In recognition of the importance of taking accurate blood cultures, there is now national guidance on procedure and policy to improve the quality of blood culture investigations and to reduce the risk of blood sample contamination (PHE [188]). Contamination can come from a number of sources: the patient's skin, the equipment used to obtain the sample, the practitioner or the general environment (Murray and Masur [151]). Failure to use an aseptic technique or careful procedures when obtaining blood cultures can cause a false‐positive result, which may lead to extensive diagnostic testing, excessive antibiotic use, prolonged hospitalization and artificially raised incidence rates (Rhodes et al. [206]).

Taking multiple blood cultures not only enhances the ability to detect bacteria but also helps to ascertain whether skin bacteria such as coagulase‐negative staphylococci are clinically significant or contamination. Coagulase‐negative staphylococci are frequent blood culture contaminants, but detection in a blood culture set from both the central line and the peripheral set increases the likelihood of it being a true pathogen. The initial samples should be obtained directly from a peripheral vein and not from existing peripheral cannulas or immediately above a cannula site, and the femoral vein should be avoided for venepuncture because it is difficult to ensure adequate skin cleansing and disinfection (DH 2010).

If the patient has an existing vascular access device (VAD) that has been in situ for more than 48 hours or is suspected to be a source of infection, a sample should also be taken from the device (Myers and Reyes [152]). Rhodes et al. ([206]) suggest that, if feasible, a set of cultures should be taken from each lumen of the VAD.

It is suggested that the blood cultures should be taken when the temperature is rising or as soon as possible following a spike in temperature as this is when the serum concentration of the bacteria is at its highest (Higgins [84]). This is because organisms in the bloodstream elicit an immune response, causing symptoms such as fever and hypotension. However, in patients with a transient bacteraemia, organisms are immediately cleared from the blood by the immune system (Mahon et al. [133]). In such patients, clinical symptoms, especially fever, may not occur until after the organisms are cleared from the bloodstream. Studies in the adult (Riedel et al. [207]) and paediatric (Kee et al. [113]) populations have shown that taking a blood culture when a patient spikes a temperature does not increase the sensitivity of the blood culture. In patients with a continuous bacteraemia (e.g. endocarditis), bacteria are constantly leaked into the blood vessels and are likely to grow from blood cultures taken at random times. The timing of taking blood cultures therefore is less important than the volume of blood taken (Miller et al. [147]).

Blood cultures should ideally be taken prior to commencing or changing antimicrobial therapy as antibiotics may delay or prevent bacterial growth, causing a falsely negative result (HPA [90]). In accordance with the National Institute for Health and Care Excellence's sepsis guidelines (NICE [164]), antimicrobial therapy should be started within the first hour of recognition of severe sepsis and blood cultures should be taken before antimicrobial therapy is initiated. This is essential to confirm infection and the responsible pathogens while not causing significant delay in antibiotic administration (Rhodes et al. [206]). If antibiotic therapy has already been commenced, blood cultures should ideally be taken immediately prior to the next dose (DH 2010).