Blood culture is the microbiological gold standard method in diagnosis of sepsis and/or fever of unknown origin. It’s important to remember that the presence of microorganisms in the blood can be: transient, as in the case of invasive procedures such as, for example, tooth extractions or bladder catheterization; intermittent, associated, for example, to infections localized; or continuous, a typical example of endovascular or CVC-related infections.
Bacteria or fungi isolation from the blood has a diagnostic value, as it allows the confirmation of a clinical suspicion, and it is essential to set a targeted therapy based on susceptibility test.
When to take a blood-culture?
The sampling should be carried out during the febrile episode, as early as possible and preferably before empirical therapy, or prior to its new administration (when the drug’s concentration in the blood should be at a minimum).
Only few studies tried to assess the correct timing of blood-cultures. Often, it is common to collect the samples at ranges of 30-60 minutes, but it is an arbitrary timing; instead, can be useful to do it close to the onset of fever or, however, whenever there is a clinical suspicion of sepsis.
In fact, there aren’t significant evidence in the positivity rate of samples taken at fever peak, and this is due to the fact that some bacteremic patients may be hypothermic or otherwise without fever, because unable to develop a febrile response to infection. Fever alone is not a useful indicator, other parameters should be always be considered, such as vital parameters (tachycardia, tachypnea, and hypotension), presence of leukocytosis, chills, increase of inflammatory markers (such as C-reactive protein and procalcitonin).
In patients with acute endocarditis may be useful the same considerations to repeat blood-cultures in monitoring therapeutic success. Instead, in sub-acute endocarditis it is recommended to perform 3 sets of blood-cultures in 30-60 minutes, and in case of negativity should run another 3 set after 24 hours.
How perform blood-culture collection?
It is recommended to carry out 2-3 sets of samples (each sample must consists of a bottle for aerobic and one for anaerobic) to have a greater sensitivity. In fact, in case of sepsis the probability of isolating a pathogen with a single sample is of 65-80%, with two samples is of 80-88%, and 96-99% with three samples.
Moreover, in cases where the microrganism could be considered a contaminant, have more samples allows to discriminate contaminations from significant infections. Different samples should be made in quick succession, at a distance of 5-10 minutes apart. In sub-acute forms of endocarditis it is preferable select at 30-60 minutes away, and the blood-sample shall be repeated after 24 hours after the first 3 set are negative.Sample should not be performed from a intra-venous device (e.g. a CVC), given the high risk of contamination, unless you suspect that there is an infection of the device. In the latter case the blood collection should be contemporary performed from intravascular device and peripheral veins. It is recommended to use at least two bottles, one for aerobic and one for anaerobic bacteria. The anaerobic bottles, designed to allow the growth of strictly anaerobic bacteria, are most efficient even in the case of staphylococci, enterococci and enterobacteria. In pedriatric patients it is recommended the use of dedicated bottles for which smaller blood volumes are needed (1-4 ml per bottle, also in relation to the weight of the child).
How much blood should be drawn? Some studies suggest that there is a direct relationship between blood volume withdrawn and the blood-culture diagnostic yield. Altogether, should be taken 20-30 ml of blood, to be divided in different phial, putting about 8 ml of blood per bottle (and not to exceed 10 ml).
Health care personnel must extend the maximum attention in the sampling phases, to minimize the possibility of contamination (despite the attentions undertaken, the expected contamination rate remains of 3%).
Precautions to be followed are as follows:
- The cap of the bottle is not sterile and must be disinfected as well as the skin.
- The disinfection of the skin with chlorhexidine is mandatory: the skin should be disinfected with a diameter of 7-8 cm, in a centrifugal way (proceeding from the center to the periphery), first by cleaning with 70% isopropyl alcohol, and subsequently by means of disinfection by body wrap with 2% chlorhexidine, to leave 30 seconds. As an alternative to chlorhexidine can be used tincture of iodine (though avoid the povidone-iodine, which requires longer exposure times).
- It is not necessary to use sterile gloves if the skin is not touched again after disinfection. In case, you can disinfect the gloves.
In case of suspected CVC-related sepsis is indispensable perform blood-cultures simultaneously from both peripheral vein and from the venous catheter, after the disinfection of the insertion of CVC with an alcohol solution (verify compatibility with the material). The first ml taken from the CVC should not be totally discarded, because it contains the highest concentration of germs. When possible, the sample from peripheral vein should be done on the arm opposite the one in which the CVC is inserted. It is important to distribute the same amount of blood in each bottle, allowing us a correct interpretation of the time to positivity. In fact in cases of CVC-associated bacteremia, the time of CVC positivity must anticipate at least two hours the positivity of bottle drawn from peripheral vein.
Delaying transport of the samples to the laboratory should be avoided. The bottles should arrive to microbiology laboratory as quickly as possible because a long time of stay outside the automatic incubation system may result in the failure of test. In fact, the microbial growth may reach the plateau outside of the incubation system, and then no longer be detectable by machine. Instead, an early incubation of bottles allows to bacteria or fungi to grow under optimal conditions, shortening the time to positivity of blood cultures.
Under special circumstances it is possible to hold the bottles up to a maximum of 16-18 hours at normal ambient temperature, while the refrigeration is absolutely contraindicated.
Why perform blood-culture?
Blood culture is the best tool for the diagnosis of septicemia. From this examination, although relatively expensive, it is used to confirm the suspected diagnosis and to have an etiologic diagnosis and sensitivity, allowing you to set a antimicrobial targeted therapy.
Learn more about learning and understanding an antibiogram
– Proposta di Percorso Diagnostico presentato durante il XXXVII Congresso Nazionale AMCLI – Stresa, 5-8 ottobre 2008 – Revisione: settembre 2014
– Wayne PA. Clinical and Laboratory Standards Institute (CLSI). Principles and Procedures for Blood Cultures; Approved guideline. CLSI M47-A, 2007.
– Baron EJ et al. A guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2013 recommendations by the Infectious Diseases Society of America (IDSA) and the American Society for Microbiology (ASM). Clin Infect Dis. 2013 Aug; 57(4): e22-e121
– Dellinger RP et al. Surviving Sepsis Campaign Guidelines Committee including the Pediatric Subgroup. Surviving sepsis campaign: international guidelines for management of severe sepsis and septic shock: 2012. Crit Care Med. 2013 Feb; 41(2): 580-63
– Jaimes F et al. Predicting bacteremia at the bedside. Clin Infect Dis. 2004 38: 357-62
– Riedel S et al. Timing of specimen collection for blood cultures from febrile patients with bacteremia. J Clin Microbiol. 2008; 46(4): 1381-5
– Grohs P et al. Relevance of Routine Use of the Anaerobic Blood Culture Bottle. J. Clin Microbiol, 2007, 45: 2711-15