Bacteremia vs contaminated blood cultures

Background

• Staphylococcus aureus and Candida sp. isolated from the blood should never be considered a contaminant
• Chances of bacteremia with the number of positive cultures a
• Final identification of the bacteria is useful

Suggestive of Contamination

1. Typical commensal organisms of the skin flora in the abscence of an intravenous catheter
   1. coagulase-negative Staphylococcal species
   2. certain Streptococci
   3. Gram-positive bacilli

1. Only 1 out of 2 or more blood cultures are positive
   1. Most commonly used when positive for coagulase-negative Staphylococcal species

1. Antibiotic susceptibility pattern of one organism is different from the pattern of the other organisms in the same or subsequent set of cultures (as long as the organisms are of the same species)
   1. For example, if 2 sets of blood cultures are both positive for Staphylococcus epidermidis but one set is sensitive to a particular antibiotic while the other set is resistant to the same antibiotics, both sets are likely contaminated

Risk Factors for Bacteremia

• Advanced age
• Corticosteroids
• Immunosuppressing medications (transplant patients, rheumatologic diseases, etc)
• Chronic liver disease
• Chronic renal failure (especially if on hemodialysis)
• Hematological malignancies
• HIV infection
• Intravenous catheters
• Intravenous drug use
• Loss of skin integrity
• Malnutrition and hypoalbuminemia
• Neutropenia
• Parenteral nutrition

Contamination Rates for Specific Organisms

Organism	False positives
Bacillus spp.	>90%
Coag-negative Staphylococcus spp.	>90%
Propionibacterium spp.	>90%
Corynebacterium spp.	>80%
Viridans-Group Streptococci	50%
Clostridium spp.	40%
Staphylococcus aureus spp.	25%
Enterococcus spp.	15%

See Also

• Microbiology (Main)