Glossary · Laboratory techniques
Blood culture
In one sentence
A blood culture incubates a blood sample in bottles of nutrient broth to find out whether live bacteria or fungi are circulating in the bloodstream. If anything grows, it is identified and tested against antibiotics to guide treatment.
Also known as: blood cultures · blood culture bottles · blood culture set · hemocultivo
What is a blood culture?
It is the reference test for suspected bloodstream infection. Blood is inoculated into bottles, usually one aerobic and one anaerobic per set, containing a medium in which bacteria and yeasts can multiply. A positive result confirms bacteraemia or fungaemia and, crucially, isolates the living organism so that antimicrobial susceptibility testing can show which antibiotics will work.
How it works
- When: whenever serious infection is suspected, such as sepsis, fever with rigors, endocarditis or a line infection, and ideally before antibiotics are started.
- How much: the CDC advises 20–30 mL of blood per set in adults and at least two sets (40–60 mL in total) per episode. Underfilled bottles lower sensitivity.
- How: by venepuncture with strict aseptic technique, to avoid introducing skin flora.
- In the laboratory: an automated system detects growth. A positive bottle undergoes a Gram stain, which already guides treatment, followed by identification and susceptibility testing.
Contamination
CLSI and the American Society for Microbiology recommend that the overall contamination rate should not exceed 3%, and the CDC notes that 1% is achievable with best practice. Typical contaminants are coagulase-negative staphylococci, Cutibacterium and corynebacteria, although in patients with devices these can be genuine pathogens.
Limitations
- Sensitivity falls when too little blood is drawn or when antibiotics have already been given.
- Some organisms grow poorly, or not at all, in standard media.
- A negative blood culture does not rule out infection.
For this reason, selected cases may also be investigated with molecular tests or clinical metagenomics, which can detect DNA from organisms that are hard to culture. These are complementary: they do not isolate a living organism and do not replace susceptibility testing.
Example
An older adult arrives in A&E with high fever and rigors following a urinary tract infection. Two blood culture sets are drawn from separate venepunctures before the first antibiotic dose. Fourteen hours later one bottle flags positive; the Gram stain shows Gram-negative rods and the team reviews the empirical antibiotic. Two days on, susceptibility testing confirms Escherichia coli and its resistance profile, and treatment is targeted accordingly.
Blood culture at Xenogene
Xenogene does not perform blood cultures. Its AMC in Blood is a metagenomic nanopore analysis of EDTA venous blood that looks for genetic material from bacteria, viruses, fungi, parasites and mycobacteria, plus resistance and virulence genes. It may add information when blood cultures are negative or the picture remains unclear, but it does not replace blood cultures or susceptibility testing. The product sheet advises collecting the sample before antibiotics where possible and interpreting results in their clinical context.
Frequently asked questions
How long do blood culture results take?
Bottles are incubated in automated systems that flag growth as soon as it is detected, and most positive cultures flag within 24 to 48 hours. Identification and susceptibility testing then take longer. A culture is usually reported as negative after about five days, depending on the laboratory's protocol.
Why are two or more sets of blood cultures taken?
The volume of blood cultured is the biggest factor in sensitivity, and multiple sets help tell true bacteraemia from contamination. If a skin organism grows in only one of several sets, it is most likely a contaminant.
What does a contaminated blood culture mean?
It means an organism that probably came from the skin or equipment during collection has grown, such as some coagulase-negative staphylococci. It does not indicate infection, but it can lead to unnecessary tests or antibiotics, which is why laboratories monitor their contamination rate.
Related terms
- Bacteraemia
Bacteraemia is the presence of viable bacteria in the bloodstream, usually confirmed by blood cultures. It can be transient and harmless or part of a serious infection, and it is not the same as sepsis, which is defined by the body's response to infection.
- Sepsis
Sepsis is life-threatening organ dysfunction caused by a dysregulated host response to infection (Sepsis-3 definition, 2016). It is a medical emergency that needs immediate treatment.
- Clinical metagenomics
Clinical metagenomics (mNGS) is the untargeted sequencing of the genetic material in a patient sample to detect bacteria, viruses, fungi or parasites without choosing the pathogen in advance. It complements culture and susceptibility testing rather than replacing them.
- Antimicrobial resistance
Antimicrobial resistance (AMR) is the ability of bacteria, viruses, fungi or parasites to stop responding to medicines that used to control them, making infections harder to treat.
- Urine culture
A urine culture grows a urine sample on culture media to see whether bacteria or fungi are present, count them in colony-forming units per millilitre (CFU/mL) and identify them. If a pathogen grows, antibiotic susceptibility testing follows.
Educational content for healthcare professionals and the general public. It does not replace assessment by a healthcare professional: ordering and interpreting any test is the responsibility of the requesting clinician.
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