⚕️ Educational content only. This article explains blood culture procedures from a microbiology laboratory science perspective. It is not medical advice. Consult a healthcare professional for personal health concerns.
Blood cultures are one of the most clinically critical tests in microbiology. They are used to detect and identify microorganisms circulating in the bloodstream — a condition known as bacteraemia (bacteria) or fungaemia (fungi). Early, accurate blood culture results directly guide antibiotic therapy and patient outcomes in sepsis.
Key Takeaways
- Blood cultures are collected as paired sets (aerobic and anaerobic bottles) before antibiotic administration.
- Continuous-monitoring automated systems (e.g. BD BACTEC, bioMérieux BacT/ALERT) detect growth by CO2 production.
- Time to positivity and Gram stain are the first results; identification and sensitivity follow via MALDI-TOF and susceptibility testing.
- Contamination (usually skin flora like CoNS) must be distinguished from true bacteraemia.
When Are Blood Cultures Taken?
Blood cultures are indicated when bacteraemia or fungaemia is suspected: fever with no obvious source, clinical signs of sepsis (hypotension, tachycardia, altered consciousness), suspected infective endocarditis, line-associated infection, pneumonia requiring hospital admission, or urinary tract infection in high-risk patients. Ideally, two sets are drawn from different venepuncture sites before starting antibiotics to maximise sensitivity and allow contamination assessment.
Collection Technique
Strict aseptic technique is essential. Skin is cleaned with 2% chlorhexidine in 70% alcohol (or povidone-iodine if chlorhexidine-sensitive). Each set consists of one aerobic bottle and one anaerobic bottle. Adult bottles typically require 8–10 mL of blood per bottle; paediatric bottles use smaller volumes. Bottles must be inoculated in the correct order (aerobic first if using a butterfly needle) to avoid air entering the anaerobic bottle. Blood should be drawn before antibiotics whenever possible, as antibiotic exposure significantly reduces culture sensitivity.
Laboratory Processing: Automated Detection Systems
Modern blood culture systems use continuous fluorescent or colorimetric CO2 detection to monitor for microbial growth. Bottles are incubated at 35–37°C for up to 5 days (or longer if fastidious organisms are suspected). When a bottle signals positive, laboratory staff are immediately alerted. The bottle contents undergo Gram stain to give rapid morphological information (Gram-positive cocci in clusters — possible Staphylococcus; Gram-negative rods — possible Enterobacterales), and a subculture is plated.
Identification and Susceptibility Testing
Isolated organisms are identified using MALDI-TOF mass spectrometry, which provides accurate identification in minutes from a small colony. Antimicrobial susceptibility testing (AST) is performed by disk diffusion (Kirby-Bauer), automated broth microdilution (e.g. VITEK 2), or gradient strip methods (e.g. Etest). Results are reported as MIC (minimum inhibitory concentration) values and clinical breakpoints (susceptible, intermediate, resistant) according to EUCAST or CLSI guidelines. Direct susceptibility testing from positive bottles can accelerate results by 12–24 hours.
Interpreting Results: True Positive vs. Contaminant
Not all positive blood cultures represent true bacteraemia. Coagulase-negative Staphylococci (CoNS, e.g. S. epidermidis) are the commonest blood culture contaminants, originating from skin flora. A single positive bottle with CoNS in a clinically well patient is likely contamination. Two or more bottles positive with the same organism, or growth of S. aureus, Streptococcus pneumoniae, Gram-negative organisms, or Candida spp. in any bottle, is considered clinically significant. Clinical context is always required for interpretation.
References
- Public Health England. UK Standards for Microbiology Investigations: Blood Cultures. gov.uk/phe
- EUCAST. Antimicrobial susceptibility testing breakpoints. eucast.org
- Weinstein MP, et al. The clinical significance of positive blood cultures in the 1990s. Clin Infect Dis. 1997;24(4):584-602.