• What are the basic principles of blood culture?
• Why do we perfom blood culture ?
  
• When do we perform blood culture ?
  
• How do we perform blood culture ?
  
• Which culture media ?
  
• How many blood samples ?
  
• What volume of blood ?
  
• Contaminants

A blood culture is systematically requested whenever septicemia is suspected: undetermined fever, shock, several local infections (meningitis, endocarditis, pneumonia, pyelonephritis, intra-abdonominal suppuration,...).The mortality rate related to septic shock can be as high as 49 %.

Samples should be collected :

1. As soon as possible after the onset of clinical symptoms.

2. Preferably when a rise in temperature is observed with shivering.

3. Before initiating antibiotic therapy.

4. Immediately before administering the next dose of antibiotics.

Sample collection is a crucial step in the blood culture process, and must be performed carefully to ensure reliable results.

• Examine the bottles and eliminate any bottles which are damaged, or show signs of contamination (turbid broth, etc.).

• It is recommended not to draw blood from a venous or arterial catheter.
  Immediately transfer the inoculated bottles and the completed blood culture request sheet to the laboratory. Otherwise, store the bottles temporarily in an incubator at 35 - 37°C.

Microorganisms causing septicemia are highly varied ( aerobes,anaerobes,yeasts,fastidious microorganisms,...) and , in addition to nuterient elements may require specific growth factors, and/or a special atmosphere. The culture medium chosen must therefore satisfy as many of these criteria as possible.
So , A blood culture must be:

1. Sensitive in order to detect

   • all microorganisms, even the most fastidious(Neisseria, Haemophilus,...),

   • microorganisms releasing small amounts of CO2(Brucella,Acinetobacter,...).

2. Easy - to - Use
   

In most instances, bacteremia and fungemia are intermittent. Since microorganisms are not constantly present in the bloodstream, the sensitivity of a single blood culture is limited between 80 & 90 %. However, with 3 blood culture sets, sensitivity can be as high as 99 %.

Adults :

Sample collection of 20ml of blood (10 ml x 2 bottles) is recommednded to take into account the numerous cases of septicemia caused by bacteremia with less than 1 CFU per ml of blood.

Pediatric :
Every extra milliter of blood collected can improve sensitivity by 3%. The difference in microorganism yield between cultures of 10 and 20 ml of blood can be as high as 40%.
Collecting a sufficient quantity of blood ensures that bacteria present in low quantities will be detected. This is essential when septicemia is suspected.

The quantity of circulating microorganism is significantly higher in newborns and infants than in adults, which means a blood culture can be performed on a volume of only 0.5 - 5 ml of blood.

Certain microorganisms such as Corynebacteria, Propionibacterium spp, coagulase-negative Staphylococcus, Bacillus spp rarely cause septicemia. They are common skin contaminants, and their detection in a single blood culture bottle can suggest contamination without clinical significance. However, it is impotant to consider that 20% of coagulase-negtive Staphylococci can be clinically significant.