Blood culture contamination

Frequency of True Bacteremia	Organism	Gram Stain
Almost never true bacteremia (almost always contaminant)	Corynebacterium, except Corynebacterium jeikeium	Gram-positive bacillus
	Bacillus, except Bacillus anthracis	Gram-positive bacillus
	Cutibacterium acnes	Gram-positive bacillus
Sometimes true bacteremia	Micrococcus	Gram-positive cocci in groups
	Clostridium perfringens	Anaerobic Gram-positive bacillus
	Coagulase-negative staphylococci, except Staphylococcus lugdenensis	Gram-positive cocci in groups
	Viridans group streptococci	Gram-positive cocci in pairs and chains
	Lactobacillus	Gram-positive bacillus
	Nutritionally variant streptococci (Abiotrophia and Granulicatella)	Gram-positive cocci in pairs and chains
Usually true bacteremia	Enterococcus	Gram-positive cocci in pairs and chains
	Aerococcus	Gram-positive cocci in pairs and chains
Always true bacteremia (or at least, always treated as such)	Staphylococcus aureus	Gram-positive cocci in groups
	Staphylococcus lugdenensis	Gram-positive cocci in groups
	Streptococcus pneumoniae	Gram-positive cocci in pairs and chains
	Escherichia coli and other Enterobacterales	Gram-negative bacilli
	Pseudomonas aeruginosa	Gram-negative bacilli
	Candida	Yeast

• Coagulase-negative staphylococci are rarely significant (12%), but because they are so common (70-80% of all positive cultures), they are the most common cause of bacteremia

Further Reading

• Laboratory approaches to determining blood culture contamination rates: an ASM Laboratory Practices Subcommittee report. J Clin Microbiol. 2023;62:e01028-23. doi: 10.1128/jcm.01028-23