Blood Culture Test

When is a blood culture ordered?

How blood cultures work

Interpreting blood culture results

What is sepsis?

Sepsis is a life-threatening organ dysfunction caused by the body's dysregulated response to infection. Bacteria in the blood (bacteraemia) trigger a massive immune response that can damage the heart, kidneys, lungs and brain. Signs include high fever or low temperature, rapid heart rate, low blood pressure, confusion and reduced urine output. Blood cultures are collected before antibiotics are started: but antibiotics must not be delayed waiting for results.

Questions to ask your doctor

• What organism grew in my blood culture?
• Is the result a true infection or a contaminant?
• Which antibiotics is the organism sensitive to?
• Do I need further imaging to find the source of infection?

Frequently Asked Questions

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

1. Febrile Neutropenia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK541102

References

Sources cited on this page. PubMed links open the original abstract.

1. Seifert H. The clinical importance of microbiological findings in the diagnosis and management of bloodstream infections. Clin Infect Dis. 2009;48(Suppl 4):S238–S245. PMID 26940000 · doi:10.1086/598188

What blood cultures actually detect – and what they don't

Blood cultures detect bacteria and fungi that have entered the bloodstream – a condition called bacteraemia (bacteria) or fungaemia (fungi). They are not a test for "infection in general" – they specifically test for bloodstream invasion, which occurs in septicaemia and bacteraemia but not in localised infections (a urinary tract infection, for example, causes a positive urine culture but blood cultures are often negative unless bacteria have spread to the blood).

Blood cultures cannot detect viruses (viral sepsis is diagnosed by PCR and serology) or parasites (malaria, for example, is diagnosed by blood film). They also do not detect bacterial toxins – so infections where toxins cause the illness (tetanus, botulism, toxic shock syndrome) have negative blood cultures despite severe systemic disease.

Why two sets (and timing) matter

Standard practice is to collect two sets of blood cultures – each set consisting of an aerobic (oxygen-rich) and anaerobic (oxygen-poor) bottle – drawn from two different venepuncture sites, taken before antibiotic administration whenever possible. Two sets are collected because:

• It improves sensitivity: a single set detects bacteraemia in approximately 80% of true positive cases; two sets increase detection to approximately 90%; three sets improve further to 95–99%.
• It distinguishes true bacteraemia from skin contamination (false positive). If a common skin commensal (coagulase-negative staphylococci, Cutibacterium acnes) grows in only one of two sets, it is likely a contaminant. If it grows in both sets drawn separately, it is more likely clinically significant.

Timing is critical: ideally, blood cultures should be taken at the time of spiking fever (bacterial load in blood peaks during fever onset) and always before starting antibiotics – antibiotics can kill bacteria within hours, rendering cultures negative even in confirmed bacteraemia.

Common organisms and what they suggest

• Staphylococcus aureus: A serious finding in any blood culture. Associated with cardiac valve infection (endocarditis), septic arthritis, osteomyelitis, and device-related infection (prosthetic joints, intravascular catheters). Requires prolonged IV antibiotic therapy and often echocardiography to exclude endocarditis.
• E. coli and other Gram-negative rods: Often originate from urinary or GI tract. Common in community-acquired sepsis. Antimicrobial sensitivity testing is crucial given increasing rates of ESBL and carbapenemase-producing Enterobacteriaceae resistance.
• Streptococcus pneumoniae: Associated with pneumonia, meningitis, and spontaneous bacterial peritonitis in cirrhosis.
• Candida species: Fungaemia in critically ill, immunocompromised, or post-surgical patients. Carries high mortality (30–50%) and requires antifungal therapy – typically an echinocandin (anidulafungin, micafungin) as first line in ICU.