The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [418]
Related theory
Blood cultures are an essential part of the management of patients with serious bloodborne infections, enabling the identification of the causative pathogens, antimicrobial susceptibility and treatment guidance (Shore and Sandoe 2008). Bacteraemia and fungaemia are associated with high morbidity and mortality amongst hospitalized patients, particularly those with compromised host defences (Panceri et al. 2004). The accurate and timely detection of these remains one of the most important functions of clinical microbiology as early detection has significant diagnostic and prognostic importance (HPA 2008f).
Micro-organisms may be present in the blood intermittently or continuously, depending upon the source of infection (HPA 2008f). The majority of bacteraemias are intermittent, so blood cultures should be taken when there are clinical signs of an infection such as fever, as this is when the concentration of bacteria in the blood is at its highest (Higgins 2007).
Evidence-based approaches
Rationale
Indications
There are many signs and symptoms in the patient which may suggest bacteraemia but clinical judgement is required. The following indicators should be taken into account when assessing a patient for signs of bacteraemia or sepsis which would then indicate the need for blood culture sampling (DH 2007a).
Core temperature out of normal range (>38°C or <36°C).
Focal signs of infection.
Abnormal heart rate (raised), blood pressure (low or raised) or respiratory rate (raised).
Chills or rigors.
Raised or very low white blood cell count.
New or worsening confusion/altered levels of consciousness.
Methods of blood culture specimen collection
In recognition of the importance of taking accurate blood cultures, there is now national guidance that implements procedure and policy to improve the quality of blood culture investigations and to reduce the risk of blood sample contamination (DH 2007a). 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. Failure to use an aseptic technique or careful process procedures when obtaining blood cultures can result in a ‘false-positive’ result which may lead to extensive diagnostic testing, excessive antibiotic use, prolonged hospitalization and artificially raised incidence rates (Dellinger et al. 2008).
In order to optimize the identification of causative organisms, the taking of a peripheral sample is recommended and two sets of cultures should be taken at separate times and from separate sites (DH 2007a). In cases of suspected endocarditis, this should be three sets within a 24-hour period. Drawing more than one culture set can help to distinguish true bacteraemia from contaminated cultures (Shafazand and Weinacker 2002). Samples should not be taken from existing peripheral cannulas but if the patient has an existing CVAD a sample can be taken from this following collection of a separate peripheral sample. The femoral vein should be avoided for venepuncture because it is difficult to ensure adequate skin cleansing and disinfection (DH 2007a).
Blood cultures should be taken prior to commencing or changing antimicrobial therapy as antibiotics may delay or prevent bacterial growth, causing a falsely negative result (Higgins 2007). However, in accordance with the Surviving Sepsis Campaign, 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 whilst not causing significant delay in antibiotic administration (Dellinger et al. 2008). If antibiotic therapy has already been commenced, blood cultures