Blood cultures are an essential part of the management of patients with serious blood‐borne infections and are known as the gold standard investigation for such diseases (PHE [188]), enabling the identification of the potential pathogens and antimicrobial susceptibility, and guiding treatment (Shore and Sandoe [228]). Bacteraemia (bacteria in the blood) and fungaemia (fungi in the blood) are associated with high morbidity and mortality among hospitalized patients, particularly those with compromised host defences (Panceri et al. [179]). The accurate and timely detection of the organism and susceptibilities has significant diagnostic and prognostic importance (PHE [188]).

Micro‐organisms may be present in the blood intermittently or continuously, depending upon the source of infection (PHE [188]). In order to determine whether there is bacteraemia or fungemia, the bacteria or fungus must be ‘grown’ or ‘cultured’. The blood sample needs to be inoculated into aerobic and anaerobic blood culture bottles and transported to the microbiology laboratory, where the blood culture bottles are incubated.

Most clinically significant bacteria are detected within the first 1 to 2 days of incubation. For fungi such as yeasts, it may take around 3 days. Blood culture bottles are usually incubated for up to 5 to 7 days before they are reported by the laboratory as having no growth. If there is evidence of bacterial growth, this will then be stained and examined under a microscope to carry out a gram stain (negative or positive) and to determine the morphology (shape – e.g. cocci or rods) before identifying the species and testing for antimicrobial drug susceptibility (Higgins [84]).