Background Increased lactate is associated with high mortality among patients with suspected infection or trauma in the emergency department (ED), but the association with patients with other aetiologies is less well described. The aim of this study was to describe the relation between lactate, aetiology and 7-day mortality in adult ED patients.

Methods A retrospective cohort study of all adult patients who had a lactate measured within 4 h after arrival to the ED at Odense University Hospital between June 2012 and May 2013. The categorisation of suspected aetiology was based on discharge diagnoses.

Results 5360 patients were included; 51.7% were men, and the median age was 67 years (IQR 50–79). 77.2% had low lactate (0–1.9 mmol/L), 16.2% intermediate lactate (2–3.9 mmol/L), and 6.6% high lactate (≥4 mmol/L). 7-day mortality was 2.9% (95% CI 2.4% to 3.5%) for patients with low lactate, 7.8% (95% CI 6.1% to 9.8%) for patients with intermediate lactate, and 23.9% (95% CI 19.6% to 28.8%) for patients with high lactate. The association between lactate level and mortality varied across different diagnostic groups. Based on Area Under the Curve in receiver operating characteristic analysis, lactate level showed to be useful in patients with infection (0.78, 95% CI 0.73 to 0.84), trauma (0.78, 95% CI 0.65 to 0.92), cardiac diseases (0.83, 95% CI 0.75 to 0.91) and gastrointestinal diseases (0.83, 95% CI 0.68 to 0.98). Lactate level was not useful in neurological (0.58, 95% CI 0.50 to 0.67) and respiratory disease (0.64, 95% CI 0.55 to 0.74), and of uncertain value in the remaining diagnostic groups.

Conclusions Among adult ED patients, the prognostic value of lactate varies between diagnostic groups.