AIMS: The National Early Warning Score (NEWS) is a scoring system that predicts increased mortality and morbidity in critical diseases. The National Early Warning Score + Lactate (NEWS + L) score was created by adding lactate values to this scoring system. In our study, we aimed to determine the value of the NEWS + L score in predicting clinical deterioration in patients presenting with acute decompensated heart failure (chronic heart failure). METHODS AND RESULTS: In this observational, cross-sectional study, patients with decompensated heart failure who were admitted to the emergency department between 1 October 2020 and 31 December 2020 were included. Patients were divided into two groups: those with and without poor prognostic outcomes. The main outcomes were in-hospital mortality, discharge after treatment in the emergency department, admission to the ward, and admission to the intensive care unit. We analysed a total of 141 applications from 130 patients. The mean age was 72.6 ± 11.8 years, and 50.8% were female. Poor prognostic outcomes were observed in 92 (65%) patients. There was no difference between the patients with and without poor prognostic outcomes in terms of mean age, gender, and comorbidities, except for atrial fibrillation. There was a statistically significant difference between the patients without and with poor prognosis outcomes in terms of NEWS {3 [interquartile range (IQR): 0-5] and 6 [IQR: 3-8]} and NEWS + L scores [4.7 (IQR: 2.3-7.2) and 8.0 (IQR: 5.2-10.4)] (P < 0.001). The area under the curve values for predicting poor prognosis were calculated as 0.719 for NEWS, 0.734 for NEWS + L, and 0.601 for lactate values. The rate of poor prognostic outcomes was higher (79%) in patients with moderate and high NEWS scores. Patients with Q1 NEWS + L scores had a lower rate of poor prognostic outcomes, while patients with Q2, Q3, and Q4 scores of NEWS + L had a higher rate of poor prognostic outcomes. CONCLUSIONS: The NEWS score and the addition of the lactate value to this score, the NEWS + L score, were higher in patients with poor prognostic outcomes who presented with decompensated heart failure in our emergency department. NEWS + L slightly outperformed the NEWS score in predicting prognosis. The NEWS + L score shows promise as a prognostic indicator for patients with decompensated heart failure presenting to the emergency department.