Blood Culture, its Procedure and Interpretations
Sample for Blood Culture
- >10 ml of blood from the patient is needed; the best time is during fever. This is collected in the culture bottle.
- Inoculation of Blood <5 ml shows markedly reduced yield.
- One reference recommends 20 to 30 mL of blood/per culture.
- Some suspected organisms may need more blood.
- Take the blood sample immediately after the onset of fever and chills.
- The best time is to take a blood sample one hour before the fever spike. This spike time is mostly the same time every day.
- It is better to get three samples during the fever. These samples may be taken from both arms.
- Take 2 to 3 cultures per septic episode or per 24-hour period.
- In continuous bacteremia like endocarditis, the sample’s timing is unimportant.
- In the case of intermittent bacteremia, take a sample at the start of the fever, and continue up to three samples in 24 hours.
- Paired culture, aerobic and anaerobic samples are taken, ideally from the opposite arms.
- Clean the site to avoid contamination by 70% Alcohol. It needs at least 2 minutes to be effective.
- Then clean with the 2% iodine.
- If the patient is sensitive to iodine, then cleaning with alcohol is enough.
- Then clean with the 2% iodine.
- For a fever of unknown origin, draw two blood samples.
- A quantity of less than one ml is insufficient to detect bacterial infection.
- The culture needs at least 72 hours for the report.
- Blood with or without microorganisms is collected into bottles for growth in an aerobic or anaerobic environment.
- The advantages of more blood samples are:
- More cultures samples increase the possibility of positive blood culture.
- Positive cultures in 2 or 3 separate samples indicate continuous bacteremia that is typically found in bacterial endocarditis.
- Positive cultures in 2 or 3 samples indicate that there is no contamination. This is very important in prophetic valve endocarditis, where S. epidermidis is common.
Precautions for blood culture
- A strict aseptic technique must be observed.
- Antimicrobial treatment before collecting the blood sample may result in a negative culture report. This may be negative even in bacteremia.
- Using inappropriate culture media or the container may interfere with aerobic growth.
- Inadequate time for the culture, temperature, or culture conditions may fail to give an adequate amount of bacteria for its identification.
- If there is patient skin contamination for the culture.
- Don’t draw blood from the I/V catheter unless no veins are found.
- In infants, take the blood from an umbilical artery catheter.
Indications for blood culture
- Blood culture is advised when there is a suspicion of bacteremia or septicemia.
- Find a bacterial infection that has spread into the blood, such as meningitis, osteomyelitis, pneumonia, a kidney infection, or sepsis.
- To find the type of bacteria on culture.
- Find a fungal infection, such as yeast, in the blood.
- Check for endocarditis, an infection of bacteria living on the heart’s valves.
- Repeated blood cultures are needed in case of endocarditis.
- Find the best antibiotics to kill the bacteria. This is called sensitivity testing.
- Find the cause of an unexplained fever or shock.
- In an unexplained fever of several days duration.
- In patients with chills.
- Infected burn.
- Urinary tract infection.
- Postoperative wound sepsis.
- Indwelling venous or arterial catheter.
- If bacteria are found in the culture, another test is often done to find the best antibiotic that will kill the bacteria. This is called sensitivity or susceptibility testing.
- Sensitivity testing is important to treat blood infections properly. This also helps prevent bacteria from becoming resistant to antibiotics.
Blood culture facts:
- Blood culture report that often identifies microorganisms needs at least 24 hours.
- 67% of the pathogens are identified in 24 hours.
- 90% of the pathogens were identified in 72 hours.
- Fungi and mycobacterium may need more time, maybe in weeks.
- Vascular catheter-related infections (bacteremia) cause at least 20% to 40% of all hospitalized patients.
- After the growth, the organism is tested with various drugs impregnated discs.
- Sensitivity will take another 24 hours or more.
- This test finds the bacteria in the blood (bacteremia).
- Bacteremia may be:
- Except in endocarditis or suppurative thrombophlebitis.
Bacteremia is seen after some of the procedures like:
|Dental extraction||18% to 85%|
|Periodontal surgery||32% to 88%|
|Tooth brushings||0% to 26%|
|Tonsillectomy||28% to 38%|
|Upper GI endoscopy||8% to 12%|
|Sigmoidoscopy||0% to 10%|
|Urethral dilatation||18% to 33%|
|Cystoscopy||0% to 17%|
|Prostatic transurethral resection||12% to 46%|
Source and causative agents of infections:
- >50% of the cases show growth of gram-negative bacilli, and mostly these are E. Coli.
- Staphylococcal aureus is the second most common organism.
- The common site of entry is mostly previous urinary tract infections.
- Other portals of entry follow surgery to instrumentation.
- Fungal endocarditis is most commonly due to Candida spp, followed by Torulopis glabrata and Aspergillus spp.
- The mortality rate in septicemia may be as high as 20% in immunocompromised and debilitated patients.
Clinical signs and symptoms of bacteremia:
- An episode of bacteremia is followed by:
- This is the best time to take the blood sample.
- S/S are accompanied by high fever and leukocytosis.
- Septic patients may be afebrile in 10% (4% to 18%) of the cases.
- These patients show leukocytosis in 60% to 65% (42% to 76%).
- Band form may be increased from 70% to 75% of the cases.
- Total WBC count increases in 75% (66% to 92%) of the cases.
- Some of the patients may show leukopenia in 10% (4% to 18%) of the cases.
Procedure for blood culture:
- Make the fresh blood film to rule out parasites like malaria, Borrelia, and Trypanosoma.
- The technique for the collection of blood for culture:
- Clean the skin from where the blood is taken.
- Can clean the skin with Betadine and then with 70% ethanol.
- Let the skin dry.
- Clean the top of the culture bottle with betadine.
- This may be followed by 70% alcohol and let it dry.
- Take 10 to 15 mL of the blood from different sites.
- Discard the needle and attach the new sterile needed to inject the blood into the culture bottle.
- Inoculate the anaerobic bottle first, then the aerobic bottle.
- Mix gently after the inoculation.
- Multiple blood samples are taken. Usually, two to three samples are adequate.
- Take the blood sample when the patient has a high fever or is shivering.
- If one sample is positive and the other is negative will indicate contamination.
- If both culture media show the same bacteria growth, it indicates bacteremia.
- If the patient takes antibiotics, it should be reported to the lab.
- In this case, take a blood sample in resin, which will inhibit the effect of the antibiotics on the growth of bacteria.
- Take the blood sample before the next dose of antibiotics is given.
- Culture from the I/V catheter is always contaminated and should be avoided.
- If catheter sepsis is suspected, only culture from the I/V catheter can be done.
- Some lab. Prefers multiple sites to collect the blood. They take blood at various intervals of 0, 30, and 90 minutes.
- Multiple sites rule out skin contamination.
- Three blood sample culture yield is 99.9%.
- A sample of more than 3 does not improve positivity.
- In the case of an adult, take 10 mL of blood per bottle.
- Some lab prefers 10 to 15 mL of blood from all the sites.
- Less than a 5 ml blood sample shows a less positive culture yield.
- Discard the syringe needle and use a new sterile needle to transfer the blood into the blood culture bottle.
- Culture the blood aerobically and anaerobically.
- First, add the blood to the anaerobic media.
- Gently mix the blood with blood culture media.
- Subculture these samples for at least 72 hours.
- After the growth, a sensitivity test determines the sensitive drugs for that growth.
- Antibiotic sensitivity tests show zones of inhibition of the growth around the disc impregnated with medicines.
- Resistance is shown by the growth of the organism right up to the disc.
Result of blood culture:
- Negative culture reports when all cultures, subcultures, and gram stain smears are negative.
- The blood culture report may be reported negative after 5 to 7 days of incubation.
- Bacteria need at least 24 hours to grow in the lab.
- By then, a preliminary report can be given.
- 48 to 72 hours are needed to identify the organism.
- Anaerobic organisms take a longer time.
- The virus can not be detected by blood culture.
- A definitive diagnosis is made in:
- Subacute bacterial endocarditis.
- Salmonellosis (Typhoid fever). The second and third week of typhoid fever is the best time for blood culture.
- After this period, positivity declines rapidly.
- Blood culture is positive in 30% to 50% of the cases.
- This organism needs CO2, and growth is very slow on ordinary media.
- There are special media that assist the growth.
- Gonococcal infection (Gonococcemia).
- Chronic meningococcemia.
- Form of rat-bite fever.
Keep in mind:
- When the first tube shows growth and the second tube is negative, then presume that the first tube may be contaminated.
- When both tubes show growth, that indicates bacteremia.
- If the patient is taking antibiotics, then take the blood before the next dose of antibiotics.
A positive culture report may show:
- Pseudomonas aeruginosa.
- H. influenzae.
- Listeria monocytogenes.
- Streptococcal pneumoniae.
- Candida albicans.
- Clostridium perfringens.
- Staphylococcus aureus.
- Staphylococcus epidermidis.
- Streptococcus species (Beta -hemolyticus).
- Positive blood culture for definitive diagnosis is seen in:
- Subacute bacterial endocarditis.
- Salmonellosis (typhoid fever).
- Blood cultures are the best means of diagnosis during the first and second weeks of the infection.
- After that, the positivity will decline rapidly.
- Chronic meningococcemia.
- Gonococcal infection (gonococcemia).
False negative blood culture:
- History of recent antibiotic therapy.
- Insufficient blood is taken for the blood culture.
- Most people take a 1:10 ratio of the blood to culture media to be adequate.
- Use of unsuitable culture media for anaerobes.
- Slow-growing bacteria may be missed in the first 72 hours.
- A blood sample was not taken at the appropriate time.
In one of the studies of bacteremia, different organisms were found:
|Type of the organism||Percent positivity|
|Strept. viridans, Bacteroides,||6%|
|Strept. epidermidis||3% to 34%|
The person with sepsis may have the following:
- Fever and chills.
- Less frequent urination.
- There may be rapid breathing and tachycardia.
- There may be a drop in blood pressure.
- There may be a failure of one or more organs.
Questions and answers:
Question 1: What is the best time to get blood culture in case of bacteremia?
The best time is when the fever is at peak level, one hour before that peak.
Question 2: How many samples are needed for blood culture?
Mostly blood for culture should be 10 to 20 mL. It is recommended to get 2 to 3 samples.