Blood Cultures

Any patient in a hospital setting who develops a fever or evidence of sepsis should have blood cultures sent e.g. rising CRP. This may need to be multiple sets. Cultures of other sites should also be considered e.g. urine, skin and throat.¹

Precautions

• Aseptic techniques should be used so that bacteria from the skin of staff does 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.² Practically this rarely occurs but every effort to use aseptic technique must be made e.g. Betadine® and/or alcohol spray and gloves.
• The commonest contaminants include Corynebacterium spp., Propionibacterium spp.and Bacillus spp. (but not B. anthracis).²

Procedure

• Need at least 20 mL of blood obtained by venepuncture.
• Standard blood culture bottles consist of two bottles - one containing aerobic and the other anaerobic culture media.³
• 10 mL should be injected in to blood culture bottles (or at least 10-15 mL).
• Some centres require a new needle to be placed on the syringe before transferring blood in to culture bottles. However this increases the risk of needle stick injuries. Furthermore, needle switching does not reduce contamination and thus it is not recommended.⁴ Newer vacuum devices allow direct venepuncture with transfer of blood into blood cultures.
• In some hospitals phlebotomists will take blood cultures. Although the rates of contamination may be lower, there are practical issues relating to availability of phlebotomists.⁵
• Multiple cultures from multiple sites increases 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 e.g. central venous pressure lines, arterial lines.
• The same rules apply especially regarding attention to asepsis.

• There are a variety of different types of blood culture systems both manual and automated. (See Internet and further reading section for more detail).
• Other culture bottles are available and their use will depend upon the clinical scenario e.g. culture bottles for tuberculosis or fungi.⁶

• Once blood cultures are taken they should be labelled and sent to the lab 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 signs of growth (e.g. turbidity, haemolysis, colonies on agar slope in Castaneda's).
• Other bottles may be incubated for 7 days or up to 3 weeks if SBE/fastidious organisms suspected.
• Observation of bottles looking for a positive result are 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 bar code).²
• 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 streptococcus grouping, coagulase testing, antigen tests for pneumococcus and Neisseria e.t.c. Modern methods such as Vitek® have helped speed up identification of organisms and can be performed directly from culture broths.
• The microbiologist will then review the results and inform the staff involved in the patients 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 usually only one organism but very rarely there may be more than one organism and the lab will perform sensitivities and further tests on all.