Blood Culture Bottles

OVERVIEW

• tubes for sampling blood for microbiological cultures

USES

• detection of circulating microorganisms in bacteraemia and septicaemia

DESCRIPTION Types of blood culture bottles and blood volumes required:

• Yellow top – paediatric aerobic (0.5–4 mL blood)
• Green top – adult aerobic (5–10 mL blood)
• Orange top – anaerobic (5–10 mL blood)
• Black top – mycobacteria (5–10 mL blood)
• Silver top – mycoplasma (3–5 mL blood)

Contents

• special nutrient broths
• contain agents which act as anticoagulants and neutralise the natural bactericidal action of blood
• cooked meat or a reducing agent is added to anaerobic bottles to aid growth of anaerobic bacteria
• ratio of blood to broth is usually 10 mL to 200 mL

METHOD OF USE Blood culture collection

• sterile blood sampling (i.e. hand hygiene, sterile dressing pack, sterile gloves and clean skin with chlorhexidine)
• take from fresh venepuncture or alternatively newly inserted peripheral cannulae and central lines
• if line sepsis is suspected cultures should be taken from the central line and peripherally
• blood placed into aerobic and anaerobic culture medium
• remove dust cap from culture bottles and top swabbed with alcohol (and allowed to dry) before blood is injected
• maximum yield if the tube is neither under nor over-filled (e.g. 9 mL in adult tubes)
• fill blood culture tubes before other blood tubes

OTHER INFORMATION Number of cultures

• two sets of cultures from separate sites before starting antibiotics is ideal
• 3 cultures having a 96% sensitivity in detecting bacteraemia
• more than 4 cultures generally offers little additional benefit
• additional cultures may be beneficial if suspected endocarditis or pre-existing antimicrobial use
• antibiotics should not be withheld pending culture in critically ill septic patients

Incubation and culture

• incubation at room temperature initially to avoid killing temperature sensitive bacteria
• turbid appearance may indicate microbial growth — bottles are placed into holders with light passed through — an alarm sounds when significant microbial growth interrupts the passage of light through bottles
• all samples are subcultured onto agar plates to test purity, allow organism identification and for antibiotic sensitivity testing

Contamination

• suspect if single, random cultures of Bacillus species, coagulase-negative staphylococci or diphtheroids
• less likely if grown in more than one bottle (another reason for taking multiple cultures from separate sites)

COMPLICATIONS

• associated with venesection
• needle stick injury
• contamination may lead to unnecessary antibiotic use (e.g. skin flora)
• failure of microbial growth due to inadequate quantity of blood in the bottle or wrong culture bottles used