Blood culture

Blood culture is an examination for bacteraemia where blood is sampled and collected in special flasks which contain a medium which provides bacteria and fungi with the resources they need to grow. It's a commonly used examination in the hospital and is indicated when there are signs of invasive bacterial or fungal infection. Clinical information should be provided with the culture so that the laboratory knows what to look for and which antibiotics to test for.

Because the bacteria are cultured, it's possible to perform antibiotic susceptibility testing of any bacteria which are present, which is very useful when guiding antibiotic therapy. Blood cultures should be obtained before antibiotic therapy is started, as the presence of any antibiotics in the blood can interfere with the bacteria's ability to grow in the flask. However, in case of life-threatening infections like sepsis, antibiotic therapy should not be postponed (at least not for long) just to obtain cultures.

Indications

• Sepsis
• Endocarditis
• Meningitis
• Osteomyelitis or discitis
• Abscess
• Septic arthritis
• Any bacterial infection where there is deemed to be a significant likelihood of bacteraemia (including pyelonephritis, pneumonia, etc)

Procedure

Multiple flasks must be taken, usually one or two for aerobe bacteria and one or two for anaerobes. Some labs require a separate flask to grow fungi, if invasive fungal infection is suspected. These flasks constitutes one "set" of blood cultures. Taking 2 or more sets of blood cultures increases the likelihood of finding the bacterium in case of bacteraemia, as a single set may be false negative. One set of blood culture has an 80% sensitivity while three has 95-98% sensitivity.

Different sets should ideally be taken from different venipuncture sites. It's also important to fill the flasks to the "fill line" on the flask, as too little or too much blood can interfere with the sensitivity.

Proper disinfection of the surface of the flasks and the patient's skin reduces risk of contamination.

Bacteraemia may be intermittent, and so performing multiple blood culture sets at different points in time may increase the sensitivity, especially in case of infections involving skin, bone, lungs, GI, urinary tract, and CNS.

Results

If anything grows in any of the flasks, the lab will usually report growth within a day or two. However, one always waits until after five days has passed until one can say for sure that there is no growth.

The lab will usually first report that there is growth of a bacterium in a certain number of flasks. Then, Gram staining will reveal the microscopic morphology of the bacterium. Later, the lab will identify the microbe species, and even later, the lab will have completed the antibiotic susceptibity testing and provide the results.

Antibiotic susceptibility

Based on the provided clinical information, the lab will test whether the bacterium is susceptible to a certain number of antibiotics which may be indicated for the infection. The lab will usually provide a table like this one for example:

Antibiotic	Enterococcus faecalis
Amoxicillin	S
Ampicillin	S
Mecillinam	R
Nitrofurantoin	S

.. where "S" means that the bacterium is sensitive to the antibiotic, and "R" if it is resistant. If there is growth of multiple bacteria (rare), the lab will add columns to the table for each bacterium.

False positive results

Blood cultures can be contaminated by pathogens, usually from the skin. These bacteria are usually staphylococcus hominis or other coagulase-negative staphylococci, Corynebacterium species, Bacillus species, Cutibacterium acnes species, etc. If one of these bacteria grow in the medium, or if there is a finding in only one of four flasks, it might be contamination rather than a sign of bacteraemia.