Relationship Between the Anion Gap and Serum Lactate in Hypovolemic Shock

Background and objectives: Previous studies evaluating patients in the Intensive Care Unit with established lactic acidosis determined that the anion gap is an insensitive screening tool for elevated blood lactate. No prior study has examined the relationship between anion gap and serum lactate within the first hours of the development of lactic acidosis. Design, setting, participants, & measurements: Data were obtained prospectively from a convenience sample of adult trauma patients at a single level 1 trauma center. Venous samples were drawn prior to initiation of intravenous fluid resuscitation. A linear regression model was constructed to assess the relationship between serum lactate and anion gap, and 95% prediction intervals were computed. Logistic regression models were constructed to determine the sensitivity and specificity for several different anion gap and lactate cutpoints. Results: 128 patients with elevated serum lactate levels (>2.1 mmol/L) and 63 patients with normal serum lactate levels (< 2.1 mmol/L) were included. The sensitivity of an elevated anion gap (> 10) to reveal hyperlactatemia was only 43% whereas specificity was 84%. Sensitivity improved if the upper limit of normal anion gap was lowered and with increasing levels of serum lactate. The coefficient of determination between serum lactate level and AG yielded an R² of 0.30 (p < 0.001) and the slope of this relationship was 2.185 with a 95% confidence interval of 2.011-2.359. The mean 95% prediction interval was + 8.9. Conclusions: Within the first hour of the development of lactic acidosis due to hypovolemic shock, the anion gap was not a sensitive indicator of an elevated serum lactate level, but it was fairly specific. The anion gap increased to a greater extent than the serum lactate, the 95% mean prediction interval was wide and approximately 70% of the change in anion gap could not be explained by increases in serum lactate, suggesting that other anions contribute to the anion gap in lactic acidosis.