Anion gap as a screening tool for elevated lactate in patients with an increased risk of developing sepsis in the Emergency Department.

Date Published:

2009 May


OBJECTIVES: Serum lactate levels are a useful tool in monitoring critically ill patients, especially those who are septic. However, lactate levels are often not routinely drawn or rapidly available in some institutions. The objective of this study was to determine if a readily available anion gap (AG) could be used as a surrogate marker for abnormal lactate level in Emergency Department (ED) patients at risk for sepsis. METHODS: Prospective, observational cohort study of consecutive ED patients seen at an urban university tertiary care referral center with 46,000 annual ED visits. ED patients aged 18 years or older presenting with clinically suspected infection were eligible for enrollment if a serum chemistry and lactate levels were drawn during the ED visit. During the 9-month study period, 1419 patients were enrolled. The initial basic chemistry panels, calculated AG, and lactate levels drawn in the ED were collected. We defined, a priori, an AG > 12 and a lactate > 4 mmol/L to be abnormal. Analysis was performed with Student's t-test, operating characteristics with 95% confidence intervals, and logistic regression. RESULTS: The mean AG was 11.8 (SD 3.6) and the mean lactate was 2.1 (SD 1.3). For an AG > 12, the mean lactate was 2.9 (SD 1.7), compared with 1.8 (SD 0.8) for an AG < 12. The sensitivity of an elevated AG (> 12) in predicting elevated lactate levels (> 4 mmol/L) was 80% (72-87%) and the specificity was 69% (66-71%). Patients with a gap > 12 had a 7.3-fold (4.6-11.4) increased risk of having a lactate > 4 mmol/L. The area under the curve was 0.84. CONCLUSION: This study suggests that an elevated AG obtained in the ED is a moderately sensitive and specific means to detect elevated lactate levels in ED patients at risk for sepsis. This information may be somewhat helpful to Emergency Physicians to risk-stratify their patients to provide more aggressive early resuscitation.