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Blood culture

Clinical use

The culture of micro-organisms from blood is essential for microbiological diagnosis of bacteraemia, fungaemia, infective endocaritis and conditions associated with a clinical presentation of pyrexia of unknown origin (PUO). Blood culture is also important for the diagnosis of prosthetic device infections (e.g. joints and vascular grafts) and intravascular line-associated sepsis. Blood cultures may also detect bloodstream infections associated with other conditions such as pneumonia, septic arthritis and osteomyelitis.

Background

Blood culture is considered the “gold standard” investigation for the detection of micro-organisms in blood. Blood cultures should only be taken when there is a reason to suspect infection. ‘Saving Lives’ guidance suggests that is bacteramia is strongly suspected then two sets of blood cultures should be taken at separate times and sites. This is particularly important for patients with septicaemia. Blood cultures should not be taken for routine assessment.

Patient preparation

Before the bottles are inoculated, ensure that the sensor at the bottom of each bottle is intact; do not use if it is yellow. The broth in the bottles should be clear, if cloudy do not use that bottle.

Kit preparation

  1. Label bottles with surname, forename, date of birth and hospital number in the spaces provided. If using printed labels, ensure they do not cover barcodes and do not tear off barcode labels.
  2. Remove caps from bottles and clean with an alcohol wipe.

Skin preparation

  1. Wash your hands with soap and water and then dry.
  2. Clean any visibly soiled skin on the patient with soap and water and then dry.
  3. Apply a tourniquet and palpate to identify vein.
  4. Clean the skin with an alcohol wipe and allow to dry. If a culture is being collected from a central venous catheter, disinfect the access port with alcohol swab and allow to dry.

Sample Collection

Needle and syringe method

  1. Wash and dry your hands again or use alcohol hand rub and apply clean examination gloves (sterile gloves not necessary)
  2. Insert needle – do not palpate again after cleaning.
  3. Collect the sample and release the tourniquet.
  4. Cover the puncture site with an appropriate dressing.
  5. If blood is being collected for other tests always inoculate the blood culture bottles first.
  6. Inoculate the blood into the culture bottles. Do not change the needle between sample collection and inoculation. Inoculate the orange blood culture first and then the green one. Ensure that as least 5-10ml is inoculated into each bottle.
  7. Discard the needle and syringe in a sharps container at the point of use.
  8. Remove gloves and wash hands with soap and water.
  9. Record the procedure in the patients medical notes including indication, date, time, site of venepuncture and any complications.

Winged blood collection method (preferred method)

  1. Wash and dry your hands again (or use alcohol hand rub) and don clean gloves.
  2. Attach the winged blood collection set to the blood collection adapter cap.
  3. Insert the needle. Do not palpate the vein again after cleaning the skin.
  4. Place the adapter cap over the blood culture bottle and pierce the rubber bung.
  5. Hold the bottle upright and use the bottle graduation lines to gauge the sample volume being collected.
  6. If blood is being collection for other tests always inoculate the blood culture bottles first. Follow the instructions for which culture bottle to inoculate first.
  7. Collect the sample and release the tourniquet.
  8. Discard the winged collection system in a sharps container at the point of use.
  9. Cover the puncture site with an appropriate dressing.
  10. Remove gloves and wash hands with soap and water.
  11. Record the procedure in the patients medical noted including indication, date, time, site of venepuncture and any complications.

Container

  • Label the bottle with the patient information.
  • Do not remove the barcode sticker from the bottle – this is essential for laboratory processing.
  • Do not cover the bottle barcode with the patient information sticker – please use the blank space provided.

Specimen requirements

Adult

  • 1 x aerobic bottle (green top)
  • 1x anaerobic bottle (orange top)

Paediatric

  • 1 x paediatric bottle (yellow top)

Minimum volume

Adult

  • Aerobic (green top): 8-10ml
  • Anaerobic (orange top): 8-10ml

Paediatric

  • Paediatric (yellow top): 5ml

Limitations and restrictions

Number and timing of samples

For most patients two blood culture sets are recommended. A second or third set taken from a different site not only increases yield but also allows recognition of contamination

Antimicrobial therapy

Ideally, blood samples should be taken prior to antimicrobial treatment. When already receiving antimicrobials, blood culture should be collected just before the next dose is due when antimicrobial concentration in the blood is at the lowest. Any recent antimicrobial therapy can have a significant effect on blood culture results by decreasing the sensitivity of the test.

Volume of blood

Blood culture volume is the most significant factor affecting the detection of organisms in bloodstream infection. 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.

Method of collection

Samples taken from central venous catheters have higher contamination rates than those taken from peripheral or arterial lines. Arterial blood offers no advantage over venous blood for detection of most microorganisms, although it has been reported as being superior detecting disseminated fungal disease.

Turnaround time

Final reports issued

  • Negative: 5-10 days
  • Positive within 48 hours: 4 days (organism dependent)

Interim reports issued

  • Adult: Negative at 48 hours
  • Paediatric: Negative at 36 hours

Positive: all positive bottles are phoned to the requesting source at the time of positivity and an interim microscopy report issued.

Analysing laboratory

Microbiology Lab, James Cook University Hospital, Marton Road, TS4 3BW

Additional information

  • If infective endocarditis is considered, please send a total of 3 sets of blood cultures and discuss with a consultant medical microbiologist. These should be taken via different venepuncture sites.
  • The blood culture bottles should reach the laboratory as soon as possible, and if delays occur, the specimen should be stored at room temperature – never refrigerate inoculated blood culture bottles.