Werapon Pongmanee,1 Veerapong Vattanavanit2 1Department of Internal Medicine, 2Division of Critical Care Medicine, Department of Internal Medicine, Faculty of Medicine, Prince of Songkla University, Hat Yai, Songkhla, Thailand Background: Lactate measurement is the key component in septic shock identification and resuscitation. However, point-of-care lactate testing is not widely used due to the lack of access to nearby test equipment. Biomarkers such as serum lactate, anion gap (AG), and base excess (BE) are used in determining shock in patients with seemingly normal vital signs. Purpose: We aimed to determine if these biomarkers can be used interchangeably in patients with septic shock in the emergency setting. Patients and methods: A prospective observational cohort study was undertaken at a tertiary hospital in southern Thailand. Baseline point-of-care BE, AG, and serum lactate were recorded in all patients presenting with septic shock at the emergency department. Overall correlations including area under the receiver operating characteristic curve (AUROC) for both BE and AG to predict serum lactate level were calculated. Results: One hundred and fifteen patients were enrolled. Pearson correlation of serum lactate to BE was -0.59 (r2 = 0.35; 95% confidence interval [CI], -0.69 to -0.44; P < 0.001) and BE to AG was -0.67 (r2 = 0.49; 95% CI, -0.76 to -0.55; P < 0.001), and serum lactate to AG was 0.64 (r2 = 0.41; 95% CI, 0.52 to 0.74; P < 0.001). A cut-off point of 15.8 for AG identified a lactate level ≥2 mmol/L (sensitivity, 71.4%; specificity, 80.7%; and AUROC, 0.76), and the best cut-off value to predict a lactate level ≥4 mmol/L was 18.5 (sensitivity, 64.2%; specificity, 85.5%; and AUROC 0.78). Conclusion: In patients with septic shock, lactate and AG showed a strong correlation with each other, whereas lactate and BE showed a moderate correlation with each other. Thus, these biomarkers can be used interchangeably to help determine septic shock earlier in patients. Keywords: base excess, anion gap, lactate, septic shock