Objectives Lactate measurements are routinely carried out in emergency departments and are associated with increased mortality in septic patients. However, no definitive research has been carried out into whether lactate measurements can be used as a prognostic marker in a clinically unwell population in the emergency department.

Methods We carried out a prospective observational cohort study in consecutive patients whose arterial lactate concentration was measured in the emergency department of a tertiary referral hospital assessing 110 000 patients per year between 11th May and 11th August 2011. The main outcome measure was 30-day mortality.

Results There were 120 deaths (16.1%) at 30 days postattendance in our cohort of 747 patients. Multivariate logistic regression revealed lower lactate levels were associated with 30-day survival: ORs for 30-day death compared with lactate ≥4 were 0.125 (95% CI 0.068 to 0.229) for lactate <2 and 0.273 (95% CI 0.140 to 0.533) for lactate 2–<4. Kaplan–Meier analysis showed a survival difference when dividing lactate concentrations into strata (p<0.0001). This survival difference was maintained when septic diagnoses were taken into account.

Conclusions A single arterial lactate measurement on presentation to the emergency department predicts 30-day mortality independent of other measures of illness severity.