Blood culture

Blood culture is a microbiological laboratory test of the blood for the presence of bacteria.

When to take blood culture
Patient suspected of a bacterial infection:
 * CRP> 60 mg / l;
 * history of fever - taking ideally when the temperature rises, preferably body temperature> 38 °, but not necessary;
 * the patient should not be given any antibiotics before taking the blood culture.

How to collect blood culture
BERAN, Ondřej:  Laboratory diagnostics of infectious diseases  is essential for sampling. [Lecture for the 5th year of the 1st Faculty of Medicine, Charles University (infectious medicine, general medicine)]. Prague, April 18, 2011. : Disinfection of the sampling site:
 * two samples are taken in the set - one for aerobic and the other for anaerobic cultivation;
 * usually 2 sets are taken, each from one hand (a total of 4 vials for blood culture, see picture);
 * the results of cultivation will not affect the use of non-coagulated blood with sodium citrate in the classical ratio (Na-citrate: blood 1: 9 to 1: 4)
 * This feature can be used to reduce the number of injections into the patient - when taking blood for basic laboratory tests, blood is taken into a special syringe and citrate is added, with a high CRP result, blood from the syringe is applied to culture vials - these are quite expensive in itself and taking blood directly into them, when we don't know if we will send them or not, seems uneconomical.
 * venous blood is most often taken from cubital fossa;
 * 2 disinfectants are used :
 * 1) alcoholic - to remove biofilm bacteria on the skin surface
 * 2) another disinfectant
 * 10 & nbsp; ml of blood is taken into each culture vial;
 * a skin swab is removed from the collection site for cultivation.

Processing and evaluation
Today, modern devices are used to cultivate and at the same time detect the presence of bacteria, which maintain a constant culture temperature of 37 ° C. If there are bacteria in the sample, the product of their metabolism is CO 2. The instrument checks the CO 2 concentration in each sample every 20 minutes. If the limit value of CO 2 is exceeded, the device triggers an alarm. Samples are usually left in the instrument for 5 & nbsp; days.

Based on the blood culture, we will only find out whether bacteria (aerobic or anaerobic) are present in the patient's blood. We will not find out the exact agent or its sensitivity to antibiotics. The subsequent classical cultivation examination on nutrient media will be used for this purpose.

Interpretation
A positive result confirms the presence of bacteria in the patient's blood ( bacteremia). To eliminate the possibility of contamination of the collected sample (false positives), a swab is cultured from the sampling site. The most common contamination is  Staphylococcus epidermidis  (coagulase negative staphylococci)

. The cause of the contamination is often collection through the venous catheter. Contamination is more usual in case of venous catheters compared to arterial catheters or collection from the peripheral vein. . The risk of contamination of the blood culture is lower during a single sampling from periphery even in comparison to the sampling from the newly inserted venous catheter. .

Related articles

 * nutrient agar
 * Inoculation of agars
 * Blood count
 * Hemocoagulation ▪ Blood clotting test ▪ Bleeding examination ▪ Erythrocyte sedimentation
 * Biochemical analysis ▪ Laboratory test for acid-base balance
 * Blood culture ▪ CRP ▪ PCT

Source

 * BERAN, Ondřej: Laboratorní diagnostika infekčních nemocí. [Přednáška pro 5. ročník 1. LF UK (infekční lékařství, všeobecné lékařství)]. Praha, 18.4.2011.