Blood culture
A blood culture is a microbiological examination of the blood in which an attempt is made to cultivate pathogens (usually bacteria ) that are in the blood to reproduce in order to detect and identify them.
Indications (situations in which the examination is carried out)
- Sepsis (ancient Greek σῆψις sēpsis "putrefaction")
- Fever of unknown cause
- Endocarditis (inflammation of the lining of the heart)
- Fever in immunocompromised patients (patients with HIV, chemotherapy)
- Pneumonia (inflammation of the lungs)
- Meningitis (inflammation of the meninges)
method
With the help of blood cultures, disease-causing bacteria have been searched for since the beginning of the 20th century. For this purpose, a certain amount of blood is taken from the patient and immediately placed in the "blood culture bottles". There are also more modern collection systems in which the blood is fed directly into the bottles filled with a nutrient medium. As always in clinical microbiology, the work must be sterile and clean. The venous blood should - depending on the indication - be collected several times at intervals of at least half an hour from different locations.
The likelihood of bacteremia is greatest just before a chills attack , but the occurrence of chills is unpredictable. For this reason, too, contrary to previous doctrines, it does not make sense to make the taking of blood cultures dependent on a specific body temperature.
In addition to a so-called bouillon, there is a gas mixture in the blood culture bottles. The latter differs between the bottle for aerobic and anaerobic pathogens.
- anaerobic bottle contains more carbon dioxide
- aerobic bottle contains more oxygen
Aerobic and anaerobic bottles each offer favorable growth conditions for different types of bacteria (so-called aerobes and anaerobes). Taking blood cultures therefore always involves filling two bottles, a so-called "pair of blood cultures".
processing
In the microbiological laboratory, the bottles are placed in the incubator and incubated at 37 ° C for several days.
In the past, people used to check daily with the naked eye to see whether the liquid was cloudy. In addition, a solid culture medium was attached to the bottle wall, on which bacterial or fungal colonies could settle.
Today there are devices that do this work for the laboratory technicians. The bottles are filled with a special gas mixture. If this mixture changes due to the metabolism of the bacteria, this can be measured by the very sensitive apparatus and it immediately gives an alarm message. The laboratory assistant can then initiate further investigations, such as bacterial identification and sensitivity / resistance of the pathogen to certain antibiotics . This enables a faster diagnosis and the patient can be treated accordingly.
Sources of error
- Asepsis : Incorrect bacterial determinations if blood is not drawn strictly aseptically or skin disinfection is insufficient . In this case, the culture result brings typical skin germs.
- Dying of sensitive pathogens: Pathogens can themselves die in the culture bottle until they are transported to the laboratory. This is v. a. if antibiotic therapy is already in progress.
- Non-cultivable or non-bacterial pathogens
Individual evidence
- ↑ Barbara I. Tshisuaka: Nice, Jules Arnold. In: Werner E. Gerabek , Bernhard D. Haage, Gundolf Keil , Wolfgang Wegner (eds.): Enzyklopädie Medizingeschichte. De Gruyter, Berlin / New York 2005, ISBN 3-11-015714-4 , p. 1030 f.
