Any patient in a hospital setting who develops a fever or evidence of sepsis should have blood cultures sent. As sepsis is one of the leading causes of deaths, rapid identification of bloodstream infection is mandatory to perform adequate antibiotic therapy. Multiple sets of blood cultures may be required and cultures of other sites should also be considered - eg, urine, skin and throat.
How to take blood cultures
- Aseptic techniques should be used so that bacteria from the skin do not contaminate the culture bottles.
- Ideally the skin should be washed with soap and rinsed with sterile water. This should be followed by the application of iodine-based solution, which should then be washed off after 30 seconds of drying, with 70% alcohol solution. Practically, this rarely occurs; however, every effort to use aseptic technique must be made - eg, Betadine® and/or alcohol spray and gloves.
- Alcoholic chlorhexidine solutions have been shown to reduce blood culture false positives compared with aqueous povidone-iodine.
- The most common contaminants include Staphylococcus epidermidis, Corynebacterium spp., Propionibacterium spp. and Bacillus spp. (but not Bacillus anthracis).
- Standard blood culture bottles consist of two bottles - one containing aerobic and the other anaerobic culture media.
- Usually at least 10-15 mL should be injected into blood culture bottles (this depends on the types of bottles used).
- Smaller volumes are often taken from babies and children. Allocating 1 mL of blood form a neonate into two bottles, aerobic and anaerobic, has been shown to improve the yield of the culture compared with 1 mL in a single aerobic bottle.
- Newer vacuum devices allow direct venepuncture with transfer of blood into blood cultures.
- In some hospitals, phlebotomists will often take blood cultures.
- Contamination results may be lower if taken by more experienced clinicians than junior staff.
- Multiple cultures from multiple sites increase the yield:
- This is especially so in infective endocarditis.
- One example is to take blood from one antecubital fossa, 30 minutes later from the opposite antecubital fossa and the last one from a third site 30 minutes later.
- Blood cultures can also be taken from lines - eg, central venous pressure lines, arterial lines.
- Studies have shown that the following may lower contamination rates:
- Adherence to a protocol.
- Sampling by peripheral venepuncture route rather than via an intravascular catheter.
- Use of sterile gloves.
- Cleaning tops of blood culture bottles with antiseptics.
- Inoculating blood culture bottles before other blood tubes.
- Samples being taken by a phlebotomy team.
- Monitoring contamination rates.
- Providing individual feedback and retraining for those with contaminants.
- One study has shown that developing a new process for blood culture collection emphasising strict attention to sterile technique, with standardised use of chlorhexidine skin antisepsis, sterile needles, sterile gloves, sterile fields and a procedural checklist, resulted in a reduction in blood culture contamination below the 3%.
- Universal decolonisation with chlorhexidine bathing has been shown to result in a significant reduction in blood culture contamination.
- One study has shown that using a new automated PCR assay for detection of Streptococcus pneumoniae, the GenomEra™ S. pneumoniae, can lead to results being available within 55 minutes, which is obviously significantly quicker than by the routinely used identification methods.
Problems with blood cultures
- Blood culture contamination is a common and preventable problem, especially in the emergency department.
- False positive blood cultures due to specimen contamination with skin bacteria are a common problem that can lead to unnecessary antibiotic use, additional laboratory tests and increased length of hospital stay, therefore leading to significant extra hospital costs.
- True and false positive rates for blood cultures vary but are commonly each in the range 5-10%.
- The use of phlebotomy teams has been shown to be effective for reducing blood culture contamination rates.
- Changing the method of blood culture collection to a more sterile process has been shown to result in significant reductions in blood culture contamination.
Types of culture bottles
- There are a various different types of blood culture systems, both manual and automated.
- Other culture bottles are available and their use depends upon the clinical scenario - eg, culture bottles for tuberculosis or fungi.
- New technologies are being introduced which can lead to the rapid identification of pathogens in positive blood cultures.[10, 11]
- Several molecular platforms can identify bacteria associated with bloodstream infections much more quickly than standard methods.
Blood culture processing
- Once blood cultures are taken they should be labelled and sent to the laboratory without delay.
- In the laboratory the bottles are agitated and incubated at body temperature. If it is out of hours then cultures are usually incubated overnight.
- Basic sets of cultures are incubated for 14 days and blind culture performed after 3, 7 and 14 days or as soon as there are signs of growth (eg, turbidity, haemolysis, colonies on agar slope in Castaneda's bottles).
- Other bottles may be incubated for 7 days or up to 3 weeks if subacute bacterial endocarditis (SBE)/fastidious organisms are suspected.
- Observation of bottles, looking for a positive result, is performed at least twice a day when using manual systems.
- Automatic systems are available and are being increasingly used. One example is the BacT/ALERT® Blood Culture System where a positive growth releases CO2 which is detected by a sensor that alerts the laboratory staff (bottles are placed in a special cabinet and linked to the patient by a barcode).
- The new BacT/ALERT® FAPlus and FNPlus BC bottles containing antibiotic-binding polymeric beads have been shown to improve the diagnosis of bacteraemia.
- If growth is detected, the bottles are subcultured and a Gram stain performed. From this, relevant sensitivities are performed. In some cases the organism can be identified within hours of detecting a positive result, using Gram stain and further tests.
- Further tests that may be performed directly on the blood culture to hasten identification include streptococcal grouping, coagulase testing, antigen tests for pneumococcus and meningococcus, etc.
- The microbiologist will then review the results and inform the staff involved in the patient's care. Thus, patients will begin provisional therapy and this will be confirmed once sensitivities are known.
- Usually, if there is to be a growth, there is generally only one organism; however, very rarely, there may be more than one organism and the laboratory will perform sensitivities and further tests on all.
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