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Blood culture is a microbiological culture of blood. It is employed to detect infections that are spreading through the bloodstream (such as bacteremia, sepsis amongst others). This is possible because the bloodstream is usually a sterile environment.
Blood cultures were pioneered in the early 20th century.
When a patient shows signs or symptoms of a systemic infection, results from a blood culture can verify that an infection is present, and they can identify the type (or types) of microorganism that is responsible for the infection. For example, blood tests can identify the causative organisms in severe pneumonia, puerperal fever, pelvic inflammatory disease, neonatal epiglottitis, sepsis, and fever of unknown origin (FUO). However, negative growths do not exclude infection.
A minimum of 10 ml of blood is taken through venipuncture and injected into two or more "blood bottles" with specific media for aerobic and anaerobic organisms. A common medium used for anaerobes is thioglycollate broth.
The blood is collected using aseptic technique. This requires that both the tops of the culture bottles and the venipuncture site of the patient are cleaned prior to collection by swabbing with 70% isopropyl alcohol (povidone and left to dry before venipuncture).
To maximise the diagnostic yield of blood cultures, multiple sets of cultures (each set consisting of aerobic and anaerobic vials filled with 3–10 mL) may be obtained. A common protocol used in US hospitals includes the following:
- Set 1 = left antecubital fossa at 0 minutes
- Set 2 = right antecubital fossa at 30 minutes
- Set 3 = left or right antecubital fossa at 90 minutes
Obtaining multiple sets of cultures increases the probability of discovering a pathogenic organism in the blood and reduces the probability of interpreting a positive culture caused by skin contaminants as being due to pathogens.
After inoculating the culture vials, advisably with new needles and not the ones used for venipuncture, the vials are sent to the clinical pathology microbiology department. Here the bottles are entered into a blood culture machine, which incubates the specimens at body temperature. The blood culture instrument reports positive blood cultures (cultures with bacteria present, thus indicating the patient is "bacteremic"). Most cultures are monitored for five days, after which negative vials are removed.
If a vial is positive, a microbiologist will perform a Gram stain on the blood for a rapid, general identification of the bacteria, which the microbiologist will report to the attending physician of the bacteremic patient. The blood is also subcultured or "subbed" onto agar plates to isolate the pathogenic organism for culture and susceptibility testing, which takes up to three days. This culture and sensitivity process identifies the species of bacteria. Antibiotic susceptibilities are then assessed on the bacterial isolate to inform clinicians with respect to appropriate antibiotics for treatment.
 Some guidelines for infective endocarditis recommend taking up to six sets of blood for culture (around 60 ml).
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