Microbiology

Blood Cultures

Indications for Blood Cultures

Blood cultures should be performed on a patient in whom there is a clinical suspicion of bacteraemia or candidaemia i.e. temperature >37.9C or other evidence of sepsis, during the investigation of deep seated infection such as infective endocarditis or discitis and on the advice of the microbiologist/ infectious diseases physician.

Blood cultures should be taken prior to the commencement of antibiotics whenever possible. If treatment has already commenced blood cultures should be taken as soon as possible after this time or at a time when blood levels of antibiotic are at their lowest.

When investigating a patient with suspected bacteraemia, it is recommended to take 2 sets (of 2 bottles) on separate occasions, from two different sites.It is rarely necessary to take more than three sets in one episode. For the microbiological diagnosis of bacterial endocarditis 3 blood cultures must be collected before starting antibiotic therapy, unless the patient is unstable.

40 ml of blood should be obtained per episode (i. e 2 sets with 20ml per set, and 10ml per bottle).

A volume of 1-2ml in neonates, 2-3ml in infants and 3-5ml in pre-teen children is required for each blood culture set (a single paediatric bottle should be used for small babies or where only a very small quantity of blood can be collected. This bottle presents technical difficulties in the laboratory and should only be used in appropriate cases).

Blood should be obtained from peripheral venous sites where possible, avoiding the femoral site. Blood cultures must be taken by trained staff, for appropriate indications, following Trust guidance.

Blood taken from a central line, Hickman line, arterial line or other lines should only be taken if there is a suspicion of a line infection. These specimens should all be collected at the same time as a carefully collected peripheral blood culture and carefully labelled with the site and time of collection.

If other venous blood samples are required to be taken during the same venepuncture procedure, the blood culture bottles must be inoculated first to avoid contamination. (Routine blood sample bottles can be used with the wide needle-holder).

Laboratory Request Form

Every blood culture should be accompanied by a request form giving details of the patient, time of collection and type of blood culture.

The request form will be ordered via Lorenzo, by an identified Registered practitioner. This practitioner must have been certified competent, and will have completed the appropriate information systems training.

Specimen types on Lorenzo under the Blood Culture button are:

  • “Peripheral” – Blood culture from peripheral vein
  • “Central line” – Blood culture from central line
  • “Arterial line” – Blood culture from arterial line
  • “Hickman line” – Blood culture from Hickman line (or other long term tunnelled line)
  • “Other” – Blood culture from other line or site that should be clearly specified in the specimen qualifier field

Accurate and relevant clinical details will help to distinguish pathogens from contaminants. Please ensure a full and accurate antibiotic history is given.

Sample bottles will be labelled immediately following sampling. This will be carried out in the patient’s presence.

Blood cultures should be taken or dispatched to the reception on the first floor of the Tower Block, HRI as soon as possible after collection.