Blood culture describes a microbiological procedure that attempts to grow bacteria found in the blood, thereby detecting or identifying them.
The procedure
One to two blood samples are taken from the patient at the time of the fever spike and mixed with a nutrient solution (blood culture bottle) in a bottle to allow the bacteria to grow optimally.
To increase the likelihood of a positive blood culture, this procedure may need to be repeated several times.
The blood culture bottles are incubated for several days at a temperature of 37 °C in an incubator. Cultivation is performed with two different gas mixtures. One blood culture bottle contains more oxygen and is used to detect aerobic pathogens (so-called aerobes); the other blood culture bottle contains more carbon dioxide and is used to detect anaerobic pathogens (so-called anaerobes).If bacteria could be cultivated, an exact determination of the pathogen as well as a resistance test is carried out.
After targeted therapy – e.g. for endocarditis: two blood samples are taken at the end of the dosing intervals.
Material required
EDTA blood; incubate blood in pre-warmed bottles; in case of vital indication, transport to the laboratory as quickly as possible.
Note: The probability of a positive microbiological result depends on the number of samples taken and the time of collection. The German Society for Hygiene and Microbiology provides the following recommendation:
- Sepsis with intermittent fever.
- Fever condition with continua
- Day 1: 2-3 collections, at least one hour apart, preferably 2 of them before starting therapy.
- Day 2: 2-3 withdrawals, at least one hour apart or at the end of antibiotic dosing intervals.
- Suspected endocarditis (endocarditis of the heart).
- Day 1: at least 3 collections before starting therapy, if possible at the onset of fever rise.
- 2nd day: at least 3 withdrawals, in refractory forms at the end of dosing intervals.
Inoculate the anaerobic bottle first, followed by the aerobic. Following the inoculation, swirl the bottles briefly.
Preparation of the patient
- Extensive disinfection of the puncture site.
- Optimal time is the rise in fever: here two blood sampling.
Interfering factors
- None known
Indications
- Sepsis (“blood poisoning”)
- Fever of unexplained cause
- Intermittent fever
- Fever in immunocompromised patients (patients with HIV or undergoing chemotherapy).
- Endocarditis (endocarditis).
- Pneumonia (pneumonia)
Interpretation
Germs that are frequently detected:
- Staphylococcus aureus
- Staphylococcus epidermidis
- Enterococcus
- Greening streptococci
- Streptococcus pneumoniae
- E. coli
- Anaerobes
Further notes
- In principle, all germs detected in blood culture are considered pathogenic (pathological). However, it should be noted that contamination (pollution) by germs of the skin flora and air can occur (e.g., Staphylococcus epidermidis, aerobic spore-formers, propionibacteria, these are gram-positive bacteria that belong to the natural microbial flora of the skin). However, if these germs are detected in more than one of the blood samples, then this speaks against contamination.
