Blood culture
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Blood culture is microbiological culture of blood. It is employed to detect infections that are spreading through the bloodstream (bacteremia, septicemia).
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[edit] Method
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.
Care needs to be taken that the bottles are not contaminated with bacteria from staff members or other patients. To that end, the patient's skin is rubbed or sprayed with denaturated alcohol and betadine applied to the sampling site. Sterile gloves should be used to minimize contamination.
To maximise the diagnostic yield of blood cultures multiple sets of cultures (each set consisting of aerobic & anaerobic vials filled with 3-10 mL) may be ordered by medical staff. A common protocol used in US hospitals includes the following:
Set 1= L. antecubital fossa at 0 minutes Set 2= R. antecubital fossa at 30 minutes Set 3= L. or R. antecubital fossa at 90 minutes
Ordering multiple sets of cultures increases the probability of discovering a pathogenic organism in the blood and reduces the probability of having a positive culture due to skin contaminants.
After inoculating the culture vials on the hospital floor, they are sent to the microbiology lab clinical pathology department. Here the cultures are entered into a blood culture machine, which keeps the samples at body temperature. The blood culture instrument reports positive blood cultures (cultures with bacteria present, thus indicating the patient is "septic") by monitoring carbon dioxide levels produced by bacteria in the vials via fluorescence detected by a light emitting diode (LED). Most cultures are monitored for 5-days, after which, if the vials are negative, they are removed.
If a vial is positive, a microbiologist will perform a Gram Stain on the blood for a rapid, general ID of the bacteria, which they will report to the attending physician of the septic patient. The blood is also subcultured or "Subed" onto agar plates to isolate the pathogenic organism for culture and suceptability testing, which takes up to 3 days time. The C&S process IDs the species of bacteria. Antibiotic sensitivities are then assessed on the isolate to inform clinicians on appropriate antibiotics for treatment.
[1] Some guidelines for infective endocarditis recommend taking up to 6 sets of blood for culture (around 60 ml).
[edit] Uses
Any infection causing fever may prompt hospital physicians to request blood cultures. Identifying the agent may aid in choosing the correct antibiotic and addressing particular risk factors.
Blood culture is essential in the diagnosis of infective endocarditis. In this elusive disease, blood cultures may have to be repeatedly taken during febrile episodes, when bacteria are shed from the heart valves into the bloodstream (bacteremia)..
[edit] History
Blood cultures were pioneered in the early 20th century. Gustav Mahler, the composer, was diagnosed with infective endocarditis close to the end of his life, but as this was before the introduction of antibiotics, it contributed to his (early) death.
