BACKGROUND:Procalcitonin (PCT), an inflammatory blood biomarker, is well studied in infectious diseases. Its prognostic value in unselected emergency department (ED) patients remains yet undefined. Herein, we investigated association of admission PCT levels and mortality in a large, international-multicenter ED patient cohort. METHODS:We prospectively enrolled 6970 unselected, consecutive, adult, medical patients seeking ED care in three tertiary-care hospitals in Switzerland, France and the USA. We used multivariable logistic regression models to examine association of admission PCT levels (as a continuous predictor and across cut-offs) and 30-day mortality. We also investigated subgroup effects by main diagnosis, comorbidities and clinical features at presentation. RESULTS:During the 30-day follow-up, 328 (4.7%) participants died. Mortality increased stepwise within higher PCT cut-offs (0.05, 0.1, 0.25, 0.5 ng/mL) from 1%, 3%, 7%, 13% to 15%, respectively. This association was also confirmed in a fully-adjusted model including age, gender, main symptom, main diagnosis and vital parameters on admission. Receiver operating characteristic (ROC) curve analysis showed that PCT differentiated well between survivors and non-survivors in the overall cohort (area under ROC curve [AUC] 0.75) with best results for patient with metabolic (AUC: 0.85) and cardiovascular disease (AUC: 0.82). Addition of PCT also improved the prognostic accuracy of the quick sequential organ failure assessment (qSOFA) score from an AUC of from 0.61 to 0.76 (p<0.001). Results were similar for other secondary endpoints including intensive care unit (ICU) admission and hospital readmission. CONCLUSIONS:In this large and heterogenous medical ED patient cohort, admission PCT was a strong and independent outcome predictor for 30-day mortality across different medical diagnoses independent of underlying infection. PCT may help to improve risk stratification in unselected medical ED patients.