Blood Culture: Key points for the laboratory diagnosis of bacteremia/fungemia
- Draw blood for culture before initiating antimicrobial therapy
Volume of blood collection: Because the number of bacteria per millilitre of blood is usually low, it is important to take a reasonable quantity of blood: 20 ml per venepuncture for adults, 2–5 ml for children infants and 1–2 ml for infants and neonates.
- Use a 2–3 bottle blood culture set for adults, at least one aerobic and one anaerobic; use 1–2 aerobic bottles for children.
Timing of blood collection: Whenever possible, blood should be taken before antibiotics are administered. The best time is when the patient is expected to have chills or a temperature spike. It is recommended that two or preferably three blood cultures be obtained, separated by intervals of approximately 1 hour.
Note: Overall volume of blood cultured is more critical to increase organism yield than the timing.
Disinfect the venipuncture site with 2% tincture of iodine, 10% polyvidone iodine, 70% alcohol, or 0.5% chlorhexidine in 70% alcohol chlorhexidine. (chlorhexidine is NOT recommended for children <2 months old).
- Catheter-drawn blood cultures have a higher risk of contamination (false positives).
Use of Anticoagulant: Anticoagulant such as sodium polyanethol sulfonate (SPS) recommended if the blood is not added to sufficient volume of broth (immediately). SPS also inhibits the antibacterial effect of serum and phagocytes. If the blood is immediately added to a sufficient volume (50 ml) of broth and thoroughly mixed to prevent clotting, no anticoagulant is needed.
Do not submit catheter tips for culture without an accompanying blood culture obtained by venipuncture.
- Never refrigerate blood prior to incubation.
A true infection of bloodstream is suspected in the following conditions:
- if the same organism grows in two bottles of the same blood specimen;
- if the same organism grows in cultures from more than one specimen;
- if growth is rapid (within 48 hours);
- if different isolates of one species show the same biotypes and antimicrobial- susceptibility profiles.
- Most common causes of bacteremia and fugemia are found in this link