Blood Cultures: Best-Practice
Published: 15 February 2024
Published: 15 February 2024
Blood cultures are considered the gold standard method of detecting pathogens related to bacteraemia and sepsis, which may be life-threatening and require immediate treatment (CEC 2021; Pathology Tests Explained 2023).
However, despite the importance of these tests, there are several disadvantages when they are used improperly or unnecessarily, including increased costs and false positives (Choosing Wisely Australia 2015).
Therefore, it’s essential that blood cultures are taken only when needed and are performed correctly.
Note: This article is intended as a refresher and should not replace best-practice care. Always refer to your organisation's policy on blood culture collection.
A blood culture is a type of phlebotomy in which samples of blood are collected via venepuncture in order to determine whether a systemic bloodstream infection (bacteraemia, fungaemia or viraemia) is present (Pathology Tests Explained 2023).
Once blood samples have been taken, they are added to cultures - substances that encourage the growth of pathogens. Any pathogens that appear in the blood sample are then detected and identified (Testing.com 2022).
If the patient’s blood culture is positive, i.e. pathogens have been found, appropriate treatments will then be prescribed (Pathology Tests Explained 2023).
If required, gram stain testing and/or susceptibility testing to identify an appropriate antimicrobial to treat the infection may also be performed (Pathology Tests Explained 2023).
Most bloodstream infections are caused by bacteria - either aerobes (bacteria that prefer oxygen) or anaerobes (bacteria that prefer reduced-oxygen environments). Fungi and viruses may also cause bloodstream infections, but this is less common (Pathology Tests Explained 2023).
Best practice is to take a minimum of two sets of blood samples from two different sites on the body. One set includes two blood culture bottles - one containing nutrients that support the growth of aerobes and the other containing nutrients that support the growth of anaerobes. Therefore, four bottles should be collected in total (CEC 2021; Pathology Tests Explained 2023).
One set alone is an inadequate sample volume and reduces the sensitivity of the test. Collecting two sets of samples helps to rule out skin contaminants that may otherwise cause false positives and detect any pathogens that are present in small numbers or are being released intermittently into the bloodstream (CEC 2021; Pathology Tests Explained 2023).
If two different culture sets return positive results for the same pathogen, it’s likely the patient’s symptoms are being caused by this pathogen (Pathology Tests Explained 2023).
A blood culture may be recommended for adult patients who have:
On one hand, blood cultures are helpful in determining specific pathogens that are present in the patient's bloodstream in order to assist in the selection of appropriate antimicrobial therapies. Furthermore, those who display a positive blood culture result are often experiencing severe and potentially life-threatening illnesses, so being able to make an accurate diagnosis and prescribe the correct treatment as soon as possible is crucial (Choosing Wisely Australia 2021).
On the other hand, however, blood cultures are prone to false positives - in fact, 50% of positives are potentially false (Choosing Wisely Australia 2015) - meaning that patients may be exposed to antibiotics when they aren’t actually required. This may contribute to antibiotic resistance. Repeated blood draws may also cause pain and blood loss for the patient (Choosing Wisely Australia 2021).
Remember that two sets of samples need to be taken from two different sites on the body. The preferred sites are the antecubital fossa or dorsum of the hand. Where possible, other sites should be avoided (especially femoral stabs), as they are associated with a higher risk of contamination (TeachMeSurgery 2021).
If the patient has a central venous access device in situ, one set of samples should be taken from a venepuncture site, and the other should be taken from a line site (Morgan 2024).
Samples should not be taken via an intravenous cannula (CEC 2021).
(CEC 2021; Choosing Wisely Australia 2021)
(CEC 2021; Ausmed 2023; TeachMeSurgery 2015, 2021; Geeky Medics 2020)
If less than 10 mL of blood is collected, the entire sample should be placed into the aerobic bottle only. The aerobic bottle is the preferred choice, as pathogens that cause bloodstream infections are more likely to be aerobic than anaerobic (CEC 2021).
Only take another set of blood cultures if there is patient deterioration that suggests a new infection or non-response to antimicrobial therapy. The second set of blood cultures must be taken at least 72 hours after the previous set (Choosing Wisely Australia 2021).
For more information about venepuncture in general, including potential complications, see the Ausmed Article Venepuncture: Phlebotomy and IV Cannula Insertion.
Question 1 of 3
How many sets of blood cultures should be taken?