Blood Cultures and Clinical Importance
SUMMARY
Detection of microorganism in the patient's blood is very important not only for diagnosis but also for treatment and prognosis. Bacteria are present in the blood in three ways: temporary, intermittent or continuous. In order to make the most of the blood cultures in the clinic, timing, number of cultures, amount of blood, content of the culture medium and the method used are very important, starting with skin antisepsis. In addition, the patient's immune deficiency or presence of an intravascular catheter should be considered carefully. Not all positive blood cultures may be clinically relevant. In the interpretation of leather flora elements, it can sometimes be misleading to comment only on the number of cultures. Good communication between the clinician and the laboratory increases the usefulness of blood cultures.
Keywords: Blood cultures, Bacteremia
SUMMARY
Blood Cultures and Clinical Significance
The detection of a microorganism in the blood of a patient. Bacteria can persist in three manners in blood; transiently To provide a benefit from blood cultures, including skin antisepsis, timing and frequency of culture, amount of blood taken, content of culture and culture method used. In addition, the patient with immunodeficiency or intravascular catheter needs to be evaluated carefully. Every positive blood culture may not be clinically significant. In particular, in interpretation of skin flora elements, evaluation of the number of cultures taken into account. Good communication between the clinician and microbiology laboratory increases the usefulness of blood cultures.
Key Words: Blood Cultures, Bacteremia
Presence and Clinical Meaning of Bacteria in Blood
Detection of microorganism in the patient's blood is important not only for diagnosis but also for treatment and prognosis. It has been shown that the risk of death is 12 times higher in patients with positive blood culture during hospitalization than in patients with negative blood culture (1).
There are three forms of bacteria in the blood. This also reflects the path of the bacterium into the blood.
a. Transient bacteremia: Infected tissues, interventions, cystoscopy, tooth extraction, urethral dilatation, sigmoidoscopy, such as the removal of burns in the blood can be temporarily mixed bacteria. In this case, the amount of bacteria in the blood is very small and the duration of its presence in the blood is very short.
b. Intermittently: Generally, bacteria from tissue and organs with closed lesions (abscess, empyema) or diffuse infections (cellulitis, peritonitis, septic arthritis) first enter the lymphatic tract and occasionally into the blood.
c. Continuous: Endocarditis, intravascular infection foci, where there are infected catheters and separate from them in the early stages of infections such as typhoid fever, brucellosis. However, the source of microorganisms cannot be determined in 1/3 of bacteremia (2).
In order to make the most of the blood cultures in the clinic, the following points should be considered.
How to Get Blood Culture?
1. Hematological, biochemical, etc. Blood tests should not be taken when taking blood for tests such as. The risk of contamination increases with multi-purpose intake.
2. Each blood culture should be taken from a different vein. If the appropriate vein is not available for blood culture collection or catheter-induced sepsis is considered, then it can be removed from the catheter.
3. After the vein to be cultured is palpated, the skin is cleaned from the center to the periphery with 70% ethyl alcohol. Then 30 sec. 1-2% iodine tincture is applied. Excess iodine is removed with 70% alcohol. Poor cleansing of the skin results in coagulase negative staphylococci, Corynebacterium spp. and Propionibacterium spp. It is the most common reason for the growth of skin contaminants such as blood cultures.
4. It is preferable to work with sterile gloves for blood culture. A new set should be used if the first attempt at blood collection has failed.
5. Bottles with blood culture should be disinfected with alcohol or iodine tincture. Iodine tincture should not be used for septum cleaning of BACTEC bottles.
6. There is no need to change the needle at the tip of the syringe during blood culture collection. After the blood is placed in the culture flask, it is mixed by shaking gently to prevent clotting.
7. The blood culture request form must be written on which vein it was taken from, at what time it was taken and whether it was taken from the catheter (3-6).
When and how many blood cultures should be taken?
Ideally, the blood culture should be taken one hour before the fever rises. Usually this is not possible in practice. There was no optimal time for the second culture to be taken within 24 hours of the first culture uptake. This is often determined by the severity of antibiotic treatment (7).
In cases such as sepsis, meningitis, osteomyelitis, arthritis, pneumonia and pyelonephritis, blood cultures should be taken from two or three different veins within 15-20 minutes. In acute endocarditis, samples should be taken from three different veins within the first 1-2 hours. In subacute endocarditis, three samples should be taken at intervals of 15 minutes or more on the first day. In the patient with endocarditis under treatment, two more cultures can be taken after three successful days. In patients receiving antibiotics, six blood cultures should be taken within 48 hours, and antibiotics should be tried at the lowest level (gutter period) (4).
In non-emergency cases, two to three blood cultures can be taken in 24 hours. 80-91.5% of the episodes are detected in the first blood culture and 99.3% are detected in the first two blood cultures (8,9).
If blood cultures taken at baseline are negative and there is no change in the patient's clinical and hemodynamic status, he should avoid taking blood culture at elevated fever several times a day. Repeated blood culture monitoring or monitoring after detection of true bacteremia is often unnecessary. Patients who do not respond to treatment or clinically worsen should be re-cultured (3).
What should be the amount of blood cultures taken?
Blood volume taken is the most important factor affecting the sensitivity of blood cultures. Generally, in 50% of bacteremia, the number of bacteria is 1 CFU or less in mL, and in 20% 0.1 CFU / mL. Each 1 mL of excess blood culture increases the positivity rate by 3% (10).
In adults, the recommended volume of blood for each different blood culture is 10-30 mL. If there is no anaerobic and fungal seeding in the protocol prepared for the patient, it is sufficient to take 10 mL for aerobic seeding. It should not be taken more than thirty mL because it causes nosocomial anemia (3).
The concentration of microorganisms during bacteremia in children is higher than in adults. It is sufficient to take 1-2 mL in newborns, 2-3 mL in infants and 3-5 mL in children (11).
What Should be the Approach in Immunodeficient Patients?
Ideally, aerobic and anaerobic bacteria, fungi, mycobacteria and some viruses (HIV, CMV) should be isolated in the culture to be taken in immunocompromised patients. This is not practical for today and is quite expensive (12).
If blood cultures remain negative despite the possibility of a strong infection in a patient with immunodeficiency, it should first be discussed whether appropriate blood culture techniques are used. It should be remembered that bone marrow and liver biopsy cultures are also very supportive in the diagnosis (13).
The higher the number of microorganisms circulating in the blood in patients with immunodeficiency, leads to an increase in the diagnostic value in direct microbiological examination of blood (14).
Blood Cultures in Catheterized Patients
When catheter-related bacteremia is considered, catheter tip culture should be performed. Maki's semi-quantitative technique is the most commonly used method (15).
Isolation of the same microorganism from both catheter tip and peripheral blood culture is highly supportive for catheter-related bacteremia (16).
For long-term catheters, bacteria are often found in the catheter. In this case, quantitative growth culture or quantitative culture of peripheral blood and catheter blood is more sensitive (17).
Clinical Microbiology Laboratory and Blood Cultures
Non-automated and automated methods for blood cultures are available in clinical microbiology laboratories.
Non-automated methods:
a. Conventional monophasic liquid media
b. Diphasic media (Castenada)
c. Lysis centrifugation method (4).
Conventional liquid media use media such as tryptic soy broth, brain-heart infusion broth, and sodium polyanethol sulfonate (SPS), also an anticoagulant. Here reproduction; turbidity, hemolysis, gas formation, Gram staining from culture or agar plate is detected by culture (18).
In the case of diphasic liquid media, in addition to the liquid culture medium, solid media containing agar is present on one side of the bottle. Thus, the liquid medium is brought into contact with the solid medium by inverting the bottle and thus the colonies are visible to the eye (5).
In the lysis centrifugation method, the blood is placed in a tube containing lysis fluid consisting of saponin, SPS, fluorinert and EDTA and centrifuged at high speed. The supernatant is discarded and the culture is made from the precipitate. It should be noted that contamination rate is high in addition to its important advantages (19).
Automatic methods can be divided into three generations:
1st generation: Radiometric blood culture systems: 14 C with radioactive label in medium. When the microorganism grows, it uses it and 14 CO 2 is formed in the medium. The system is based on radiometric reading of this.
2nd generation: Non-radiometric blood culture systems: The color change caused by the CO 2 released by the microorganism in the sensor dye in the bottle is measured spectrophotometrically or the resulting fluorescence is read.
3rd generation: Blood culture systems that monitor reproduction continuously: An important advantage of this method is the possibility of continuous monitoring of cultures under computer support. In addition, one of the important advantages of automatic methods is that they are very fast (20).
Microscopic Examination of Blood
Microscopic examination of blood blood protozoa, Borrelia and Leptospira can be searched (21).
Direct microscopic screening of blood or y buffy coat bakteri for bacteria is a time-consuming and insensitive method (22).
Gram staining from blood taken from the catheter of patients receiving total parenteral nutrition may help in the diagnosis of sepsis.
The presence of intracellular bacteria in peripheral blood smear taken from central venous catheters in immunosuppressive patients may be significant in terms of catheter infection (23,24).
How should blood culture results be evaluated in the clinic?
Not all positive blood cultures are clinically relevant. Even under optimal conditions, 2-3% contamination occurs. It is not always easy to distinguish true positivity from false positivity (3).
The following points should be observed to separate contaminant bacteria.
a. Type of microorganism
b. Compliance with the clinic
c. Reproduction time
D. How many blood cultures are produced
to. Presence of microorganisms belonging to many skin flora
f. Whether blood culture was taken during effective antibiotherapy
g. The number of non-reproductive blood cultures (25).
Generally, contaminant bacteria are skin flora elements. They do not exist in other or followed cultures. They are isolated after a long incubation (3,10).
Skin flora bacteria such as coagulase negative staphylococci, Corynebacterium spp, Propionibacterium acnes are considered contaminants when isolated from one of several blood cultures. However, accepting coagulase-negative staphylococci as contaminant in this way may also lead to the omission of a clinically significant episode (3,8).
Coagulase negative staphylococci are considered significant when isolated from two or more blood cultures. However, it is not always possible to say that this is true bacteremia without strains typing phenotypically and genotypically. These may be due to unrelated contaminated strains or recontamination. Therefore, when coagulase negative staphylococci are isolated from two or more blood cultures, at least antibiotic susceptibility should be compared. As can be seen, it can be misleading to adhere to the number of cultures in the interpretation of leather flora elements (25).
Generally, isolation of several different microorganisms from the blood culture promotes contamination. However, in one study, polymicrobial bacteremia was reported in 21% of septic episodes and it was shown to have higher mortality compared to monomicrobial bacteremia. Polymicrobial bacteremia is more common in patients with underlying serious disease, intraabdominal infections or obstructions and non-hematologic malignancies (9,26,27).
As a result, blood cultures go through different periods, each of which may have an impact on the diagnosis and treatment of the patient, starting with sampling from the patient, evaluating in the laboratory, monitoring and clinical interpretation of the results. Compliance with the rules and clinical-laboratory cooperation in all these periods increases the usefulness of blood cultures.
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