A critical microbiological test that detects microorganisms in the bloodstream when bacteremia, sepsis, or endocarditis is suspected; collected as two separate sets during fever spikes.
Indication
- Suspected sepsis: fever, chills and rigors, hypotension, tachycardia
- Unexplained high fever (>38°C)
- Suspected endocarditis (murmur, fever, embolic findings)
- Identification of the causative agent in serious infections such as pneumonia, meningitis, pyelonephritis
- Fever in immunosuppressed or neutropenic patients
- Suspected catheter-related infection
- Infection unresponsive to antibiotic therapy
Preparation
- Whenever possible, the sample is taken before antibiotics are started
- Sampling during a fever spike (chill phase) increases yield
- The skin is carefully disinfected with chlorhexidine or iodine
- Bottle caps are also wiped with antiseptic
- Two separate sets (aerobic and anaerobic) are taken from two different sites in the same session
How it's performed
- Blood is drawn under sterile technique from two different peripheral veins
- Approximately 8-10 mL per set is collected from adults; 1-3 mL from infants
- Samples are transferred directly into aerobic and anaerobic blood culture bottles
- Drawing blood from a catheter is generally not recommended (contamination risk); if needed, it is done together with a peripheral sample
- Bottles are loaded into automated systems (BACTEC, BacT/ALERT) and monitored for 5-7 days
- When growth is detected, Gram staining, subculture, and antibiogram are performed
Post-procedure
- When a positive signal occurs, the Gram stain result is rapidly reported to the physician (within hours)
- Identification and antibiogram are completed within 24-72 hours
- Coagulase-negative staphylococci growing in only one bottle are usually considered contamination; growth in both sets supports true bacteremia
- Results are interpreted with the clinical picture and antibiogram, and empirical therapy is adjusted
- In endocarditis, follow-up cultures document conversion to negative within 48-72 hours
Risks
- Transient bruising or tenderness at the venipuncture site
- Rare vasovagal dizziness
- Very rare phlebitis or local infection
- Contamination from improper antiseptic technique may produce false positive results
- False negatives are more likely when antibiotics have already been started
FAQ
Why are two separate sets collected?
When the same organism grows in both sets taken from two separate sites, it indicates true infection; growth in only one set usually represents skin contamination.
Can the test be done without fever?
In continuous bacteremias such as endocarditis, fever is not required. In intermittent bacteremias, sampling during a fever spike improves yield.
How long does the result take?
A positive signal usually appears within 24-72 hours; final identification and antibiogram are completed in 2-4 days. If the culture remains negative for 5-7 days, a 'no growth' report is issued.
Should antibiotics be delayed until samples are taken?
In severe presentations such as sepsis, empirical antibiotics are started immediately after samples are obtained. Delaying treatment may worsen the prognosis.
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