Sepsis is a huge global health burden, with a recent Lancet paper reporting 11 million deaths in 2017 (1). Each year approximately 7 million sepsis cases (14%) are triggered by more than one pathogen (polymicrobial sepsis), which often leads to significantly worse patient outcomes, including longer hospital stays and higher mortality rates compared to single-pathogen sepsis (2). Despite polymicrobial sepsis being a more severe manifestation of sepsis, recent epidemiological data is lacking. Most of the data available is based on the routine culture-based method for diagnosing sepsis. In addition to slow turnaround times (24-72 hours), the method often yields inaccurate results due to contamination, low pathogen load, unculturable organisms, and interference from antibiotics administered prior to sample collection (1). When sepsis is polymicrobial, only the pathogen present at the highest abundance is usually detected in the first instance, leading to delayed identification of all causative organisms and incorrect initial antimicrobial therapy (3–5). Therefore, the development of an alternative culture-independent tool for detecting polymicrobial infections is highly warranted. Our group is currently validating a highly sensitive and rapid multiplex polymerase chain reaction (PCR) test that detects the 12 most common sepsis pathogens. This project aims to optimise this PCR test for the detection of polymicrobial infections, which will then be used to retrospectively analyse positive blood cultures frozen at PathWest Laboratories (diagnostic laboratory). Concordance between our PCR test and the routine method (PathWest data) will be assessed to determine whether PCR can detect polymicrobial infections missed by routine testing. The clinical characteristics of the patients will also be assessed in relation to bacterial composition. Ultimately the project will allow us to assess the true prevalence and clinical significance of polymicrobial sepsis.