When is a blood culture ordered?
| Situation | Why blood culture is needed |
|---|---|
| Suspected sepsis | Bacteria in the blood require urgent antibiotics |
| High fever with no obvious source | Identify if bacteria are circulating |
| Suspected endocarditis | Heart valve infection — multiple cultures needed |
| Line infection (IV catheter) | Catheters can introduce bacteria into the bloodstream |
| Meningitis workup | Blood culture alongside CSF culture |
| Immunocompromised patients | Higher risk of unusual organisms |
How blood cultures work
The collection and incubation process
Blood is collected under strict sterile technique into two special bottles — one for aerobic organisms (need oxygen) and one for anaerobic organisms (grow without oxygen). The bottles are placed in an automated incubator that monitors for bacterial growth up to 5 days. If growth is detected, the organism is identified and antibiotic sensitivity testing (susceptibility testing) is performed. Results are reported in 24–72 hours if positive, or after 5 days if negative.
Interpreting blood culture results
| Result | Meaning |
|---|---|
| Positive — true bacteraemia | Pathogenic bacteria confirmed — requires antibiotics |
| Positive — contaminant | Skin bacteria (eg. coagulase-negative Staph) — usually from collection, not infection |
| Negative after 5 days | No bacterial or fungal growth detected |
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?