Blood Culture is a common investigation that is done everyday in the Emergency Departments. It plays an integral role in the evaluation of sepsis. Culture media used in the blood culture bottles can grow most of the important species of bacteria/fungi. Our goal is to find out if the organisms are present in the patient’s blood stream or not.
If proper technique is not used, false positive results occur where skin commensals grow instead of the pathogens. Therefore, maintaining asepsis is vital when taking blood culture. Bacteremia is often associated with fever, so it is always recommended to take blood culture during an episode of fever. But overall, the indications of blood culture are broad and not clearly defined.
Indications for blood culture:
1. Clinical features of sepsis including tachycardia, tachypnea, increased or sub-normal temperature and change in sensorium, hypotension
2. Suspicion of infective endocarditis
3. Pyrexia of unknown origin
4. Unexplained leucocytosis or leucopenia
5. Systemic and localised infections including suspected meningitis, osteomyelitis, septic arthritis, acute untreated bacterial pneumonia or other possible bacterial infection
Types of Blood Culture Bottles
- Yellow top – paediatric aerobic
- Green top – adult aerobic
- Orange top – anaerobic
- Black top – mycobacteria
- Silver top – mycoplasma
Procedure
1 Verify the patient’s identity and obtain verbal consent
2 Assemble the correct materials required for blood culture:
• blood culture bottle(s), syringe (10 ml or more), needle (22 gauge or more)
• sterile gloves, tourniquet, adhesive strip, skin disinfectant
• sterile pack containing cotton/gauze swabs, sterile paper x2 and waste bag
• patient labels, sharps waste disposal bin.
3 Open the sterile blood culture pack onto the trolley.
4 Apply tourniquet and select a suitable vein. Wash hands with soap and water or disinfect with alcohol hand disinfectant. Dry your hands or rub the hand disinfectant in until dry. Apply sterile gloves.
5 Clean the puncture site three times with skin disinfectant using aseptic technique. Allow 1 - 2 minutes for the disinfectant to dry. Drape the area.
6 Collect a minimum of 10 ml of blood (adults). If using the vacutainer system, the blood culture must be the first blood specimen to be collected. The rate of isolation of micro-organisms from blood is directly related to the volume of blood collected.
7 Release tourniquet. Remove needle and syringe from puncture site and place dry swab on puncture site and apply pressure. Before inoculating the sample into the culture bottle, first disinfect the top of the blood culture bottle with an alcohol swab. If blood is being collected for other tests, always inoculate blood culture bottle first. Changing needles between blood sample collection and inoculation of blood culture bottle is controversial (I don’t recommend it), and if you are doing this beware of needle-stick injury.
8 Gently rotate the blood culture bottle to mix the blood and culture medium (do not shake vigorously).
9 Label the blood culture bottle and complete a laboratory request form. Remember to include the site, date and time of collection, full clinical information regarding the suspected diagnosis, and contact details for the clinician responsible for the patient.
10 Deliver the blood culture bottle to the laboratory ASAP. If there is a delay in getting the sample to the laboratory, do not refrigerate the bottle; rather leave it at room temperature.
Issues to contemplate:
1. Can we collect blood cultures from indwelling catheters?
Blood should not be collected from indwelling arterial or venous lines unless an infected intravenous line is suspected. Should blood be drawn for culture from an indwelling line, a second specimen should be obtained from a peripheral site.
2. How many sets we need?
Ideally 2 sets i.e. 2 aerobic (10+10=20ml) and 2 anaerobic (10+10=20ml) cultures, although routine use of anaerobic culture bottles in controversial. Two or more blood specimens should be collected using sterile technique at separate sites, if possible before administering antibiotics. If all 4 sets are negative after 24 hours and sepsis is still suspected, more cultures may be collected. A larger number of cultures may have to be collected from persons already receiving antimicrobials although, if clinical condition allows, stopping antibiotics and re-culturing after 48 hours is preferred.
3. How much blood is enough?
Ideally, a minimum of 10 ml of blood, depending on the blood culture system used, should be inoculated into a culture bottle when taking blood from adults. For Pediatrics, draw 1 mL/year of age. Adequate volumes of blood improve detection of pathogenic organisms and reduce time to detection. The rate of isolation of micro-organisms from blood is directly related to the volume of blood collected. The yield in adults increases approximately by 3% per ml of blood cultured.
4. Is there a difference between arterial and venous blood culture samples?
Arterial blood culture provides no advantage over venous samples.
5. What are ways by which we can reduce contamination?
Important ways to decrease contamination of blood culture bottles include the use of tincture of iodine as a disinfectant, avoidance of drawing blood through existing intravenous lines, disinfecting the membrane of the blood culture bottle and using sterile gloves during the procedure. Also, the initial collection should be with a peripheral venipuncture (not through a line) if possible.
Key Points:
- Use aseptic technique or else don't do it!
- Take a minimum of 10ml/culture bottle for adults.
- Do not withhold antibiotics due to pending cultures in critically ill septic patients.
Further Reading:
