Blood Culture, its Procedure and Interpretations

Sample

1. >10 ml of blood of the patient is needed, the best time is during fever. This is collected in the culture bottle.
   1. Inoculation of Blood <5 ml, shows markedly reduced yield.
   2. One reference recommends 20 to 30 mL of the blood/per culture.
   3. Some suspected organisms may need more blood.
   4. Take the blood sample as soon as possible after the onset of fever and chills.
   5. 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.
      

      Blood culture sample best time

2. It is better to get three samples during fever. These samples may be taken from both arms.
   1. Take 2 to 3 cultures per septic episode or per 24 hours period.
   2. In continuous bacteremia like endocarditis, the timing of the sample is not important.
   3. In the case of intermittent bacteremia, take a sample at the start of fever, continue up to three samples in 24 hours.
   4. Paired culture,  aerobic and anaerobic samples are taken, ideally from the opposite arms.
3. Clean the site to avoid contamination by 70%  Alcohol.
   1. Then clean with the 2% iodine.
      1. If the patient is sensitive to iodine, then cleaning with alcohol is enough.
4. For fever of unknown origin draw two blood samples.
5. A quantity of less than one ml is insufficient to detect bacterial infection.
6. This culture needs at least 72 hours for the report.
   1. Blood with or without microorganisms is collected into bottles for growth in an aerobic or anaerobic environment.
7. Advantages of more blood samples are:
   1. More cultures samples increase the possibility of positive blood culture.
   2. Positive cultures in 2 or 3 separate samples indicate continuous bacteremia that is typically found in bacterial endocarditis.
   3. 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

1. A strict aseptic technique must be observed.
2. 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.
3. Use of inappropriate culture media or the container may interfere with aerobic growth.
4. Inadequate time for the culture, temperature, or culture conditions may fail to give an adequate amount of bacteria for its identification.
5. If there is patient skin contamination for the culture.
6. Don’t draw blood from the I/V catheter unless no veins are found.
   1. In infants take the blood from umbilical artery catheter.

Indications

1. Blood culture is advised when there is a suspicion of bacteremia or septicemia.
2. Find a bacterial infection that has spread into the blood, such as meningitis, osteomyelitis, pneumonia, a kidney infection, or sepsis.
   

   Blood culture indications

3. To find the type of bacteria on culture.
4. Find a fungal infection, such as yeast, in the blood.
5. Check for endocarditis, which is an infection of bacteria living on the valves of the heart.
6. Find the best antibiotics to kill the bacteria. This is called sensitivity testing.
7. Find the cause of an unexplained fever or shock.
8. In an unexplained fever of several days duration.
   1. In patients with chills.
   2. Infected burn.
   3. urinary tract infection.
   4. Postoperative wound sepsis.
   5. Indwelling venous or arterial catheter.
9. 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.
   1. Sensitivity testing is important to treat the blood infection properly. This also helps prevent bacteria from becoming resistant to antibiotics.

Blood culture facts:

1. Blood culture report that often identifies microorganisms needs at least 24 hours.
   1. 67% of the pathogens are identified in 24 hours.
   2. 90% of the pathogens were identified in 72 hours.
   3. Fungi and mycobacterium may need more time, maybe in weeks.
2. Vascular catheter-related infections (bacteremia) cause at least 20% to 40% of all hospitalized patients.
3. After the growth, the organism is tested with a variety of drugs impregnated discs.
   1. Sensitivity will take another 24 hours or more.
4. This test is done to find the bacteria in the blood (bacteremia).
5. Bacteremia may be:
   1. Intermittent.
   2. Transient.
      1. Except in endocarditis or suppurative thrombophlebitis.
   3. 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%

6. Source and causative agents of infections:
   1. >50% of the cases show growth of gram-negative bacilli and mostly these are E.Coli.
   2. Staphylococcal aureus is the second common organism.
   3. The common site of the entry is mostly previous urinary tract infection.
   4. Other portals of entry follow surgery to instrumentation.
   5. Fungal endocarditis is most commonly due to Candida spp, followed by Torulopis glabrata, and Aspergillus spp.
7. The mortality rate in septicemia may be as high as 20% in immunocompromised and debilitated patients.
8. Clinical signs and symptoms:
   1. An episode of bacteremia is followed by:
   2. Chills.
   3. Fever.
      1. This is the best time to take the blood sample.
   4. S/S are accompanied by high fever and leukocytosis.
   5. Septic patients may be afebrile in 10% (4% to 18%) of the cases.
   6. These patients show leukocytosis in 60% to 65% (42% to 76%).
      1. Band form may be increased 70% to 75% of the cases.
      2. Total WBC count increases in 75% (66% to 92%) of the cases.
   7. Some of the patients may show leukopenia in 10% (4% to 18%) of the cases.

Procedure

1. Make the fresh blood film to rule out parasites like malaria, Borrelia, and Trypanosoma.
2. The technique for the collection of blood for culture:
   1. Clean the skin from where the blood is taken.
      1. Can clean the skin with Betadine and then with 70% ethanol.
      2. Let the skin dry.
      3. Clean the top of the culture bottle with betadine.
         1. This may be followed by 70% alcohol and let it dry.
      4. Take 10 to 15 mL of the blood from different sites.
      5. Discard the needle and attach the new sterile needed to inject the blood into the culture bottle.
      6. Inoculate the anaerobic bottle first then the aerobic bottle.
      7. Mix gently after the inoculation.
         

         Blood culture procedure

3. Multiple blood samples are taken. Usually, two to three samples are adequate.
   1. Take the blood sample when the patient has a high fever or is shivering.
   2. If one sample is positive and the other is negative will indicate contamination.
   3. If both culture media shows the same growth of bacteria, indicates bacteremia.
   4. If the patient is taking antibiotics, it should be reported to the lab.
      1. In this case, take a blood sample in resin, which will inhibit the effect of the antibiotics on the growth of bacteria.
      2. Take the blood sample before the next dose of antibiotics is given.
   5. Culture from the I/V catheter is always contaminated and should be avoided.
   6. The only culture from the I/V catheter can be done if the catheter sepsis is suspected.
4. Some lab prefers multiple sites to collect the blood. They take blood at various intervals of 0, 30, and 90 minutes.
   1. Multiple sites rule out skin contamination.
   2. Three blood sample culture yield is 99.9%.
   3. Sample more than 3 does not improve positivity.
5. In the case of an adult take 10 mL of blood per bottle.
   1. Some lab prefers 10 to 15 mL of the blood from all the sites.
   2. Less than a 5 ml sample of blood shows a less positive yield of culture.
6. Discard the syringe needle and use a new sterile needle to transfer the blood into the blood culture bottle.
7. Culture the blood aerobically and anaerobically.
   1. First, add the blood to anaerobic media.
8. Gently mix the blood with blood culture media.
9. Subculture these samples for at least 72 hours.
10. After the growth, a sensitivity test is done to find out the sensitive drugs for that growth.
    1. Antibiotic sensitivity tests show zones of inhibition of the growth around the disc impregnated with medicines.
    2. Resistance is shown by the growth of the organism right up to the disc.
       

       Blood culture sensitivity

Result of blood culture

1. Negative culture report when all cultures and subcultures and gram stains smears are negative.
2. The blood culture report may be reported negative after 5 to 7 days of incubation.
   1. Bacteria need at least 24 hours to grow in the lab.
      1. By then a preliminary report can be given.
      2. 48 to 72 hours are needed to identify the organism.
      3. Anaerobic organisms take a longer time.
3. The virus can not be detected by blood culture.
4. Definitive diagnosis is made in:
   1. Subacute bacterial endocarditis.
   2. Salmonellosis (Typhoid fever).
   3. Brucellosis.
   4. Gonococcal infection (Gonococcemia).
   5. Chronic meningococcemia.
   6. Tularemia.
   7. Form of rat-bite fever.

Keep in mind:

1. When the first tube shows growth and the second tube is negative, then presume that the first tube may be contaminated.
2. When both tubes show growth that indicates bacteremia.
3. If the patient is taking antibiotics then take the blood before the next dose of antibiotics.

A positive culture report may show:

1. Bacteroides.
2. Brucella
3. Enterobacteriaceae.
4. Pseudomonas aeruginosa.
5. H. influenzae.
6. Listeria monocytogenes.
7. Enterococcus.
8. Streptococcal pneumoniae.
9. Candida albicans.
10. Clostridium perfringens.
11. Staphylococcus aureus.
12. Staphylococcus epidermidis.
13. Streptococcus species (Beta -hemolyticus).
    1. Positive blood culture for definitive diagnosis is seen in:
       1. Subacute bacterial endocarditis.
       2. Salmonellosis (typhoid fever).
          1. During the first and second weeks of the infection, blood cultures are the best means of diagnosis.
          2. After that, the positivity will decline rapidly.
       3. Brucellosis.
       4. chronic meningococcemia.
       5. Gonococcal infection (gonococcemia).
       6. Tularemia.

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:

1. Fever and chills.
2. Less frequent urination.
3. Nausea.
4. There may be rapid breathing and tachycardia.
5. Confusion.
6. There may be a drop in blood pressure.
7. There may be a failure of one or more organs.

 

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