Ram Benny Dessau ^1,2,3* Christian Østergaard Andersen ⁴ John Coia ⁵ Svend Ellermann-Eriksen ⁶ Sophie Gubbels ² Thøger Jensen ⁷ Jenny Dahl Knudsen ⁸ Jonas Kähler ² Steen Lomborg ⁹ Lisbeth Lützen ¹⁰ Marc Trunjer Kusk Nielsen ⁵ Bente Olesen ¹¹ Mette Pinholt ¹² Flemming Scheutz ¹³ Kirstine Kobberøe Søgaard ¹⁴ Marianne Voldstedlund ² Kåre Mølbak ¹⁵

• ¹ Department of Clinical Microbiology, Zealand University Hospital, Køge, Denmark
• ² Department of Data Integration and Analysis, Statens Serum Institut, Copenhagen, Denmark
• ³ Institute of Regional Health, University of Southern Denmark, Odense, Denmark
• ⁴ Department of Diagnostic and Infectious disease preparedness, Statens Serum Institut, Copenhagen, Denmark
• ⁵ Department of Regional Health Research, Research Unit of Clinical Microbiology, Esbjerg, Denmark
• ⁶ Department of Clinical Microbiology, Aarhus University Hospital, Aarhus, Denmark
• ⁷ Department of Clinical Microbiology, Odense University Hospital, Odense, Denmark
• ⁸ Department of Clinical Microbiology, Rigshospitalet, Copenhagen, Denmark
• ⁹ Department of Clinical Microbiology, Sygehus Sønderjylland, Aabenraa, Denmark
• ¹⁰ Department of Clinical Microbiology, Sygehus Lillebælt, Vejle, Denmark
• ¹¹ Department of Clinical Microbiology, Herlev Hospital, Herlev, Denmark
• ¹² Department Of Clinical Microbiology, Hvidovre Hospital, Copenhagen University Hospital, Hvidovre, Denmark
• ¹³ The International Escherichia and Klebsiella Centre, Department of Bacteria, Parasites and Fungi, Statens Serum Institut, Copenhagen, Denmark
• ¹⁴ Department of Clinical Microbiology, Aalborg University Hospital, Aalborg, Denmark
• ¹⁵ Epidemiological Infectious Disease Preparedness, Statens Serum Institut, Copenhagen, Denmark

An increasing incidence of positive blood cultures has been reported from several countries. This represents an important challenge for public health due to the large number of hospital admissions with high mortality among these patients. The aim of this study is to characterize the epidemiology and mortality associated with positive blood cultures.We performed a nationwide cohort study, identifying all individuals with a blood culture drawn in Denmark from 2010 to 2022. We estimated the incidence of positive blood cultures and 30-day mortality and per 100,000 persons and modelled case fatality rate (CFR) and population incidences with regression techniques.We identified almost 4 million blood cultures drawn during the study period, with increasing 64% numbers from 220,757 in 2010 to 362,239 in 2022. During this period the underlying population increased only 10%, whereas the population of seniors above 85 years increased 3.3 times. The positivity rate was 9.8% and the most common pathogens were Escherichia coli (29%) and other pathogens associated with urinary tract infection, whereas Staphylococcus aureus accounted for 12%, and Enterococcus faecium for 4.1%. The 30day CFR was 17% with a small reduction during the study period, with notable differences according to species.We found a substantial rise in blood culture activity and the incidence of positive cultures. This rise is driven by an increasing number of older citizens, which is expected to increase further the coming years. Considering the high risk of mortality and morbidityincreasing incidence, a national surveillance of bacteremia is warranted. We show that data for electronic surveillance based on health records is readily available.