A blood culture is a test that checks whether bacteria, fungi or other microorganisms are present in the bloodstream — a condition called bacteraemia or septicaemia. It's one of the most consequential tests in acute medicine because its result directly guides antibiotic treatment for potentially life-threatening infections.
When blood cultures are ordered
- Suspected sepsis — fever, rapid heart rate, low blood pressure, confusion
- Unexplained fever, particularly in hospitalised or immunocompromised patients
- Suspected endocarditis (infection of the heart valves)
- Suspected meningitis — blood cultures are taken alongside a lumbar puncture
- After prolonged high fever that hasn't responded to initial treatment
- Patients with central venous lines or other indwelling devices at high infection risk
What the process involves
Two or more samples are taken from different sites — this reduces the chance of contamination from skin bacteria being misidentified as a pathogen. Blood is injected into aerobic and anaerobic culture bottles and incubated for up to 5 days. Modern automated systems alert laboratory staff as soon as growth is detected — typically within 12–72 hours if bacteria are present.
If growth occurs, the lab performs Gram staining to identify the type of organism and then susceptibility testing to determine which antibiotics it responds to. This guides a targeted antibiotic switch from the broad-spectrum cover given empirically on admission.
Interpreting results
A negative blood culture after 48–72 hours with no growth is reassuring but not definitive — some organisms are fastidious and slow to grow, and early antibiotic treatment before sampling reduces sensitivity. A positive culture with a known pathogen and matching susceptibility is highly significant. A positive culture with a common skin contaminant (like coagulase-negative Staphylococcus) in only one of multiple bottles is often a contamination, not a true infection.