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Blood Cultures (Bacteriology)

Bloodstream Infection, Sepsis, Neonatal Sepsis, Infective endocarditis, Prosthetic valve endocarditis (PVE), Bacteraemia

The Blood Culture system can also be used for small volumes of sterile fluid to aid the recovery of fastidious organisms, for example but not limited to Vitreous aspirates, Joint Fluids (Prosthetic and Natural), Cardiac pacemaker site aspirates, Stem Cell fluids. The FAN (green top) bottle contains a charcoal neutralising agent and is suitable for PD fluids.

General information

Collection container (including preservatives): Collect specimens in BactAlert bottle. Use aseptic technique.

Specimen type: Venous blood, arterial blood, peripheral blood, sterile fluids

blood cultures

Collection: A blood culture set is defined as one aerobic (blue top) and one anaerobic (purple top) bottle. For infants and neonates, a single aerobic (yellow top) bottle may be requested. Small volume sterile fluids such as Pacemaker fluids and stem cells can use single aerobic bottle (Yellow top).

Take two consecutive sets from two separate venepuncture sites during any 24hr period for each septic episode. For neonates, take a single aerobic bottle or special low volume bottle.

Take two sets during the first hour in cases of severe sepsis prior to commencing antibiotic treatment, provided this does not significantly delay antibiotic administration.

Take at least three sets during a 24hr period where the patient has suspected infective endocarditis.

If patient is on antibiotics, or fungaemia is suspected use a FAN (green top bottle) and anaerobic (purple top) bottle.

Specimen transport (eg at room temperature, or within 4 hrs): Collect specimens before antimicrobial therapy where possible. Samples should be taken as soon as possible after a spike of fever. Samples should not be refrigerated.

Inoculated bottles should be incubated as soon as possible, and within a maximum of four hours. The four hour turnaround time from collection to incubation for blood culture samples reflects their clinical significance.

Type and volume of sample:

Adults - Purple top and Blue top bottles. Inoculate up to 10mL to each bottle.

Children - Yellow top bottle. Inoculate up to 4mL

Neonates - Yellow top bottle. Inoculate preferably 1-2mL.

Do not exceed the manufacturer's recommended maximum volume for each bottle as shown on label.

Special precautions:

  • Use aseptic technique
  • Inspect the blood culture bottles for damage
  • Ensure that the blood culture bottles have not exceeded their expiry date
  • Do not re-sheathe needles

Laboratory information

Measurement units: Growth detected or not detected

Biological reference units: Not applicable

Turn round time for provisional result (calendar days): 24 hrs

Turn round time to final result (calendar days): 6 days

Clinical information

Clinical decision points: Not applicable

Factors known to significantly affect the results: Any recent antimicrobial therapy can have a significant effect on blood culture results by decreasing the sensitivity of the test. This may be of particular importance in those patients receiving prophylactic antibiotics and who are at high risk of bloodstream infections. If patients have received previous antimicrobial treatment, bacteraemia should be considered even if blood culture results are negative.  There is a direct relationship between blood volume and yield, with approximately a 3% increase in yield per mL of blood cultured. False negatives may occur if inadequate blood culture volumes are submitted.

Limitations

It is estimated that 2-5% of positives samples may be missed if bottles are pre-incubated, these organisms may fail to trip the threshold algorithm of the continuous monitoring blood culture machine. This may occur with Abiotrophia species (nutritionally variant streptococci), S. pneumoniae which have undergone a degree of autolysis, and fastidious organisms which are unable to grow on routine solid culture media.

Organisms may include:

  • Campylobacter species
  • Helicobacter species
  • Capnophilic organisms
  • Slow-growing anaerobes