In the era of antibiotics, infections are still the main "killers" of human health. According to statistics, infectious diseases account for 25% of the causes of death in the world's population. Clinical pathogenic microbial detection is an important guarantee for the rational application of antibacterial drugs and the monitoring of bacterial resistance, of which blood culture is the most direct and reliable method for diagnosing bloodstream infections.
Blood culture is an artificial culture method that inoculates fresh and isolated blood samples on nutrient medium, and under certain temperature, humidity and other conditions, the bacteria with higher nutritional requirements grow and multiply and identify them, so as to determine the pathogenic bacteria.
Standardized blood cultures are the gold standard for the diagnosis of bloodstream infections, and a positive result can not only confirm the presence and type of infectious pathogen, but also guide the clinical optimization of antibiotic selection and improve patient outcomes through subsequent susceptibility test results.
Accurate blood culture results require clinicians, caregivers, and laboratory staff to work together for quality control. So, how do we properly collect blood cultures?
First, clarify the indications for blood culture collection
1. Body temperature > 38 °C or
2. Chills;
3. Peripheral blood white blood cell count is increased (count > 10.0 ×109/L, especially when there is "nuclear left shift") or decreased (count 9/L);
4. Respiratory rate > 20 times/min or partial pressure of carbon dioxide in arterial blood (PaCO2)
5. Heart rate > 90 beats/min;
6. Bleeding from the skin and mucous membranes;
7. Coma;
8. Multi-organ dysfunction;
9. Decrease in blood pressure;
10. Inflammatory response parameters such as C-reactive protein, procalcitonin (PCT), 1,3-β-D-dextran (G test) increased.
Second, the appropriate collection time
Optimal acquisition time
Collection at the beginning of chills or fever (best collection within 2 hours); best collection before application of antibacterial drugs (those who have been taken before collection should be noted, and it is best to collect after 6 to 8 hours after stopping the drug or before the next dose)
Correct number of collection units
Aerobic bottles - anaerobic bottles
"A set of blood cultures" refers to blood samples taken from the same puncture site, usually injected into aerobic and anaerobic flasks, respectively.
Adults should collect 2 to 3 sets each time (to improve the detection rate of pathogenic bacteria and determine whether there are contaminated bacteria), and each set should be collected from different puncture points and marked sites. In addition, since bacteria in the blood are not removed immediately after antibiotics are started, there is no need to repeat the collection within 5 days.
If infective endocarditis or acute endocarditis is suspected, blood cultures should be collected immediately. It is advisable to collect 2 to 3 sets of blood cultures at different sites within 30 minutes before the empiric use. Subacute endocarditis should be collected every 0.5h to 1 hour with one set of blood cultures, and a total of 3 sets of blood cultures should be collected from different parts. If the culture is negative at 24h, it is advisable to add 2 sets of blood cultures. Studies have shown that among patients with bloodstream infection, the positive detection rate of 1 set is 73.2%, 93.3% of 2 sets, and 96.9% of 3 sets.
Children usually collect only aerobic bottles. Anaerobic bottle culture should be considered when there are the following risk factors: children with peritonitis in the maternal puerperium, or chronic stomatitis or sinusitis, cellulitis, signs and symptoms of intraperitoneal infection, bites, and agranulocytosis receiving steroid therapy.
When streptococcus pneumoniae bacteremia is considered, CSF culture should be done at the same time.
Third, sufficient collection amount and incubation time
Specimen collection volume
The total amount of blood collected is one of the important factors affecting the detection of pathogens. Each bottle should be collected 8 to 10 ml of blood, because of the high concentration of pathogens in the blood in children, the amount of blood culture does not need to be equivalent to adults, but it is also recommended that the amount of blood collected is not less than 2 ml per bottle.
Incubation time
Blood cultures are recommended for 7 days at 35°C to 37°C, and some slow-growing caustic bacteria (such as Bartonella, Listeria, Brucella, and Nukas) and bidirectional fungi require longer incubation time.
Yeast is easy to grow in aerobic bottles, most yeast incubation can be detected after 2 to 5 days, and bipolar fungal or filamentous fungal infection is suspected, and the incubation time needs to be extended by 2 to 4 weeks. Due to slow multiplication of mycobacteria, culture flasks targeting them should be incubated for at least 6 weeks.
Detection rate of blood cultures over time
Fourth, the correct collection method
Collection of venous blood (collection of catheter blood only when evaluating catheter-associated bloodstream infections). Blood culture should be collected separately, at the same time as other test items, and blood culture flasks should be collected first to avoid contamination.
Do hand hygiene before collection, after the venipuncture point is selected, remove the plastic cap of the blood culture bottle, do not open the metal sealing ring and rubber stopper, use 75% ethanol or 70% isopropanol to disinfect, and dry naturally for 60s. Pay attention to check whether the blood culture flask is intact and expired before blood collection.
Before or during puncture, to prevent vein slippage, sterile latex gloves should be worn to fix the vein.
Puncture point skin disinfection
1. Three-step method
Step 1: 75% alcohol to wipe the venipuncture site, to dry for more than 30 seconds;
The second step: 1% to 2% iodine tincture action 30s or 1% iodine volt action 60s, from the puncture point outward circle disinfection, disinfection area diameter of more than 3cm;
Step 3: 75% ethanol wipes the disinfected area of iodine tincture or iodine volt for deiodine.
Patients who are allergic to iodine, on the basis of the first step, disinfect with 75% ethanol for 60s, and collect blood after the alcohol volatilization is dried.
2. One-step method
0.5% gluconotype acting 30s (not suitable for newborns up to 2 months), or 70% isopropanol is naturally dried after sterilization (for newborns up to 2 months). Note that puncture points should not be touched after disinfection.
5. Blood culture collection process
Hand hygiene (can be hand-disinfected) Wear a mask 75% alcohol disinfection of the rubber stopper of the culture bottle, to be dried 60s 75% alcohol disinfection puncture site, from the inside to the outside of the circle, the role (to be dried) 30s with iodine tincture or iodine volt disinfection puncture site (iodine tincture action 30s, iodine volt action 1.5-2min), from the puncture point from the inside to the outside, disinfection range diameter of more than 3cm 75% alcohol deiodization action 60s, to be dried Holding the puncture needle according to the conventional method into the vein, the other end into the corresponding blood culture bottle.
Blood collection volume is 8-10 ml / bottle for adults, 1-5 ml / bottle for infants (strict sterile operation, it is not allowed to touch the veins with the hands after skin disinfection).
After injecting blood into the blood culture flask, gently shake the bottle, mark it and send it for testing immediately (do not refrigerate or freeze the culture flask).
Precautions
After taking blood with a syringe sterile puncture, do not change the needle (e.g., for a second puncture, needle change), inject directly into the blood culture flask, and anticoagulant blood should not be injected into the blood culture flask. After the blood is inoculated into the culture flask, mix gently upside down to prevent blood from clotting.
Blood culture flasks should be sent to the laboratory for incubation or on the machine within 2 hours; if they cannot be sent for testing in time, the blood culture flasks should be placed at room temperature and should not be refrigerated or frozen. Specimens are transported in sealed bags and hard, leak-proof containers. If shipped to a reference laboratory, biosafety-compliant packaging should be used.
【Reference】
[1] Consensus of Chinese experts on the standardized collection and submission of clinical microbial specimens for examination.2017
[2] WS/T 503-2017 Code of Practice for Blood Cultures in Clinical Microbiology Laboratories
[3] Weinstein et al. Detection of Bloodstream Infections in Adults: How Many BloodCultures Are Needed. J ClinMicrobiol. 2007; 45:3546-3548.
[4] F.R.Cookerill III et al. Clinical Infectious Diseases 2004; 38:1724-30.
[5] Mermel LA et al. Ann Intern Med. 1993; 119:270-272.
Source: Department of Aviation Medical Laboratory
Edited by: Yeah Reviewer: Xiao Ran