The Rational Use of Blood Cultures in Adult Patients
Lab Update No. 54 – September 2024
Why It Matters
Blood cultures are the gold standard for diagnosing bloodstream infections – a major global cause of death.
However:
- Over 90% of blood cultures show no growth
- 30–50% of positives are contaminants
This leads to: - Unnecessary antibiotics
- Longer hospital stays
- Increased costs
- Unneeded tests and procedures
When Should Blood Cultures Be Taken?
Blood cultures are indicated when:
- High likelihood of bacteraemia
- Delay in culturing primary infection site
- Risk of endovascular infection (e.g. prosthetic valves, pacemakers, vascular grafts)
- Results will influence treatment decisions (e.g. resistant organisms, severe cellulitis in immunocompromised patients)
Use clinical judgment, especially for:
- Immunocompromised patients
- Those with diabetes, liver or kidney disease
Clinical Scenarios for Blood Cultures
High Diagnostic Value
Send 2 sets of peripheral blood cultures:
- Severe sepsis or septic shock
- Infective endocarditis
- Central line-associated bloodstream infection (CLABSI)
- Discitis or vertebral osteomyelitis
- Native septic arthritis
- Epidural abscess
- Meningitis
- Ventriculoatrial shunt infections
Intermediate Diagnostic Value
Send 2 sets if:
- Endovascular risk
- Primary site not available
- Results will guide treatment
Examples: - Acute pyelonephritis
- Cholangitis
- Severe CAP
- Cellulitis with comorbidities
- VAP
- Prosthetic vertebral osteomyelitis
Low Diagnostic Value
Blood cultures not recommended for:
- Isolated fever or leukocytosis
- Non-severe cellulitis
- Lower UTI (e.g. cystitis, prostatitis)
- Non-severe CAP or HAP
- Post-op fever within 48 hours
How Many Sets Should Be Sent?
Key Factors:
- Proper skin antisepsis
- Volume of blood collected
Single sets can miss 10–40% of pathogens and can't distinguish contamination from true infection.
Recommended:
- At least 2 sets (each with 10 mL in aerobic + anaerobic bottles)
- Total: 40 mL of blood
- More sets = higher yield:
- 1 set → 61.4%
- 2 sets → 78.2%
- 3 sets → 93.1%
Timing of Sampling
Important:
- Collect before antibiotics
- No need to space over 24 hours
- No need to time with fever spikes
When to Repeat Blood Cultures?
Repeat if:
- Pathogen is Staphylococcus aureus, S. lugdunensis, or Candida
- Suspected endocarditis or endovascular infection
- Persistent bacteraemia
- Need to confirm contamination vs. true infection
Key Points Summary
-Volume of blood is most critical
-Collect 2–3 sets (40–60 mL total)
-Sample before antibiotics
-Don’t space samples over 24 hours
-Don’t wait for fever spikes
References
- Fabre et al., J Clin Microbiol, 2022
- Papavarnavas et al., SAMJ, 2022
- Fabre et al., Clin Infect Dis, 2020
- Snyder, Clin Microbiol Newsletter, 2015