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.

Because the bacteria are cultured, it's possible to perform antibiotic resistance analysis of any bacteria which are present, which is very useful when guiding antibiotic therapy.

Procedure

Multiple flasks must be taken, usually two for aerobe bacteria and two for anaerobes. Some labs require a separate flask to grow fungi, if invasive fungal infection is suspected. These 4 (or 5) 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. 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.

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.

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,