Background: Point-of-care tools are invaluable in the emergency department. Arterial lactate has been used for prognostication in subsets of population in the emergency department but not often for a heterogeneous population. Objectives: We aimed to study the use of arterial lactate as a prognostication and disposition tool in an undifferentiated population presenting to the emergency department. Methods: We conducted a prospective study among all consenting emergency department patients with age >18 years, who had an arterial blood gas performed as a part of routine care and had a lactate value ⩾2 mmol/L. We collected data on demographics, comorbidities and patient disposition from the emergency department and 28-day mortality as a follow-up telephonically. Results: We included 469 patients with a median age of 37 years. Sixteen provisional diagnoses were made in the emergency department, and pneumonia/lower respiratory tract infection was relatively higher (13.6%). The median lactate was 4.6 (interquartile range = 3.2–7) with 155 patients (33%) being transferred to intensive care unit and 62 deaths (13.2%) recorded at 28 days. Furthermore, we observed optimum values for lactates at 5 mmol/L predicted intensive care unit admissions and 6 mmol/L predicted mortality. A unit increase in arterial lactate in the emergency department significantly increased mortality by 66% (95% confidence interval = 1.45–1.88; p < 0.001) and had a 2.15 times (95% confidence interval = 1.63–2.83; p < 0.001) significantly higher chance of being transferred to the intensive care unit. Conclusion: Arterial lactate can be used as a prognostication tool for a heterogeneous population presenting to the emergency department. Clinical significance: Point-of-care investigations such as arterial lactate can help the emergency physician make quick decisions on the floor and guide prognostication and disposition.