p < 0.0001); and all relative to patients with delta anion gap 0-5 mEq/L. Similar significant robust associations post multivariable adjustments are seen with death by days 90 and 365 as well as in-hospital mortality. Correcting for albumin or limiting the cohort to patients with standard anion gap at critical care initiation of 10-18 mEq/L did not materially change the delta anion gap-mortality association. Delta anion gap has similarly moderate discriminative ability for 30-day mortality in comparison to standard base excess and strong ion gap. CONCLUSION: An increase in standard anion gap at critical care initiation relative to prehospital admission standard anion gap is a predictor of the risk of all cause patient mortality in the critically ill.

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