Blood Culture, its Procedure and Interpretations

Sample
- >10 ml of blood of 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 the blood/per culture.
- Some suspected organisms may need more blood.
- Take the blood sample as soon as possible 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 fever. These samples may be taken from both arms.
- Take 2 to 3 cultures per septic episode or per 24 hours period.
- In continuous bacteremia like endocarditis, the timing of the sample is not important.
- In the case of intermittent bacteremia, take a sample at the start of fever, 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.
- 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 fever of unknown origin draw two blood samples.
- A quantity of less than one ml is insufficient to detect bacterial infection.
- This 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.
- 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
- A strict aseptic technique must be observed.
- Antimicrobial treatment prior to the collection of the blood sample may result in a negative culture report. This may be negative even in the bacteremia.
- Use of 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 umbilical artery catheter.
Indications
- 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, which is an infection of bacteria living on the valves of the heart.
- 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 the blood infection 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 a variety of drugs impregnated discs.
- Sensitivity will take another 24 hours or more.
- This test is done to find the bacteria in the blood (bacteremia).
- Bacteremia may be:
- Intermittent.
- Transient.
- Except in endocarditis or suppurative thrombophlebitis.
- Bacteremia is seen after some of the procedures like:
Procedure Incidence (occurrence) Dental extraction 18% to 85% Periodontal surgery 32% to 88% Tooth brushings 0% to 26% Bronchoscopy 15% 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 common organism.
- The common site of the entry is mostly previous urinary tract infection.
- 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:
- An episode of bacteremia is followed by:
- Chills.
- Fever.
- 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 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
- 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.
- Clean the skin from where the blood is taken.
- 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 shows the same growth of bacteria, indicates bacteremia.
- If the patient is taking 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.
- The only culture from the I/V catheter can be done if the catheter sepsis is suspected.
- 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%.
- Sample 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 the blood from all the sites.
- Less than a 5 ml sample of blood shows a less positive yield of culture.
- 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 anaerobic media.
- Gently mix the blood with blood culture media.
- Subculture these samples for at least 72 hours.
- After the growth, a sensitivity test is done to find out 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 report when all cultures and subcultures and gram stains 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.
- Bacteria need at least 24 hours to grow in the lab.
- The virus can not be detected by blood culture.
- Definitive diagnosis is made in:
- Subacute bacterial endocarditis.
- Salmonellosis (Typhoid fever).
- Brucellosis.
- Gonococcal infection (Gonococcemia).
- Chronic meningococcemia.
- Tularemia.
- 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:
- Bacteroides.
- Brucella
- Enterobacteriaceae.
- Pseudomonas aeruginosa.
- H. influenzae.
- Listeria monocytogenes.
- Enterococcus.
- 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).
- During the first and second weeks of the infection, blood cultures are the best means of diagnosis.
- After that, the positivity will decline rapidly.
- Brucellosis.
- chronic meningococcemia.
- Gonococcal infection (gonococcemia).
- Tularemia.
- Positive blood culture for definitive diagnosis is seen in:
In one of the studies of bacteremia different organisms were found:
Type of the organism | Percent positivity |
E.coli | 20% |
S.aureus | 10% |
Klebsiella | 6% |
Pneumococcus | 6% |
Strept. viridans, Bacteroides, | 6% |
Pseudomonas | 6% |
Strept. epidermidis | 3% to 34% |
The person with sepsis may have:
- Fever and chills.
- Less frequent urination.
- Nausea.
- There may be rapid breathing and tachycardia.
- Confusion.
- There may be a drop in blood pressure.
- There may be a failure of one or more organs.