Blood Culture

See Also

Sepsis

Indications
Evaluation of Bacterial Infection

Bacteremia
Sepsis

Precautions

Blood Cultures have high False Positive Rates (contamination) resulting in hospital admissions, Antibiotic Overuse
1. Only obtain Blood Cultures when absolutely needed and will affect management
2. Viral symptoms in immunompetent febrile patients with positive Blood Cultures most often have contaminants
3. History, symptoms and comorbidities are most helpful in the interpretation of positive cultures
  1. Not all patients need to return emergently for reevaluation
  2. Give return precautions for all patients
4. Reexamine patients with Blood Cultures positive for significant pathogens (see below)
  1. Staphylococcus Aureus
  2. Enterobacteriaceae (Gram Negative Rods)
Obtain Blood Cultures before systemic Antibiotics have been given
Obtain samples at 2 different venipuncture sites and 8-10 ml per bottle
1. Ideally space venipunctures by 15 min apart
At least 2 culture bottle sets (2 bottles each, 4 bottle total) are obtained in adults (1 set in children)
Central Lines and PICC Lines are prone to colonization and not recommended as culture sources EXCEPT:
1. Central Lines placed within prior 48 hours
2. Suspected line infection or CLABSI (obtain 1 culture set from line, and 2 additional peripheral cultures)
Repeat culture indications
1. Wait at least 48 hours after a positive culture result (not thought to be contaminant)
2. Fever of Unknown Origin cultures are repeated at 36-48 hours after the first set
  1. Perform culture sets immediately before expected fever spike (in late afternoon)
Typical contaminants may be true pathogens in those with intravascular or intracardiac devices
1. Multiple Blood Culture sets may be needed to distinguish device infection vs contamination

Technique

Preparation of skin
1. Alcohol swabs
2. Chlorhexidine (preferred agent for antiseptic)
  1. Superior to Povidone-Iodine
  2. Superior to Alcohol
3. Povidone-Iodine
  1. Wait to draw blood for 2-3 minutes after applying
  2. High contamination rate compared with Chlorhexidine
4. References
  1. Mimoz (1999) Ann Intern Med 131:834-7 [PubMed]
Culture Bottle Sets
1. Each culture bottle requires 8-10 ml blood
  1. Test Sensitivity increases 3% for each ml obtained
2. Culture bottle sets are each composed of 2 bottles
  1. Aerobic culture bottle (fill first)
  2. Anaerobic culture bottle (fill second)
Two Venipuncture - 2 Culture Sets (Standard Protocol)
1. Two culture sets is used in most adult cases (single culture set is typically used in children)
Three Venipuncture - 3 Culture Sets
1. Indicated in Bacterial Endocarditis evaluation
2. Collect up to 3-4 sets of Blood Culture bottles
  1. Two Blood Culture bottles in each set
  2. Obtain at least 5 ml blood per bottle (8-10 ml is preferred)
3. Draw from up to 3 different venipuncture sites
4. Ideally space blood collections by 1 hour intervals

Efficacy

Test Sensitivity
1. Increases with number of Blood Cultures drawn
2. With 3 Blood Cultures, Sensitivity approaches 90%
Study of hospitalized patients with persistent fever
1. Post-Antibiotic culture unlikely to yield pathogen
2. Obtain pre-antibiotic Blood Culture and await results
3. Grace (2001) Clin Infect Dis 32:1651-5 [PubMed]

Causes
Positive Cultures

Common Pathogens
Common Contaminants (but pathogens with intravascular devices, mechanical valves)
1. Bacillus
2. Carnobacterium
3. Diphtheroids
4. Coagulase Negative Staphylococcus
5. Staphylococcus Epidermidis
6. Staphylococcus hominis

Resources

Blood Cultures (Henry Ford Lab Users Guide)
1. https://lug.hfhs.org/bloodCult.htm

References

Claudius, Miller, Adler-Shohet (2025) EM:Rap, 2/10/2025