Blood culture is one of the most simple and commonly used investigation to establish the etiology of bloodstream infections (i.e. detection of bacteremia, sepsis and fungemia). Early diagnosis and accurate identification of the bacteria or fungi causing blood stream infections provides vital information to clinicians to start appropriate antimicrobial therapy. Providing timely and adequate antimicrobial therapy not only decrease the infection related mortality and cost but also reduce risk of emergence/spread of drug resistance.
What is a blood culture?
A blood culture is a laboratory test in which blood, taken from the patient, is inoculated into bottles containing appropriate culture media to determine whether infection-causing microorganisms (bacteria or fungi) are present in the patient’s bloodstream.
Aims of Blood Culture:
- To confirm presence of microorganisms in the bloodstream
- To identify the etiology of the bloodstream infection
- To guide appropriate antimicrobial therapy by providing susceptibility information of isolates
When should a blood culture be performed?
Blood cultures should always be requested when a bloodstream infection or sepsis is suspected. Some of the most common clinical symptoms in a patient which may lead to a suspicion of a bloodstream infection are:
- Undetermined fever ( ≥ 38oC) or hypothermia ( ≤ 36oC).
- Shock, chills, rigors
- Severe local infections (meningitis, endocarditis, pneumonia, pyelonephritis, intra-abdominal suppuration etc)
Blood cultures should be collected:
- as soon as possible after the onset of clinical symptoms;
- ideally, prior to the administration of antimicrobial therapy.
If the patient is already on antimicrobial therapy, recovery of microorganisms may be increased by collecting blood sample immediately before administering the next dose and by inoculating the blood into bottles containing specialized antimicrobial neutralization media.
What volume of blood should be collected?
Sufficient blood volume is critical for the successful recovery of organisms causing bloodstream infection. It has been found that most bacteremias in adults have a low number of colony-forming units (CFU) per milliliter (mL) of blood.
“More the blood volume cultured greater the chance of isolating pathogen causing bloodstream infection”.
Recommended blood-to-broth-ratio is 1:5 to 1:10, commercial blood culture systems may use a smaller blood-to-broth ratio (<1:5) due to the addition of Sodium polyanetholesulfonate (SPS).
For an adult, the recommended volume of blood to be obtained per culture is 20-30 ml. Since each blood culture set includes an aerobic and an anaerobic bottle, each bottle should be inoculated with approximately 10 ml of blood (If anaerobic culture is not available in your settings, replacing it with an additional aerobic bottle ensures culture of sufficient blood volume and increases the chance of recovery of pathogens).
For infants and children, the recommended blood volume should be based on the weight of the patient, and an aerobic bottle should be used, unless an anaerobic infection is suspected.
It is not safe to take large samples of blood from children, particularly infants. It is safe to obtain as much as 4-4.5% of known total blood volume for culture from infants and children.
Specially formulated blood culture bottles are commercially available for use in children <2 years of age. They are specifically designed to maintain the usual blood-to-broth ratio (1:5 to 1:10) with smaller blood volumes, and have shown to improve microbial recovery.
For more information refer to table below:
How many blood culture sets should be collected?
Standards for the timing and number of blood cultures depends on the periodicity of microorganisms in the blood stream (random, intermittent, or continuous.
It is recommended to collect two, or preferably three blood culture sets for each septic episode. Since the pathogens may not be constantly present in the bloodstream (intermittent bacteremia), the sensitivity of a single blood culture set is always limited. Using single blood culture/bottle or set will result in an inadequate volume of blood cultured and a substantial number of bacteremias may be missed. It’s also difficult to confirm organism grown in a single blood culture bottle/set is contaminant or a true pathogen.
A contaminant will usually be present in only one bottle of a set of blood culture bottles, but if it’s a true bloodstream infection, multiple blood culture bottles/set will be positive.
Timing of Blood Collection
Timing of blood cultures is not considered a critical factor in determining blood stream infections, as the diagnostic yield remains the same. Most of the researchers/authors concluded that “the overall volume of blood cultured was more critical in increasing organism yield than was timing”.
Guidelines recommend that the first two/three sets (2 bottles/set) of blood culture be obtained either at one time or over a brief time period (e.g. within 1 hour) from multiple venipuncture sites.
- Blood Culture: A key Investigation for Diagnosis of Bloodstream Infections (BioMérieux)
- Bailey & Scott’s Diagnostic Microbiology, 14th Edition