Blood culture (automated systems)

Principle

• Blood is inoculated into broth bottles that are incubated and monitored continuously by an instrument.
• Growing organisms produce CO2 (and consume O2), which changes a sensor in the base of each bottle.
• BacT/ALERT uses a colorimetric CO2 sensor; BACTEC uses a fluorescent sensor; VersaTREK detects pressure changes.
• The instrument flags a bottle positive when the signal rate or level crosses its algorithm threshold.

Specimen

• Adults: 2–3 sets from separate venipunctures before antimicrobials; each set is one aerobic plus one anaerobic bottle.
• Typical adult volume 20–30 mL per set (about 8–10 mL per bottle); volume is the single most important factor for yield.
• Children: smaller, weight-based volumes into pediatric bottles.
• Skin antisepsis with chlorhexidine–alcohol (or iodine tincture); disinfect the bottle septum with alcohol and let it dry.

Reagents and equipment

• Aerobic, anaerobic and pediatric bottles; many contain resins or charcoal to neutralize antibiotics.
• Sodium polyanethol sulfonate (SPS) is the usual anticoagulant; it inhibits complement and phagocytosis but can inhibit some Neisseria spp., Gardnerella vaginalis and Peptostreptococcus anaerobius.

Procedure

• Label bottles, do not cover barcodes, and load promptly; do not refrigerate.
• Standard protocol is 5 days of incubation at about 35 °C with continuous agitation.
• For a flagged bottle: Gram stain, subculture to solid media (aerobic and anaerobic as indicated) and phone the Gram stain result as a critical value.
• Rapid molecular panels or MALDI-TOF on short-incubation growth speed identification.

Quality control and pitfalls

• Monitor contamination rate; the traditional target is below 3%, with ≤1% now encouraged.
• Single positive sets with coagulase-negative staphylococci, Cutibacterium, Micrococcus, diphtheroids or Bacillus (non-anthracis) often indicate contamination.
• Underfilled bottles give false negatives; overfilled bottles may give false positives.
• Instrument-positive, Gram stain-negative bottles need subculture (and acridine orange if used).

Clinical use

• Diagnosis of bacteremia, sepsis, endocarditis and fungemia; guides antimicrobial therapy.