Blood culture is an experimental method used to detect the presence of bacteria or fungi in the blood. Blood is collected from the patient and placed in a bottle containing culture medium. By analyzing the microorganisms in the blood, various diseases related to blood infection, such as bacteremia, sepsis, sepsis, catheter-related bloodstream infection, etc., can be diagnosed and treated. Early blood culture testing and early correct antimicrobial treatment are the first measures to control sepsis.
Why is blood culture important?
Microorganisms in the blood reach every organ within 3 minutes. Bacteremia is an important challenge for us during critical care. Early diagnosis is one of the decisive factors for patient prognosis.
Bacteremia is the most common potential cause of death. Severe bacteremia or septic shock has a mortality rate of 32% to 54%.
Sources of severe bacteremia:
Most severe bacteremia is diagnosed based on positive blood culture results
Respiratory tract infection and genitourinary system infection are the main sources of bacteremia in men and women respectively
Mortality rate of bacteremia caused by different system infections
Respiratory system infection and endocarditis have the highest mortality rate among bacteremia infections of known sites, exceeding 30%
Three purposes of blood culture
1. Confirm infectious pathogens
2. Identify pathogens
3. Guide antimicrobial drug treatment
When to perform blood culture?
When bloodstream infection or bacteremia is suspected, routine blood culture should be performed.
Symptoms and conditions that may indicate a patient has a bloodstream infection include:
1. Unexplained fever (>38°C)
2. Severe local infection, such as meningitis, endocarditis, pneumonia, pyelonephritis, abdominal postoperative infection, etc.
3. Invasive procedures, such as intravenous catheterization, urinary catheterization, mechanical ventilation, etc.
4. Shock, chills, rigidity
5. Abnormally fast heart rate
6. Hypotension or hypertension
7. Faster respiratory rate
Time for blood culture collection
Blood collection before estimated chills or peak temperature
Blood collection before antimicrobial drugs are used
When suspicious clinical symptoms appear, blood culture should be collected as early as possible. It is ideal to collect blood culture before the application of antimicrobial drugs. If the patient has already started antimicrobial treatment, blood culture should be collected before the next medication
Blood culture volume
The recommended blood collection volume for adults is 20 ml of blood per pair of culture bottles. Each pair of culture bottles includes an aerobic bottle and an anaerobic bottle. Each bottle should be inoculated with 8-10 ml of blood.
The amount of blood drawn from children should not exceed 1% of their total blood volume. The recommended amount of blood drawn is 2-4 ml.
Number of blood culture collections
It is usually recommended to draw blood continuously at different locations within a short period of time (e.g. within 1 hour) and do 2 to 3 pairs (2 bottles/pair) of blood cultures.
When infective endocarditis or other intravascular infections (catheter-related) bacteremia and fungemia are suspected, blood is drawn at intervals (e.g. every 1-2 hours) for continuous monitoring.
Studies have shown that using a continuous blood culture monitoring system to accumulate and statistically analyze blood culture sensitivity results, we can observe that the cumulative detection rate of multiple pairs of blood culture combinations (20 ml of blood drawn each time, 10 ml per bottle) increases with the number of blood draws.
Since bacteria and fungi are transiently present in the blood in most cases, the sensitivity of a single blood culture test is limited. In order to ensure the detection rate of bacteremia, it is necessary to take more than two pairs of blood cultures.
Blood culture inoculation order
If the amount of blood drawn is less than the recommended amount, the blood should be inoculated into the aerobic bottle first, and the remaining blood should be inoculated into the anaerobic bottle. Most bacteremia is caused by aerobic and facultative aerobic bacteria.
If blood is collected with a syringe and needle, the anaerobic bottle should be inoculated first to avoid air entering
If blood is collected with a butterfly needle, the aerobic bottle should be inoculated first to prevent the air in the device from conveying the anaerobic bottle.
Skin disinfection
Skin disinfection before collecting specimens is very important
It is recommended to use iodine tincture, hypochlorous acid and chlorhexidine or iodine tincture
The disinfection effect of iodine tincture, hypochlorous acid and chlorhexidine is better than iodine tincture
The disinfectant needs to have a sufficient action time to ensure the disinfection effect
Iodine tincture acts for 30 seconds;
Iodine tincture acts for 1.5~2 minutes
The action time of chlorhexidine is the same as iodine tincture, but since there is no associated allergic reaction, it does not need to be wiped off, but it cannot be used for skin disinfection of infants less than 2 months old.
Disinfection of blood culture bottles
The blood culture medium contains an anticoagulant called SPS (sodium polydisulfide propane sulfonate), which is a substance containing disulfide bonds and free sulfonate groups. It is reductive and easily undergoes oxidation and addition reactions with oxidants such as iodine, thus causing the molecular structure to be destroyed and the anticoagulation to fail. The blood culture bottle stopper needs to be disinfected with alcohol.
Key points for collecting high-quality blood cultures
First check the blood culture bottles for signs of damage or contamination
Check the expiration date printed on each bottle
Each pair of bottles of blood culture should be collected from different parts of the body
The blood used for culture should be peripheral venous blood rather than arterial blood, which is of little diagnostic significance
According to recommendations, blood should be drawn from venous or arterial catheters, because these devices usually have a higher contamination rate. If blood needs to be drawn, the attached devices must be removed first
If blood must be drawn from an indwelling catheter, another blood culture specimen should also be collected from a peripheral vein at the same time to help interpret positive results
Transport standards for blood cultures
Blood culture bottles should be sent to the laboratory immediately after collection
Blood culture bottles should not be refrigerated or frozen before and after inoculation
Send the specimen to the clinical microbiology laboratory for culture as quickly as possible (preferably within 2 hours). If there is any delay, it should be temporarily stored at room temperature
