BLOOD CULTURE: WHAT IS IT FOR, RATIONALE, PROCEDURE, RESULTS - BIOLOGY - 2024

The blood culture is a bacteriological test that seeks to detect the presence of microorganisms in the blood. Blood is a sterile liquid by nature and thus must be kept under physiological conditions, so the presence of bacteria or fungi in the blood is always pathological.

When bacteria or fungi are found in the blood, but the multiplication does not exceed the elimination of microorganisms by the immune system, it is called bacteremia (for bacteria) or fungemia (for fungi); But if the microorganisms increase uncontrollably in number, it is called septicemia.

Blood culture bottles (yellow: pediatric aerobiosis, green: adult aerobiosis, and red: adult anaerobiosis). Source: photograph taken by the author MSc Marielsa Gil.

Bacteremia, fungemia and septicemia endanger the life of the patient and therefore must be treated immediately. That is why, when there is suspicion of infection in the blood, doctors request a blood culture study.

This bacteriological analysis allows to know whether or not there is an infection in the blood and which is the microorganism involved. In addition, if it is positive, a sensitivity test is performed to know which antibiotic or antifungal could be used in the treatment.

If, on the other hand, the blood culture is negative at 24 hours of incubation, it should not be discarded until it is negative for 240 hours. This ensures that there are no slow-growing microorganisms.

For a blood culture to be reliable, extreme aseptic measures must be adopted when taking the sample, and to increase the reliability and sensitivity of the test, a minimum of two samples should be taken during or near the peak of fever.

What is it for?

Blood is a sterile liquid and when microorganisms are found in it it is 100% pathological. This situation represents a very delicate clinical picture that compromises the life of the patient.

Blood culture is an important bacteriological test that detects the presence of microorganisms in the bloodstream.

The microorganisms can reach the blood by different routes, which could be extravascular infections such as: pneumonia, intra-abdominal infections, pyelonephritis, serious skin infections, soft tissues or arthritis, among others.

Or it can also be intravenously, for example, contamination of intravenous or arterial catheters, endocarditis, intravenous drug addiction, administration of contaminated drugs or solutions, etc.

Detecting and treating the causative agent of sepsis in time is essential to ensure patient survival.

In this sense, the doctor should indicate the realization of a blood culture when he observes signs and symptoms that make suspicion of septicemia, such as: fever (greater than 38 ° C) without an apparent infectious focus or, on the contrary, hypothermia (<de 36 ° C).

Other signs may be: chills, increased white blood cell count (> 10,000 cells / mm ³) or significant decrease in polymorphonuclear cells (<1,000 PMN / mm ³). As well as multiorgan deterioration or loss of vitality suddenly, among other warning signs.

Bacteremia can be constant, transient, or intermittent. This is important when taking a sample, since it is necessary to take it when there is a greater probability of finding the microorganism circulating.

Therefore it is recommended to take at least 2 samples in different places. In addition, the ideal is that the sample is taken in feverish peaks or when the patient presents shivering, extreme hypothermia, sweating or tachycardia.

However, for blood culture to be a truly useful tool, the sample must be taken with extreme care. A bad manipulation or bad asepsis at the moment of taking the sample can invalidate the test, obtaining false positives.

Basis

The study consists of taking two or three blood samples aseptically and placing it in special bottles.

Special devices for culturing blood samples are called blood culture bottles. These are classified into:

According to the age of the patient

-Pediatric use

-For adults.

According to the type of microorganism

-Flasks for aerobic microorganisms (aerobic bacteria, facultative bacteria and fungi).

-Blood culture bottles for anaerobic microorganisms (strict anaerobic bacteria).

Some contain a liquid culture medium and some contain a solid and liquid culture medium at the same time. They also exist with activated carbon particles.

Process

Recommendations for sampling

- The sample to be taken by highly trained and trained personnel in the area of ​​microbiology.

- Asepsis or thorough cleaning of the sample collection site is undoubtedly the most important step.

- Like all sampling, health personnel must fully comply with biosafety measures during the process (use of gloves, gown, glasses, among others).

- Take care that all the necessary implements for taking samples are available.

- Label the vials with the patient's full name, date, medical record number, sample collection time, and laboratory sequence number.

-The ideal is to take the sample before the patient begins antimicrobial therapy. It is only indicated in the case that the non-functioning of the treatment in progress is suspected. In this case, the sample must be taken before the drug change, using blood culture bottles with antibiotic inhibitors (activated carbon particles).

- At least 2 samples must be taken in different anatomical sites, such as the right arm and the left arm. In suspected endocarditis, 3 samples are recommended. Two bottles will be included in each sample (one for aerobiosis and one for anaerobiosis).

Sample quantity

The amount of sample varies according to the age of the patient, but the ratio 1: 5 to 1:10 should always be maintained with respect to the blood / culture broth dilution.

In newborns, the recommended sample quantity is 1 ml of blood per vial. Pediatric bottle is used.

In the case of infants between one month and one year, it can be increased to 1.5 ml of blood per bottle. Pediatric bottle is used.

In children older than 2 years the appropriate sample amount is 2.5 ml of blood per vial. Pediatric bottle is used.

From adolescence it can be increased to a blood volume between 5 - 10 ml per bottle. An adult bottle is used.

Finally, in the adult stage, the necessary amount is 8-10 ml per bottle. An adult bottle is used.

Sampling

- The blood sample can be venous or arterial. However, arterial blood is taken only when venous sampling is impossible.

- It is not recommended to take a sample from a central venous catheter unless:

1. It is impossible to take the sample peripherally (venous or arterial).
2. Patients at risk of bleeding.
3. When the doctor suspects bacteremia due to contamination of the central venous catheter.
4. When the fever reappears after a febrile cessation of 4 to 5 days, regardless of whether or not the patient is on antimicrobial treatment.

Asepsis before sampling

- Choose the anatomical sites for sampling. Generally, the veins of the best caliber are chosen (basilic or cephalic vein).

- According to the Centers for Disease Control (CDC) in Atlanta (USA), the operator should wash their hands with 2% chlorhexidine or 10% povidone iodine before sampling, in addition to wearing gloves.

-Palpate and locate the vein to be used.

-Clean the puncture area in a rotating way, making movements from the center outwards using soapy chlorhexidine or antiseptic soap. Rinse with sterile saline.

Subsequently, apply an antiseptic and let it act. Example chlorhexidine gluconate 0.5% for 1 minute or povidone iodine 10% for 2 minutes. For the latter, first ask if the patient is allergic to iodine. If you are allergic, you can substitute 70% alcohol.

Sample extraction

- Place the tourniquet to exacerbate blood flow and sprout the vein.

- Do not touch the puncture site with your finger again. If this is strictly necessary, the finger should be washed in the same way as the puncture area.

-Insert the injector needle or the scalp into the vein and extract the necessary amount of blood.

-Do not put cotton or gauze on the needle when removing it if it is not sterile.

-Remove the security seal from the bottles very carefully and without touching the cap. Some authors recommend performing a disinfection of the cap before inoculating the sample.

- Distribute the appropriate amount of blood in the vials. If the sample is taken with an injector, the necessary quantity is first poured into the anaerobic flask and then into the aerobic flask. If the taking is done with scalp (butterfly) it is poured in the opposite way.

- Mix the blood culture bottle gently by inversion.

- Change gloves and repeat the previous steps for the second sample collection.

-If the second sample is taken from a different site, it can be done immediately, but if it is from the same site, you must wait 30 to 90 minutes between one sample and another.

- The sample must be taken to the laboratory as soon as possible, if this is not possible it must be left at room temperature for up to 18 hours.

Culture

Once in the laboratory, the flasks are incubated at 37 ° C under the conditions of each flask, that is, in aerobiosis and anaerobiosis respectively.

Under the manual method, ringing should be started at 24 hours of incubation and then ringing inter-daily. The rings are performed as follows: first the bottle cap is disinfected and the needle of a sterile injector is inserted. Liquid is extracted from the flask and sown on blood agar and chocolate agar.

If there is growth, a Gram is performed, subcultures in specific media, biochemical tests and antibiogram.

In automated methods the Bact / Alert equipment emits an alarm when it detects that a vial is positive. In the same way, it should be tapped on blood agar and chocolate agar.

Another method that is gaining ground is to analyze the flask after 6 hours of incubation through mass spectrometry. This method has helped increase the sensitivity and speed of diagnosis.

Results

As long as the blood culture bottle is negative, preliminary intermediate reports can be given to the treating physician. The report indicates that it is negative in the hours it has been incubating. For example, if it goes negative until the fourth day, it will be reported as follows:

Preliminary result: negative culture at 96 hours of incubation.

Note: study continues for 240 hours.

If the blood culture is positive, the treating physician is immediately informed and a report is sent with at least the gram of the colony. Example:

Preliminary result: in positive culture at 48 hours of incubation, gram negative bacilli and negative oxidase are observed. Identification and sensitivity testing are in process.

This information guides the treating physician to start an empirical therapy towards the possible microorganism, while waiting for the final result of the laboratory.

Upon completion of the bacteriological study, that is, the microorganism has been identified and the antibiogram is available, the final report should be sent as soon as possible.

Special care should be taken if the target microorganism is Neisseria gonorrhoeae or Neisseria meningitidis, as these bacteria are inhibited in the presence of high concentrations of sodium polyanethosulfonate (SPS).

That is why this compound should not exceed 0.025% in blood culture bottles.

On the other hand, if the blood culture sample is first taken in Vacutainer tubes, these tubes have concentrations of SPS toxic to meningococci and gonococci, so the blood must be transferred within 1 hour to the broth culture system.

How to tell if a blood culture is positive or contamination

A blood culture is considered contaminated when there is growth in only one blood culture vial from the total taken. And the suspicion of contamination increases if the isolated microorganism is a habitual skin microbiota: example: Staphylococcus coagulase negative, Propionibacterium spp, among others.

However, in immunocompromised patients, no microorganism should be neglected, but in this case the microorganism should appear in several samples.

On the other hand, if the sensitivity to antibiotics of the same microorganism isolated in two different samples is the same, the infection is real.

Another characteristic is the bacterial load, since contaminated blood cultures grow late, while real infections in untreated patients are generally positive at 14 hours of incubation when the microorganism is non-bothersome.

On the other hand, in patients treated with antimicrobials the microorganism involved may take time to grow because the load is very low.

The appearance of more than one microorganism may suggest contamination, but if the same result is repeated in several shots from different sites, then it is real.

References

1. "Blood culture." Wikipedia, The Free Encyclopedia. 3 Jul 2019, 17:28 UTC. 14 Jul 2019, 19:05 en.wikipedia.org
2. Hervé B. New technologies in microbiological diagnosis: automation and some applications in microbial identification and susceptibility study. Rev. Med. Clin. Counts. 2015; 26 (6) 753-763. Available at: reader.elsevier.com
3. Villarroel P. Chapter 20: Sepsis and risk of cardiovascular disease. Cardiovascular health. pp 187-194. Available at: fbbva.es
4. Sánchez R, Rincón B, Cortés C, Fernández E, Peña S, Heras EM Blood cultures: What have they told you and what do you do? Sick glob. 2012; 11 (26): 146-163. Available at: scielo.isc
5. Pardinas-Llergo M, Alarcón-Sotelo A, Ramírez-Angulo C, Rodríguez-Weber F, Díaz-Greene E. Probability of success of obtaining a positive blood culture. Med. Interna Mex. 2017; 33 (1): 28-40. Available at: scielo.org